Abstract: This paper reports on an investigation conducted to establish the influence of Wire Electrical Discharge Machining (WEDM) on the fracture toughness of aluminum 7075-T6511. The main objective was to determine if WEDM can be used to introduce a pre-crack into a compact tension specimen instead of the ASTM E-1820-11 specified fatigue pre-crack method. Fracture tests were conducted on four specimens which were pre-cracked using the WEDM technique. The rest of the fracture toughness evaluation followed the ASTM E-1820-11 guidelines. Results obtained from the experimental data were found to be inconsistent with the theoretical expectations. The fracture toughness was found to be significantly dependent on the effect of the WEDM on the material. The WEDM introduced a Heat Affected Zone (HAZ) on the surface of the pre-crack which modifies the fracture behavior of the material. It was concluded that WEDM is not a viable alternative to create a pre-crack in a compact tension specimen to perform fracture toughness testing of aluminum 7075-T6511

Madyira, Daniel M.

English

Madyira, Daniel, M.

Name not available

Effect of wire EDM on microstructure and fracture toughness of 7075-t6511 aluminum alloy

University of Johannesburg Institutional Repository

  Abstract - This paper reports on an investigation conducted to establish the influence of Wire Electrical Discharge Machining (WEDM) on the fracture toughness of aluminum 7075-T6511. The main objective was to determine if WEDM can be used to introduce a pre-crack into a compact tension specimen instead of the ASTM E-1820-11 specified fatigue pre-crack method. Fracture tests were conducted on four specimens which were pre-cracked using the WEDM technique. The rest of the fracture toughness evaluation followed the ASTM E-1820-11 guidelines. Results obtained from the experimental data were found to be inconsistent with the theoretical expectations. The fracture toughness was found to be significantly dependent on the effect of the WEDM on the material. The WEDM introduced a Heat Affected Zone (HAZ) on the surface of the pre-crack which modifies the fracture behavior of the material. It was concluded that WEDM is not a viable alternative to create a pre-crack in a compact tension specimen to perform fracture toughness testing of aluminum 7075-T6511.  Key words: EDM, Compact tension specimen, Fracture toughness, 7075-T6511 aluminum alloy, microstructure, Wire EDM I. INTRODUCTION luminum alloys offer attractive strength-to-weight ratios especially when compared to other structural materials such as steels.  Aluminum is manufactured in many different alloys and heat treatments such as 2014-T6, 7075-T6, and 7079-T6 [1]. They have therefore found extensive use in many applications ranging from automotive to aerospace. In addition, aluminum alloys have good corrosion resistance, formability and machinability. They can therefore be made into many components of different sizes ranging from screws to aircraft wing sections. 7075 T6511 aluminum alloy is a special heat temper grade of the 7075 alloy. It is the workhorse alloy for high performance applications. Its main alloying element is zinc. It has good fatigue resistