Structure and properties of laser beam welding zone of dissimilar materials, AZ31 magnesium alloy and A5754 Aluminum alloy, are investigated. The microstructure and quality of the Mg/Al weld were studied by metallography, microhardness and optical microscopy. Differences in physical and mechanical properties of both materials, magnesium and aluminum, affect weldability and resistance of this combination, and lead to the formation of intermetallic compounds in the welded metal

Fatih H.

H. Mhiri

J. E. Masse

K. Abderrazak

M. Autric

Nakada K.

Peng L.

S. Bannour

S. Mattei

Stodolsky R.

Yoo M. S.

English

International audienceStructure and properties of laser beam welding zone of dissimilar materials, AZ31 magnesium alloy and A5754 Aluminum alloy, are investigated. The microstructure and quality of the Mg/Al weld were studied by metallography, microhardness and optical microscopy. Differences in physical and mechanical properties of both materials, magnesium and aluminum, affect weldability and resistance of this combination, and lead to the formation of intermetallic compounds in the welded metal

Bannour, Sana

Abderrazak, Kamel

Mattei, Simone

Masse, Jean-Eric

Autric, Michel

Mhiri, Hatem

HAL-uB

The influence of position in overlap joints of Mg and Al alloys on microstructure and hardness of laser welds

BANNOUR, Sana

ABDERRAZAK, Kamel

MATTEI, Simone

MASSE, Jean-Eric

AUTRIC, Michel

MHIRI, Hatem

SAM;  the Arts et Métiers ParisTech open access repository

Science Arts & Métiers (SAM)
is an open access repository that collects the work of Arts et Métiers ParisTech
researchers and makes it freely available over the web where possible.
This is an author-deposited version published in: https://sam.ensam.eu
Handle ID: .http://hdl.handle.net/10985/6857
To cite this version :
Sana BANNOUR, Kamel ABDERRAZAK, Simone MATTEI, Jean-Eric MASSE, Michel AUTRIC,
Hatem MHIRI - The influence of position in overlap joints of Mg and Al alloys on microstructure
and hardness of laser welds - The influence of position in overlap joints of Mg and Al alloys on
microstructure and hardness of laser welds - Vol. 25, n°3, p.032001 - 032008 - 2013
Any correspondence concerning this service should be sent to the repository
Administrator : archiveouverte@ensam.eu
1 
 
The influence of position in overlap joints of Mg and Al 
alloys on microstructure and hardness of laser welds. 
S. BANNOURa,b *, K. ABDERRAZAKa, S. MATTEIc, J.E. MASSEd, M. AUTRICb, H. 
MHIRIa. 
a
Unité de Thermique et Thermodynamique des Procédés Industriels, Ecole Nationale d’Ingénieurs de Monastir, Avenue Ibn Jazzar, 5019 
Monastir, Tunisia, sana_bannour@yahoo.fr, Tel: +33 646 11 62 71, Fax +33 491 11 38 38. 
b
Université de la Méditerranée – Aix-Marseille Université, Institut de Mécanique de Marseille, Laboratoire IUSTI, UMR CNRS 6595, 
Technopôle de Château- Gombert, 60 rue Joliot Curie, 13453 Marseille Cedex 13, France 
c
Université de Bourgogne, Laboratoire Interdisciplinaire Carnot de Bourgogne, Equipe Laser et Traitement des Matériaux, UMR CNRS 
5209, 12 rue de la fonderie, 71200, Le Creusot, France. 
d
ENSAM, Ecole Nationale Supérieure d’Arts et Métiers, Laboratoire Mécasurf, 2 cours des arts et métiers, 13617 Aix en Provence,  France. 
 
Abstract 
Structure and properties of laser beam welding zone of dissimilar materials, AZ31 
magnesium alloy and A5754 Aluminum alloy, are investigated. The microstructure and 
quality of the Mg/Al weld were studied by metallography, microhardness and optical 
microscopy. Differences in physical and mechanical properties of both materials, magnesium 
and aluminum, affect weldability and resistance of this combination, and lead to the formation 
of intermetallic compounds in the welded metal. 
 
Keywords: laser welding, dissimilar materials, AZ31 magnesium alloy, A5754 aluminum 
alloy, microstructure evolution. 
 
1. Introduction 
The procedures mostly described for obtaining quality welds in the literature have been 
elaborated for identical metals or, at least, for metals of similar compositions and properties. 
However, there are many applications in which assembly is made from metals of different 
compositions. This may be the case when one has to solve a mechanical wear problem, a 
high-temperature situation, or other conditions in which different properties are required from 
different parts of the same assembly. Therefore, the dissimilar-metal welding process has 
been identified as top priority for materials joining technologies recently [1]. 
Use of lightweight materials improves the energy efficiency and fuel economy of 
automotive vehicles. Thus, because of their extremely low weight in combination with their 
good castability and workability, the use of Mg alloys and Al alloys in specific structural 
applications has increased, replacing often the steels [2-3]. 
2 
 
Welding of Mg and Al to form a compound structure can reduce the weight of the 
structure and the cost of the workpiece. However, it is difficult to obtain good dissimilar 
welds of these two kinds of alloys through conventional fusion welding techniques, such as 
arc and electron beam [4], since large intermetallic compounds, which have a strong negative 
effect on the mechanical properties of the weld, are easy to form in these processes [5]. 
In the past, the dissimilar combination of magnesium and aluminum has been realized by 
vacuum diffusion welding [6] or even by explosion welding [7]. These methods allow to 
obtain promising results but are difficult to implement in the context of industrialization. 
Other techniques, like friction stir welding [8] are particularly suitable for joining very 
dissimilar materials, by reducing the amount of interfacial brittle phases promoted by high 
temperature gradients. But again, the process used does not allow flexibility in the case of 
complex shapes or when the processing speed is a key factor in the success of the assembly. 
Among the welding processes, laser techniques are now widely used for industrial 
production of homogeneous joints in the automotive industry because of a rather high 
flexibility of laser tool, and because of high welding speed. However, few studies have stated 
the limit of such techniques to join the Mg alloys to Al alloys [9]. So it is necessary to study 
the microstructure of laser welded joint Mg/Al.  
The main objective of this research is to investigate the microstructure and the effect of 
the position in overlap joints of Al and Mg on the intermetallic compounds formation in the 
fusion zone. Experiments were conducted to provide a better understanding of the process. 
The microstructure and micro-hardness distribution were observed and analyzed by means of 
metallography and microhardness tests. 
 
2. Experimental details 
AZ31 magnesium alloy and A5754 aluminum alloy were selected as the base metal for 
welding experiments. AZ31 magnesium alloy was chosen due to its good rolling capabilities. 
2 mm thick plates were hot rolled by several passes and then leveled and annealed at 300°C. 
The A5754 aluminum alloy belongs to the 5xxx series. This group includes magnesium as the 
major alloying element (up to 5.6%). These alloys derive their strength from the solid solution 
strengthening due to magnesium. The nominal alloying elemental compositions of AZ31 and 
A5754 alloy are shown in Table 1. The welding technological parameters are listed in Table 
2. 
 
3 
 
Materials Chemical Composition (Wt. %)     
 Mg Al Mn Zn Fe Cu Si 
AZ31 96.67 2.65 0.17 0.52 - - - 
A5754 2.68 96.63 - - 0.4 0.14 0.13 
Table 1. Chemical composition of test materials. 
 
 
 
Table 2. Welding technological parameters used in test. 
 
The sheet was cut and machined into rectangular samples, 160 mm long by 110 mm wide. 
Two kinds of assemblies were realized: (1) with aluminum placed upon magnesium and (2) 
with the magnesium placed upon aluminum in an overlap configuration, Fig.1 (a) and (b) 
respectively. For the investigation, the experiments were carried out with a 6kW diode-
pumped disk laser TRUMPF 6002. The source is a Yb: YAG continuous laser with 1032 nm-
wavelength. The spot size of the laser beam on the plates was 0.4 mm with a Gaussian 
intensity distribution. The workpieces are moved using a Computer Numerically Controlled 
table. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.1. Laser welding of Mg alloy AZ31 to Al alloy A5754. 
 
Joint Laser power 
(W) 
Velocity  
m.s-1 
Linear Energy 
KJ.m-1 
Al to Mg 2500 0.067 37.5 
Mg to Al 4000 0.1 40 
(a) 
(b) 
2 mm 
Magnesium
Aluminum 
20 mm
Laser
beam 
2 mm 
20 mm
Magnesium 
Aluminum
Laser
beam 
4 
 
As magnesium and aluminum are highly susceptible to oxidation, a protective atmosphere 
is required during welding. The use of shielding gas is also used to protect the optics from 
metal slag. It is well known that the choice of the shielding gas has an influence on the 
formation of the plasma. Three types of shielding gases are commonly used in laser welding: 
helium, argon and nitrogen. Many authors have investigated the effectiveness of these three 
shielding gases and have reported that an argon gas flow was a good choice for these two 
materials, in case of Nd:YAG welding [10]. The use of a back shielding gas can reduce the 
sag, which often occurs due to the low viscosity and surface tension of magnesium. 
The influence of gas flow rate on weld width was also studied and it was reported that 
increasing gas flow up to 20 l/min is needed to affect the susceptibility to oxidation [10]. Next 
to these works, we decide to use 20 l/min argon gas flow to protect the molten pool of the top 
and back from oxidation. The focal point was fixed on the surface of workpiece. 
After welding, the laser-welded joints were ground with abrasive paper, mechanically 
polished with a final polishing step, comprising use of a 0.3 µm diamond paste. 
Microstructural characterizations of welded samples were performed on their cross-
sections by using both Optical Microscopy (OM, Leitz RZD-DO) and Scanning Electron 
microscopy (SEM, Jeol JSM-7001F) accompanied by Energy Dispersive X-ray Spectroscopy 
(EDX, Oxford). The measurement of hardness was performed on a Leica VMHT digit 
hardness tester with a load of 1.96 N and dwelling time of 15 s to estimate mechanical 
properties of dissimilar welds. 
 
3. Results and discussion 
Fig.2 shows an optical macrograph of a transverse section of dissimilar welds. The mixing 
of the two metals was very clear. It could be seen that the molten metal of upper sheet 
penetrated in the lower sheet in the two welded samples, the penetration depth of the weld 
pool within the lower sheet was 0.45 mm and 2 mm, Fig.2 (a) and (b) respectively.  
 
 
 
 
 
5 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.2. Optical macrographs of cross-section of dissimilar welds, (a) A5754 on the top,          
(b) AZ31 on the top. 
 
Fig.3 shows SEM micrographs at the interface between the fusion zone and the Mg 
substrate, Fig.3(a), and between the fusion zone and the Al substrate, Fig.3(b). The 
microstructure consists of three different areas: base metal, heat-affected zone and fusion 
zone. From the microstructure analysis, the heat-affected area could be identified only 
between fusion zone and Mg substrate, Fig.3(a), however no difference could not be observed 
between the heat-affected zone and the Al substrate, Fig.3(b). Also, it can be seen from the 
Fig.3(a) that the structure close to the weld metal was columnar crystals which grew into the 
base metal.  
Fig.4 shows the micrograph of Mg-Al interface. The welded metal is mainly composed of 
dendrite crystals, which were eutectic in structure formed during the laser beam welding [11]. 
From this figure, it can be observed that the microstructure of the fusion zone is composed of 
a rather complex and heterogeneous solidification structure. This microstructure was 
promoted by hydrodynamic movements in the Mg-Al interface. A recrystallization 
phenomenon was also evidenced in the fusion zone of dissimilar laser welds, with a grains 
refinement generated during cooling thermal cycles. 
 
6 
 
Fig.3. The SEM image and the distribution of Mg, Al, in the joint, with magnesium placed 
upon aluminum. 
 
Mg base metal 
Weld metal 
Weld metal 
Al base metal
(b) 
(a) 
7 
 
Quantitative analyses of the Mg and Al elements were done in different areas (A, B, C and 
D) of the joint, Fig. 2(a) and (b). For convenience of discussion, the binary Al-Mg phase 
diagram [12] is shown in Fig.5. The analysis results of the chemical composition of various 
representative areas of the welds are given in Table 3. 
Materials Chemical Composition (Wt. %)   
 Mg Al Zn Fe Si 
Area A1 20.12 79.88    
Area A2 32.71 66.94  0.35  
Area B1 63.59 36.13 0.28   
Area B2 66.66 32.92 0.42   
Area C1 66.21 30.73 0.64  2.41 
Area C2 82.69 16.29   1.02 
Area D1 36.62 63.38    
Area D2 20.88 79.12    
Table 3. Chemical composition in the different areas representative of the welds. 
 
Areas A and B relate to the configuration in which the Al alloy A5754 is above the Mg 
alloy AZ31. From the EDS analysis, Fig.6, in the A area which is at the top of the fusion 
zone, parts A1 and A2 have an aluminum concentration well above the concentration of 
magnesium. These results suggest that the area A consists of intermetallic compounds Al3Mg2 
and of a α-Al solid solution, in proportions that vary depending on the location of the 
measurement and thus from the cooling of the melted zone. In the B area, which is at the 
bottom of the fusion zone, analysis of part B1 indicating that this location may contains 
Al12Mg17 intermetallic compound, while part B2 is composed of a mixture of intermetallic 
compounds Al12Mg17 and of a α-Mg solid solution. It may be noted that the area A contains 
iron, an element present in the aluminum alloy in similar proportions. 
8 
 
 
Fig.4. Microstructure of fusion zone in dissimilar weld when Mg was on the top. 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.5. Al-Mg phase diagram [12]. 
9 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.6. Chemical analyses of Al and Mg distribution in (a) region A and (b) region B, 
figure 2(a). 
B1
B2
A2 
A1
(a) 
(b) 
10 
 
Figures 7 and 8 show the element spatial distribution at the top and the bottom of the 
fusion zone, respectively, in the configuration in which the Mg alloy AZ31 is above the Al-
alloy A5754.  
From EDS analysis of C area, measurements results suggest that the grey phase (part C1) 
and black phase (part C2) could be composed of the mixture of the intermetallic compound 
Al12Mg17 and the α-Mg solid solution. Also, a large number of precipitated particles 
containing Mn (white dots) are observed in the fusion zone. 
The microstructure of the area D is shown in Fig.8. The structure showed some 
precipitates containing Fe. From the EDS analysis, it is noted that the composition of the 
interdendritic part D1 corresponds to the intermetallic compound Al3Mg2. The element 
analysis of part D2 is consistent with a mixture of the intermetallic compound Al3Mg2 and the 
α-Al solid solution. 
We can conclude from qualitative and quantitative EDS micro-analyses that during the lap 
welding process of aluminum-base alloy A5754 and magnesium-base alloy AZ31, the α-Al 
solid solution and Al3Mg2 phase were the first intermetallic compounds formed adjacent to Al 
side. However, the α-Mg solid solution and Al12Mg17 phase were the first intermetallic 
compounds formed adjacent to Mg.  
Particular attention is also given to the mechanical properties of dissimilar welds by 
monitoring the hardness in the different zones (Fusion zone and base metals). The 
microhardness distribution of the fusion zone has an effect on the performance of the weld.  
Fig.9 shows the hardness profile of the dissimilar weld AZ31/A5754. As can be seen, the 
hardness of the fusion zone is significantly higher than that of Mg substrate (65 HV) and Al 
substrate (64 HV). The microstructure of the fusion zone of dissimilar weld AZ31/A5754 
depends on the chemical composition of both AZ31 and A5754 base metals. So, the structure 
transformation may be reflected by the microhardness distribution.  
According to previous observation, Fig.6 and Fig.7, the fusion zone was characterized by 
complex vortex flow so this enhances the intermetallic compounds formations, which were 
composed of the brittle phases, Al3Mg2 and Al12Mg17, as confirmed by much higher hardness 
in the fusion zone relative to the base metals, Fig.9 (a) and (b). It was shown that the phases 
Al3Mg2 and Al12Mg17 can reach hardness levels greater than 220 Vickers [13]. These 
important hard phases, combined with a refinement of the grain, can explain the values 
reached in the melted zones. 
11 
 
Fig.7. Quantitative EDS maps of Al, Mg and Mn distribution in region C, figure 2(b). 
C1 
C2
12
Fig.8. Quantitative EDS maps of Al, Mg and Fe distribution in region D, figure 2(b). 
 
 
 
 
D1 
D2
13
0
50
100
150
200
250
-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500
H
ar
dn
es
s(
 H
V
)
Distance from joint center (µm)
 
 
 
 
 
 
 
 
 
 
 
 
Fig.9. Microhardness profiles of microstructure from (a) Mg to Al and (b) Al to Mg. 
 
 
4. Conclusion 
This study could be a helpful document to understand the microstructure and the 
performance of a Mg/Al laser beam weld, when the aluminum is placed above the magnesium 
and also when the magnesium is placed above the aluminum. 
1- The microstructure in the fusion zone of dissimilar welds between Mg/Al was 
observed and analyzed. The welded metal was mainly composed of dendrite crystals. 
2- A variety of Al-Mg intermetallic compounds, Al12Mg17 and Al3Mg2, was generated in 
the irregular shaped region in the welded zone. 
0
50
100
150
200
250
300
350
-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500
H
ar
dn
es
s (
 H
V
)
Distance from joint center (µm)
(a)
(b) 
14 
 
3- The microhardness measurements in the fusion zone were higher than that in the base 
metals. The test results indicate that the Mg-Al intermetallic with high hardness could 
be formed near the fusion zone, and this caused much higher hardness in the weld 
metal. 
 
Acknowledgements 
The authors wish to place their sincere thanks to Mr. Henri Andrzejewski for his technical 
assistance in laser experiments.  
 
References 
[1] K. Nakada, M. Ushio. Needs and properties of dissimilar metal joining and welding. 
Journal of Japon Welding Society, (2002) 71(6):6-9. 
[2] R. Stodolsky, A. Vyas, R. Cuenca, L. Gaines. Life-cycle energy savings potential from 
aluminum-intensive vehicles. In SAE Conference 1995. Argonne National Laboratory. 
[3] M.J. Torkamany, S. Tahamtan, J. Sabbaghzadeh. Dissimilar welding of carbon steel to 
5754 aluminum alloy by Nd:YAG pulsed laser. Materials Design, (2010) 31:458–65. 
[4] H. Fatih. The effects of the welding current on heat input, nugget geometry, and the 
mechanical and fractural properties of resistance spot welding on Mg/Al dissimilar materials. 
Materials and Design, (2011) 32:2476–2484. 
[5] L. Peng, L. Yajiang, G. Haoran, W. Juan. Microstructure characteristics in TIG welded 
joint of Mg/Al dissimilar materials. Materials Letters, (2007) 61; 1288–1291. 
[6] L. Peng, L. Yajiang, G. Haoran, W. Juan. A study of phase constitution near the interface 
of Mg/Al vacuum diffusion bonding. Materials Letters, (2005) 59;2001–2005. 
[7] Y.B. Yan, Z.W. Zhang, W. Shen, J.H. Wang, L.K. Zhang, B.A. Chin. Microstructure and 
properties of Magnesium AZ31B–Aluminum 7075 explosively welded composite plate. 
Materials Science and Engineering A, (2010) 527;2241–2245. 
[8] R.S. Mishra, Z.Y. Ma. Friction stir welding and processing. Materials Science and 
Engineering A, (2005) 50:1–78. 
[9] R. Borrisutthekul, Y. Miyashita, Y. Mutoh. Dissimilar material laser welding between 
magnesium alloy AZ31B and aluminum alloy A5052-O. Science and Technology of 
Advanced Materials, (2005) 6:199–204. 
[10] X. Cao, M. Jahazi, J.P. Immarigeon, Wallace W. A review of laser welding techniques 
for magnesium alloys. J Mater Proc Technol, (2006) 171:188–204. 
[11] Y.J. Quan, Z.H. Chen, X.S. Gong, Z.H. Yu. Materials Characterization, (2008). 
15 
 
[12] J.L. Murray. The Al-Mg (Aluminum-Magnesium) System. Bull. Alloy Phase Diagrams,  
(1982) 3:60–74. 
[13] M.S. Yoo, K.S. Shin, N.J. Kim. Effect of Mg2Si particles on the elevated temperature 
tensile properties of squeeze-cast Mg-Al alloys. Metallurgical and Materials Transactions A, 
(2004) 35;1629-1632. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 


SAM : Science Arts et Métiers

Science Arts & Métiers (SAM)is an open access repository that collects the work of Arts et Métiers ParisTechresearchers and makes it freely available over the web where possible.This is an author-deposited version published in: http://sam.ensam.euHandle ID: .http://hdl.handle.net/10985/6857To cite this version :Sana BANNOUR, Kamel ABDERRAZAK, Simone MATTEI, Jean-Eric MASSE, Michel AUTRIC,Hatem MHIRI - The influence of position in overlap joints of Mg and Al alloys on microstructureand hardness of laser welds - The influence of position in overlap joints of Mg and Al alloys onmicrostructure and hardness of laser welds - Vol. 25, n°3, p.032001 - 032008 - 2013Any correspondence concerning this service should be sent to the repositoryAdministrator : archiveouverte@ensam.eu1  The influence of position in overlap joints of Mg and Al alloys on microstructure and hardness of laser welds. S. BANNOURa,b *, K. ABDERRAZAKa, S. MATTEIc, J.E. MASSEd, M. AUTRICb, H. MHIRIa. aUnité de Thermique et Thermodynamique des Procédés Industriels, Ecole Nationale d’Ingénieurs de Monastir, Avenue Ibn Jazzar, 5019 Monastir, Tunisia, sana_bannour@yahoo.fr, Tel: +33 646 11 62 71, Fax +33 491 11 38 38. bUniversité de la Méditerranée – Aix-Marseille Université, Institut de Mécanique de Marseille, Laboratoire IUSTI, UMR CNRS 6595, Technopôle de Château- Gombert, 60 rue Joliot Curie, 13453 Marseille Cedex 13, France cUniversité de Bourgogne, Laboratoire Interdisciplinaire Carnot de Bourgogne, Equipe Laser et Traitement des Matériaux, UMR CNRS 5209, 12 rue de la fonderie, 71200, Le Creusot, France. dENSAM, Ecole Nationale Supérieure d’Arts et Métiers, Laboratoire Mécasurf, 2 cours des arts et métiers, 13617 Aix en Provence,  France.  Abstract Structure and properties of laser beam welding zone of dissimilar materials, AZ31 magnesium alloy and A5754 Aluminum alloy, are investigated. The microstructure and quality of the Mg/Al weld were studied by metallography, microhardness and optical microscopy. Differences in physical and mechanical properties of both materials, magnesium and aluminum, affect weldability and resistance of this combination, and lead to the formation of intermetallic compounds in the welded metal.  Keywords: laser welding, dissimilar materials, AZ31 magnesium alloy, A5754 aluminum alloy, microstructure evolution.  1. Introduction The procedures mostly described for obtaining quality welds in the literature have been elaborated for identical metals or, at least, for metals of similar compositions and properties. However, there are many applications in which assembly is made from metals of different compositions. This may be the case when one has to solve a mechanical wear problem, a high-temperature situation, or other conditions in which different properties are required from different parts of the same assembly. Therefore, the dissimilar-metal welding process has been identified as top priority for materials joining technologies recently [1]. Use of lightweight materials improves the energy efficiency and fuel economy of automotive vehicles. Thus, because of their extremely low weight in combination with their good castability and workability, the use of Mg alloys and Al alloys in specific structural applications has increased, replacing often the steels [2-3]. 2  Welding of Mg and Al to form a compound structure can reduce the weight of the structure and the cost of the workpiece. However, it is difficult to obtain good dissimilar welds of these two kinds of alloys through conventional fusion welding techniques, such as arc and electron beam [4], since large intermetallic compounds, which have a strong negative effect on the mechanical properties of the weld, are easy to form in these processes [5]. In the past, the dissimilar combination of magnesium and aluminum has been realized by vacuum diffusion welding [6] or even by explosion welding [7]. These methods allow to obtain promising results but are difficult to implement in the context of industrialization. Other techniques, like friction stir welding [8] are particularly suitable for joining very dissimilar materials, by reducing the amount of interfacial brittle phases promoted by high temperature gradients. But again, the process used does not allow flexibility in the case of complex shapes or when the processing speed is a key factor in the success of the assembly. Among the welding processes, laser techniques are now widely used for industrial production of homogeneous joints in the automotive industry because of a rather high flexibility of laser tool, and because of high welding speed. However, few studies have stated the limit of such techniques to join the Mg alloys to Al alloys [9]. So it is necessary to study the microstructure of laser welded joint Mg/Al.  The main objective of this research is to investigate the microstructure and the effect of the position in overlap joints of Al and Mg on the intermetallic compounds formation in the fusion zone. Experiments were conducted to provide a better understanding of the process. The microstructure and micro-hardness distribution were observed and analyzed by means of metallography and microhardness tests.  2. Experimental details AZ31 magnesium alloy and A5754 aluminum alloy were selected as the base metal for welding experiments. AZ31 magnesium alloy was chosen due to its good rolling capabilities. 2 mm thick plates were hot rolled by several passes and then leveled and annealed at 300°C. The A5754 aluminum alloy belongs to the 5xxx series. This group includes magnesium as the major alloying element (up to 5.6%). These alloys derive their strength from the solid solution strengthening due to magnesium. The nominal alloying elemental compositions of AZ31 and A5754 alloy are shown in Table 1. The welding technological parameters are listed in Table 2.  3  Materials Chemical Composition (Wt. %)      Mg Al Mn Zn Fe Cu Si AZ31 96.67 2.65 0.17 0.52 - - - A5754 2.68 96.63 - - 0.4 0.14 0.13 Table 1. Chemical composition of test materials.    Table 2. Welding technological parameters used in test.  The sheet was cut and machined into rectangular samples, 160 mm long by 110 mm wide. Two kinds of assemblies were realized: (1) with aluminum placed upon magnesium and (2) with the magnesium placed upon aluminum in an overlap configuration, Fig.1 (a) and (b) respectively. For the investigation, the experiments were carried out with a 6kW diode-pumped disk laser TRUMPF 6002. The source is a Yb: YAG continuous laser with 1032 nm-wavelength. The spot size of the laser beam on the plates was 0.4 mm with a Gaussian intensity distribution. The workpieces are moved using a Computer Numerically Controlled table.               Fig.1. Laser welding of Mg alloy AZ31 to Al alloy A5754.  Joint Laser power (W) Velocity  m.s-1 Linear Energy KJ.m-1 Al to Mg 2500 0.067 37.5 Mg to Al 4000 0.1 40 (a) (b) 2 mm MagnesiumAluminum 20 mmLaserbeam 2 mm 20 mmMagnesium AluminumLaserbeam 4  As magnesium and aluminum are highly susceptible to oxidation, a protective atmosphere is required during welding. The use of shielding gas is also used to protect the optics from metal slag. It is well known that the choice of the shielding gas has an influence on the formation of the plasma. Three types of shielding gases are commonly used in laser welding: helium, argon and nitrogen. Many authors have investigated the effectiveness of these three shielding gases and have reported that an argon gas flow was a good choice for these two materials, in case of Nd:YAG welding [10]. The use of a back shielding gas can reduce the sag, which often occurs due to the low viscosity and surface tension of magnesium. The influence of gas flow rate on weld width was also studied and it was reported that increasing gas flow up to 20 l/min is needed to affect the susceptibility to oxidation [10]. Next to these works, we decide to use 20 l/min argon gas flow to protect the molten pool of the top and back from oxidation. The focal point was fixed on the surface of workpiece. After welding, the laser-welded joints were ground with abrasive paper, mechanically polished with a final polishing step, comprising use of a 0.3 µm diamond paste. Microstructural characterizations of welded samples were performed on their cross-sections by using both Optical Microscopy (OM, Leitz RZD-DO) and Scanning Electron microscopy (SEM, Jeol JSM-7001F) accompanied by Energy Dispersive X-ray Spectroscopy (EDX, Oxford). The measurement of hardness was performed on a Leica VMHT digit hardness tester with a load of 1.96 N and dwelling time of 15 s to estimate mechanical properties of dissimilar welds.  3. Results and discussion Fig.2 shows an optical macrograph of a transverse section of dissimilar welds. The mixing of the two metals was very clear. It could be seen that the molten metal of upper sheet penetrated in the lower sheet in the two welded samples, the penetration depth of the weld pool within the lower sheet was 0.45 mm and 2 mm, Fig.2 (a) and (b) respectively.       5                Fig.2. Optical macrographs of cross-section of dissimilar welds, (a) A5754 on the top,          (b) AZ31 on the top.  Fig.3 shows SEM micrographs at the interface between the fusion zone and the Mg substrate, Fig.3(a), and between the fusion zone and the Al substrate, Fig.3(b). The microstructure consists of three different areas: base metal, heat-affected zone and fusion zone. From the microstructure analysis, the heat-affected area could be identified only between fusion zone and Mg substrate, Fig.3(a), however no difference could not be observed between the heat-affected zone and the Al substrate, Fig.3(b). Also, it can be seen from the Fig.3(a) that the structure close to the weld metal was columnar crystals which grew into the base metal.  Fig.4 shows the micrograph of Mg-Al interface. The welded metal is mainly composed of dendrite crystals, which were eutectic in structure formed during the laser beam welding [11]. From this figure, it can be observed that the microstructure of the fusion zone is composed of a rather complex and heterogeneous solidification structure. This microstructure was promoted by hydrodynamic movements in the Mg-Al interface. A recrystallization phenomenon was also evidenced in the fusion zone of dissimilar laser welds, with a grains refinement generated during cooling thermal cycles.  6  Fig.3. The SEM image and the distribution of Mg, Al, in the joint, with magnesium placed upon aluminum.  Mg base metal Weld metal Weld metal Al base metal(b) (a) 7  Quantitative analyses of the Mg and Al elements were done in different areas (A, B, C and D) of the joint, Fig. 2(a) and (b). For convenience of discussion, the binary Al-Mg phase diagram [12] is shown in Fig.5. The analysis results of the chemical composition of various representative areas of the welds are given in Table 3. Materials Chemical Composition (Wt. %)    Mg Al Zn Fe Si Area A1 20.12 79.88    Area A2 32.71 66.94  0.35  Area B1 63.59 36.13 0.28   Area B2 66.66 32.92 0.42   Area C1 66.21 30.73 0.64  2.41 Area C2 82.69 16.29   1.02 Area D1 36.62 63.38    Area D2 20.88 79.12    Table 3. Chemical composition in the different areas representative of the welds.  Areas A and B relate to the configuration in which the Al alloy A5754 is above the Mg alloy AZ31. From the EDS analysis, Fig.6, in the A area which is at the top of the fusion zone, parts A1 and A2 have an aluminum concentration well above the concentration of magnesium. These results suggest that the area A consists of intermetallic compounds Al3Mg2 and of a α-Al solid solution, in proportions that vary depending on the location of the measurement and thus from the cooling of the melted zone. In the B area, which is at the bottom of the fusion zone, analysis of part B1 indicating that this location may contains Al12Mg17 intermetallic compound, while part B2 is composed of a mixture of intermetallic compounds Al12Mg17 and of a α-Mg solid solution. It may be noted that the area A contains iron, an element present in the aluminum alloy in similar proportions. 8   Fig.4. Microstructure of fusion zone in dissimilar weld when Mg was on the top.              Fig.5. Al-Mg phase diagram [12]. 9                                Fig.6. Chemical analyses of Al and Mg distribution in (a) region A and (b) region B, figure 2(a). B1B2A2 A1(a) (b) 10  Figures 7 and 8 show the element spatial distribution at the top and the bottom of the fusion zone, respectively, in the configuration in which the Mg alloy AZ31 is above the Al-alloy A5754.  From EDS analysis of C area, measurements results suggest that the grey phase (part C1) and black phase (part C2) could be composed of the mixture of the intermetallic compound Al12Mg17 and the α-Mg solid solution. Also, a large number of precipitated particles containing Mn (white dots) are observed in the fusion zone. The microstructure of the area D is shown in Fig.8. The structure showed some precipitates containing Fe. From the EDS analysis, it is noted that the composition of the interdendritic part D1 corresponds to the intermetallic compound Al3Mg2. The element analysis of part D2 is consistent with a mixture of the intermetallic compound Al3Mg2 and the α-Al solid solution. We can conclude from qualitative and quantitative EDS micro-analyses that during the lap welding process of aluminum-base alloy A5754 and magnesium-base alloy AZ31, the α-Al solid solution and Al3Mg2 phase were the first intermetallic compounds formed adjacent to Al side. However, the α-Mg solid solution and Al12Mg17 phase were the first intermetallic compounds formed adjacent to Mg.  Particular attention is also given to the mechanical properties of dissimilar welds by monitoring the hardness in the different zones (Fusion zone and base metals). The microhardness distribution of the fusion zone has an effect on the performance of the weld.  Fig.9 shows the hardness profile of the dissimilar weld AZ31/A5754. As can be seen, the hardness of the fusion zone is significantly higher than that of Mg substrate (65 HV) and Al substrate (64 HV). The microstructure of the fusion zone of dissimilar weld AZ31/A5754 depends on the chemical composition of both AZ31 and A5754 base metals. So, the structure transformation may be reflected by the microhardness distribution.  According to previous observation, Fig.6 and Fig.7, the fusion zone was characterized by complex vortex flow so this enhances the intermetallic compounds formations, which were composed of the brittle phases, Al3Mg2 and Al12Mg17, as confirmed by much higher hardness in the fusion zone relative to the base metals, Fig.9 (a) and (b). It was shown that the phases Al3Mg2 and Al12Mg17 can reach hardness levels greater than 220 Vickers [13]. These important hard phases, combined with a refinement of the grain, can explain the values reached in the melted zones. 11  Fig.7. Quantitative EDS maps of Al, Mg and Mn distribution in region C, figure 2(b). C1 C212Fig.8. Quantitative EDS maps of Al, Mg and Fe distribution in region D, figure 2(b).     D1 D213050100150200250-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500Hardness( HV)Distance from joint center (µm)            Fig.9. Microhardness profiles of microstructure from (a) Mg to Al and (b) Al to Mg.   4. Conclusion This study could be a helpful document to understand the microstructure and the performance of a Mg/Al laser beam weld, when the aluminum is placed above the magnesium and also when the magnesium is placed above the aluminum. 1- The microstructure in the fusion zone of dissimilar welds between Mg/Al was observed and analyzed. The welded metal was mainly composed of dendrite crystals. 2- A variety of Al-Mg intermetallic compounds, Al12Mg17 and Al3Mg2, was generated in the irregular shaped region in the welded zone. 050100150200250300350-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500Hardness ( HV)Distance from joint center (µm)(a)(b) 14  3- The microhardness measurements in the fusion zone were higher than that in the base metals. The test results indicate that the Mg-Al intermetallic with high hardness could be formed near the fusion zone, and this caused much higher hardness in the weld metal.  Acknowledgements The authors wish to place their sincere thanks to Mr. Henri Andrzejewski for his technical assistance in laser experiments.   References [1] K. Nakada, M. Ushio. Needs and properties of dissimilar metal joining and welding. Journal of Japon Welding Society, (2002) 71(6):6-9. [2] R. Stodolsky, A. Vyas, R. Cuenca, L. Gaines. Life-cycle energy savings potential from aluminum-intensive vehicles. In SAE Conference 1995. Argonne National Laboratory. [3] M.J. Torkamany, S. Tahamtan, J. Sabbaghzadeh. Dissimilar welding of carbon steel to 5754 aluminum alloy by Nd:YAG pulsed laser. Materials Design, (2010) 31:458–65. [4] H. Fatih. The effects of the welding current on heat input, nugget geometry, and the mechanical and fractural properties of resistance spot welding on Mg/Al dissimilar materials. Materials and Design, (2011) 32:2476–2484. [5] L. Peng, L. Yajiang, G. Haoran, W. Juan. Microstructure characteristics in TIG welded joint of Mg/Al dissimilar materials. Materials Letters, (2007) 61; 1288–1291. [6] L. Peng, L. Yajiang, G. Haoran, W. Juan. A study of phase constitution near the interface of Mg/Al vacuum diffusion bonding. Materials Letters, (2005) 59;2001–2005. [7] Y.B. Yan, Z.W. Zhang, W. Shen, J.H. Wang, L.K. Zhang, B.A. Chin. Microstructure and properties of Magnesium AZ31B–Aluminum 7075 explosively welded composite plate. Materials Science and Engineering A, (2010) 527;2241–2245. [8] R.S. Mishra, Z.Y. Ma. Friction stir welding and processing. Materials Science and Engineering A, (2005) 50:1–78. [9] R. Borrisutthekul, Y. Miyashita, Y. Mutoh. Dissimilar material laser welding between magnesium alloy AZ31B and aluminum alloy A5052-O. Science and Technology of Advanced Materials, (2005) 6:199–204. [10] X. Cao, M. Jahazi, J.P. Immarigeon, Wallace W. A review of laser welding techniques for magnesium alloys. J Mater Proc Technol, (2006) 171:188–204. [11] Y.J. Quan, Z.H. Chen, X.S. Gong, Z.H. Yu. Materials Characterization, (2008). 15  [12] J.L. Murray. The Al-Mg (Aluminum-Magnesium) System. Bull. Alloy Phase Diagrams,  (1982) 3:60–74. [13] M.S. Yoo, K.S. Shin, N.J. Kim. Effect of Mg2Si particles on the elevated temperature tensile properties of squeeze-cast Mg-Al alloys. Metallurgical and Materials Transactions A, (2004) 35;1629-1632.                   

A review of laser welding techniques for magnesium alloys. J Mater Proc Technol,

A study of phase constitution near the interface of Mg/Al vacuum diffusion bonding. Materials Letters,

Dissimilar material laser welding between magnesium alloy AZ31B and aluminum alloy

Dissimilar welding of carbon steel to 5754 aluminum alloy by Nd:YAG pulsed laser. Materials Design,

Effect of Mg2Si particles on the elevated temperature tensile properties of squeeze-cast Mg-Al alloys. Metallurgical and Materials Transactions A,

Friction stir welding and processing.

Life-cycle energy savings potential from aluminum-intensive vehicles.

Materials Characterization,

Microstructure and properties of Magnesium AZ31B–Aluminum 7075 explosively welded composite plate. Materials Science and Engineering A,

Microstructure characteristics in TIG welded joint of Mg/Al dissimilar materials. Materials Letters,

Needs and properties of dissimilar metal joining and welding.

The Al-Mg (Aluminum-Magnesium) System. Bull. Alloy Phase Diagrams,

The effects of the welding current on heat input, nugget geometry, and the mechanical and fractural properties of resistance spot welding on Mg/Al dissimilar materials. Materials and Design,

HAL AMU

The influence of position in overlap joints of Mg and Alalloys on microstructure and hardness of laser weldsSana Bannour, Kamel Abderrazak, Simone Mattei, Jean-Eric Masse, MichelAutric, Hatem MhiriTo cite this version:Sana Bannour, Kamel Abderrazak, Simone Mattei, Jean-Eric Masse, Michel Autric, et al.. Theinfluence of position in overlap joints of Mg and Al alloys on microstructure and hardness oflaser welds. The influence of position in overlap joints of Mg and Al alloys on microstructureand hardness of laser welds, 2013, 25 (3), pp.032001 - 032008. <10.2351/1.4792615>. <hal-00799202>HAL Id: hal-00799202https://hal.archives-ouvertes.fr/hal-00799202Submitted on 11 Mar 2013HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.1  The influence of position in overlap joints of Mg and Al alloys on microstructure and hardness of laser welds. S. BANNOURa,b *, K. ABDERRAZAKa, S. MATTEIc, J.E. MASSEd, M. AUTRICb, H. MHIRIa. aUnité de Thermique et Thermodynamique des Procédés Industriels, Ecole Nationale d’Ingénieurs de Monastir, Avenue Ibn Jazzar, 5019 Monastir, Tunisia, sana_bannour@yahoo.fr, Tel: +33 646 11 62 71, Fax +33 491 11 38 38. bUniversité de la Méditerranée – Aix-Marseille Université, Institut de Mécanique de Marseille, Laboratoire IUSTI, UMR CNRS 6595, Technopôle de Château- Gombert, 60 rue Joliot Curie, 13453 Marseille Cedex 13, France cUniversité de Bourgogne, Laboratoire Interdisciplinaire Carnot de Bourgogne, Equipe Laser et Traitement des Matériaux, UMR CNRS 5209, 12 rue de la fonderie, 71200, Le Creusot, France. dENSAM, Ecole Nationale Supérieure d’Arts et Métiers, Laboratoire Mécasurf, 2 cours des arts et métiers, 13617 Aix en Provence,  France.  Abstract Structure and properties of laser beam welding zone of dissimilar materials, AZ31 magnesium alloy and A5754 Aluminum alloy, are investigated. The microstructure and quality of the Mg/Al weld were studied by metallography, microhardness and optical microscopy. Differences in physical and mechanical properties of both materials, magnesium and aluminum, affect weldability and resistance of this combination, and lead to the formation of intermetallic compounds in the welded metal.  Keywords: laser welding, dissimilar materials, AZ31 magnesium alloy, A5754 aluminum alloy, microstructure evolution.  1. Introduction The procedures mostly described for obtaining quality welds in the literature have been elaborated for identical metals or, at least, for metals of similar compositions and properties. However, there are many applications in which assembly is made from metals of different compositions. This may be the case when one has to solve a mechanical wear problem, a high-temperature situation, or other conditions in which different properties are required from different parts of the same assembly. Therefore, the dissimilar-metal welding process has been identified as top priority for materials joining technologies recently [1]. Use of lightweight materials improves the energy efficiency and fuel economy of automotive vehicles. Thus, because of their extremely low weight in combination with their good castability and workability, the use of Mg alloys and Al alloys in specific structural applications has increased, replacing often the steels [2-3]. 2  Welding of Mg and Al to form a compound structure can reduce the weight of the structure and the cost of the workpiece. However, it is difficult to obtain good dissimilar welds of these two kinds of alloys through conventional fusion welding techniques, such as arc and electron beam [4], since large intermetallic compounds, which have a strong negative effect on the mechanical properties of the weld, are easy to form in these processes [5]. In the past, the dissimilar combination of magnesium and aluminum has been realized by vacuum diffusion welding [6] or even by explosion welding [7]. These methods allow to obtain promising results but are difficult to implement in the context of industrialization. Other techniques, like friction stir welding [8] are particularly suitable for joining very dissimilar materials, by reducing the amount of interfacial brittle phases promoted by high temperature gradients. But again, the process used does not allow flexibility in the case of complex shapes or when the processing speed is a key factor in the success of the assembly. Among the welding processes, laser techniques are now widely used for industrial production of homogeneous joints in the automotive industry because of a rather high flexibility of laser tool, and because of high welding speed. However, few studies have stated the limit of such techniques to join the Mg alloys to Al alloys [9]. So it is necessary to study the microstructure of laser welded joint Mg/Al.  The main objective of this research is to investigate the microstructure and the effect of the position in overlap joints of Al and Mg on the intermetallic compounds formation in the fusion zone. Experiments were conducted to provide a better understanding of the process. The microstructure and micro-hardness distribution were observed and analyzed by means of metallography and microhardness tests.  2. Experimental details AZ31 magnesium alloy and A5754 aluminum alloy were selected as the base metal for welding experiments. AZ31 magnesium alloy was chosen due to its good rolling capabilities. 2 mm thick plates were hot rolled by several passes and then leveled and annealed at 300°C. The A5754 aluminum alloy belongs to the 5xxx series. This group includes magnesium as the major alloying element (up to 5.6%). These alloys derive their strength from the solid solution strengthening due to magnesium. The nominal alloying elemental compositions of AZ31 and A5754 alloy are shown in Table 1. The welding technological parameters are listed in Table 2.  3  Materials Chemical Composition (Wt. %)      Mg Al Mn Zn Fe Cu Si AZ31 96.67 2.65 0.17 0.52 - - - A5754 2.68 96.63 - - 0.4 0.14 0.13 Table 1. Chemical composition of test materials.    Table 2. Welding technological parameters used in test.  The sheet was cut and machined into rectangular samples, 160 mm long by 110 mm wide. Two kinds of assemblies were realized: (1) with aluminum placed upon magnesium and (2) with the magnesium placed upon aluminum in an overlap configuration, Fig.1 (a) and (b) respectively. For the investigation, the experiments were carried out with a 6kW diode-pumped disk laser TRUMPF 6002. The source is a Yb: YAG continuous laser with 1032 nm-wavelength. The spot size of the laser beam on the plates was 0.4 mm with a Gaussian intensity distribution. The workpieces are moved using a Computer Numerically Controlled table.               Fig.1. Laser welding of Mg alloy AZ31 to Al alloy A5754.  Joint Laser power (W) Velocity  m.s-1 Linear Energy KJ.m-1 Al to Mg 2500 0.067 37.5 Mg to Al 4000 0.1 40 (a) (b) 2 mm MagnesiumAluminum 20 mmLaserbeam 2 mm 20 mmMagnesium AluminumLaserbeam 4  As magnesium and aluminum are highly susceptible to oxidation, a protective atmosphere is required during welding. The use of shielding gas is also used to protect the optics from metal slag. It is well known that the choice of the shielding gas has an influence on the formation of the plasma. Three types of shielding gases are commonly used in laser welding: helium, argon and nitrogen. Many authors have investigated the effectiveness of these three shielding gases and have reported that an argon gas flow was a good choice for these two materials, in case of Nd:YAG welding [10]. The use of a back shielding gas can reduce the sag, which often occurs due to the low viscosity and surface tension of magnesium. The influence of gas flow rate on weld width was also studied and it was reported that increasing gas flow up to 20 l/min is needed to affect the susceptibility to oxidation [10]. Next to these works, we decide to use 20 l/min argon gas flow to protect the molten pool of the top and back from oxidation. The focal point was fixed on the surface of workpiece. After welding, the laser-welded joints were ground with abrasive paper, mechanically polished with a final polishing step, comprising use of a 0.3 µm diamond paste. Microstructural characterizations of welded samples were performed on their cross-sections by using both Optical Microscopy (OM, Leitz RZD-DO) and Scanning Electron microscopy (SEM, Jeol JSM-7001F) accompanied by Energy Dispersive X-ray Spectroscopy (EDX, Oxford). The measurement of hardness was performed on a Leica VMHT digit hardness tester with a load of 1.96 N and dwelling time of 15 s to estimate mechanical properties of dissimilar welds.  3. Results and discussion Fig.2 shows an optical macrograph of a transverse section of dissimilar welds. The mixing of the two metals was very clear. It could be seen that the molten metal of upper sheet penetrated in the lower sheet in the two welded samples, the penetration depth of the weld pool within the lower sheet was 0.45 mm and 2 mm, Fig.2 (a) and (b) respectively.       5                Fig.2. Optical macrographs of cross-section of dissimilar welds, (a) A5754 on the top,          (b) AZ31 on the top.  Fig.3 shows SEM micrographs at the interface between the fusion zone and the Mg substrate, Fig.3(a), and between the fusion zone and the Al substrate, Fig.3(b). The microstructure consists of three different areas: base metal, heat-affected zone and fusion zone. From the microstructure analysis, the heat-affected area could be identified only between fusion zone and Mg substrate, Fig.3(a), however no difference could not be observed between the heat-affected zone and the Al substrate, Fig.3(b). Also, it can be seen from the Fig.3(a) that the structure close to the weld metal was columnar crystals which grew into the base metal.  Fig.4 shows the micrograph of Mg-Al interface. The welded metal is mainly composed of dendrite crystals, which were eutectic in structure formed during the laser beam welding [11]. From this figure, it can be observed that the microstructure of the fusion zone is composed of a rather complex and heterogeneous solidification structure. This microstructure was promoted by hydrodynamic movements in the Mg-Al interface. A recrystallization phenomenon was also evidenced in the fusion zone of dissimilar laser welds, with a grains refinement generated during cooling thermal cycles.  6  Fig.3. The SEM image and the distribution of Mg, Al, in the joint, with magnesium placed upon aluminum.  Mg base metal Weld metal Weld metal Al base metal(b) (a) 7  Quantitative analyses of the Mg and Al elements were done in different areas (A, B, C and D) of the joint, Fig. 2(a) and (b). For convenience of discussion, the binary Al-Mg phase diagram [12] is shown in Fig.5. The analysis results of the chemical composition of various representative areas of the welds are given in Table 3. Materials Chemical Composition (Wt. %)    Mg Al Zn Fe Si Area A1 20.12 79.88    Area A2 32.71 66.94  0.35  Area B1 63.59 36.13 0.28   Area B2 66.66 32.92 0.42   Area C1 66.21 30.73 0.64  2.41 Area C2 82.69 16.29   1.02 Area D1 36.62 63.38    Area D2 20.88 79.12    Table 3. Chemical composition in the different areas representative of the welds.  Areas A and B relate to the configuration in which the Al alloy A5754 is above the Mg alloy AZ31. From the EDS analysis, Fig.6, in the A area which is at the top of the fusion zone, parts A1 and A2 have an aluminum concentration well above the concentration of magnesium. These results suggest that the area A consists of intermetallic compounds Al3Mg2 and of a α-Al solid solution, in proportions that vary depending on the location of the measurement and thus from the cooling of the melted zone. In the B area, which is at the bottom of the fusion zone, analysis of part B1 indicating that this location may contains Al12Mg17 intermetallic compound, while part B2 is composed of a mixture of intermetallic compounds Al12Mg17 and of a α-Mg solid solution. It may be noted that the area A contains iron, an element present in the aluminum alloy in similar proportions. 8   Fig.4. Microstructure of fusion zone in dissimilar weld when Mg was on the top.              Fig.5. Al-Mg phase diagram [12]. 9                                Fig.6. Chemical analyses of Al and Mg distribution in (a) region A and (b) region B, figure 2(a). B1B2A2 A1(a) (b) 10  Figures 7 and 8 show the element spatial distribution at the top and the bottom of the fusion zone, respectively, in the configuration in which the Mg alloy AZ31 is above the Al-alloy A5754.  From EDS analysis of C area, measurements results suggest that the grey phase (part C1) and black phase (part C2) could be composed of the mixture of the intermetallic compound Al12Mg17 and the α-Mg solid solution. Also, a large number of precipitated particles containing Mn (white dots) are observed in the fusion zone. The microstructure of the area D is shown in Fig.8. The structure showed some precipitates containing Fe. From the EDS analysis, it is noted that the composition of the interdendritic part D1 corresponds to the intermetallic compound Al3Mg2. The element analysis of part D2 is consistent with a mixture of the intermetallic compound Al3Mg2 and the α-Al solid solution. We can conclude from qualitative and quantitative EDS micro-analyses that during the lap welding process of aluminum-base alloy A5754 and magnesium-base alloy AZ31, the α-Al solid solution and Al3Mg2 phase were the first intermetallic compounds formed adjacent to Al side. However, the α-Mg solid solution and Al12Mg17 phase were the first intermetallic compounds formed adjacent to Mg.  Particular attention is also given to the mechanical properties of dissimilar welds by monitoring the hardness in the different zones (Fusion zone and base metals). The microhardness distribution of the fusion zone has an effect on the performance of the weld.  Fig.9 shows the hardness profile of the dissimilar weld AZ31/A5754. As can be seen, the hardness of the fusion zone is significantly higher than that of Mg substrate (65 HV) and Al substrate (64 HV). The microstructure of the fusion zone of dissimilar weld AZ31/A5754 depends on the chemical composition of both AZ31 and A5754 base metals. So, the structure transformation may be reflected by the microhardness distribution.  According to previous observation, Fig.6 and Fig.7, the fusion zone was characterized by complex vortex flow so this enhances the intermetallic compounds formations, which were composed of the brittle phases, Al3Mg2 and Al12Mg17, as confirmed by much higher hardness in the fusion zone relative to the base metals, Fig.9 (a) and (b). It was shown that the phases Al3Mg2 and Al12Mg17 can reach hardness levels greater than 220 Vickers [13]. These important hard phases, combined with a refinement of the grain, can explain the values reached in the melted zones. 11  Fig.7. Quantitative EDS maps of Al, Mg and Mn distribution in region C, figure 2(b). C1 C212Fig.8. Quantitative EDS maps of Al, Mg and Fe distribution in region D, figure 2(b).     D1 D213050100150200250-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500Hardness( HV)Distance from joint center (µm)            Fig.9. Microhardness profiles of microstructure from (a) Mg to Al and (b) Al to Mg.   4. Conclusion This study could be a helpful document to understand the microstructure and the performance of a Mg/Al laser beam weld, when the aluminum is placed above the magnesium and also when the magnesium is placed above the aluminum. 1- The microstructure in the fusion zone of dissimilar welds between Mg/Al was observed and analyzed. The welded metal was mainly composed of dendrite crystals. 2- A variety of Al-Mg intermetallic compounds, Al12Mg17 and Al3Mg2, was generated in the irregular shaped region in the welded zone. 050100150200250300350-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500Hardness ( HV)Distance from joint center (µm)(a)(b) 14  3- The microhardness measurements in the fusion zone were higher than that in the base metals. The test results indicate that the Mg-Al intermetallic with high hardness could be formed near the fusion zone, and this caused much higher hardness in the weld metal.  Acknowledgements The authors wish to place their sincere thanks to Mr. Henri Andrzejewski for his technical assistance in laser experiments.   References [1] K. Nakada, M. Ushio. Needs and properties of dissimilar metal joining and welding. Journal of Japon Welding Society, (2002) 71(6):6-9. [2] R. Stodolsky, A. Vyas, R. Cuenca, L. Gaines. Life-cycle energy savings potential from aluminum-intensive vehicles. In SAE Conference 1995. Argonne National Laboratory. [3] M.J. Torkamany, S. Tahamtan, J. Sabbaghzadeh. Dissimilar welding of carbon steel to 5754 aluminum alloy by Nd:YAG pulsed laser. Materials Design, (2010) 31:458–65. [4] H. Fatih. The effects of the welding current on heat input, nugget geometry, and the mechanical and fractural properties of resistance spot welding on Mg/Al dissimilar materials. Materials and Design, (2011) 32:2476–2484. [5] L. Peng, L. Yajiang, G. Haoran, W. Juan. Microstructure characteristics in TIG welded joint of Mg/Al dissimilar materials. Materials Letters, (2007) 61; 1288–1291. [6] L. Peng, L. Yajiang, G. Haoran, W. Juan. A study of phase constitution near the interface of Mg/Al vacuum diffusion bonding. Materials Letters, (2005) 59;2001–2005. [7] Y.B. Yan, Z.W. Zhang, W. Shen, J.H. Wang, L.K. Zhang, B.A. Chin. Microstructure and properties of Magnesium AZ31B–Aluminum 7075 explosively welded composite plate. Materials Science and Engineering A, (2010) 527;2241–2245. [8] R.S. Mishra, Z.Y. Ma. Friction stir welding and processing. Materials Science and Engineering A, (2005) 50:1–78. [9] R. Borrisutthekul, Y. Miyashita, Y. Mutoh. Dissimilar material laser welding between magnesium alloy AZ31B and aluminum alloy A5052-O. Science and Technology of Advanced Materials, (2005) 6:199–204. [10] X. Cao, M. Jahazi, J.P. Immarigeon, Wallace W. A review of laser welding techniques for magnesium alloys. J Mater Proc Technol, (2006) 171:188–204. [11] Y.J. Quan, Z.H. Chen, X.S. Gong, Z.H. Yu. Materials Characterization, (2008). 15  [12] J.L. Murray. The Al-Mg (Aluminum-Magnesium) System. Bull. Alloy Phase Diagrams,  (1982) 3:60–74. [13] M.S. Yoo, K.S. Shin, N.J. Kim. Effect of Mg2Si particles on the elevated temperature tensile properties of squeeze-cast Mg-Al alloys. Metallurgical and Materials Transactions A, (2004) 35;1629-1632.                   

The influence of position in overlap joints of Mg and Al alloys on microstructure and hardness of laser welds

Abstract

Similar works

Full text

Available Versions

HAL-uB

SAM; the Arts et Métiers ParisTech open access repository

SAM : Science Arts et Métiers

HAL AMU

Crossref

Name not available

HAL: Hyper Article en Ligne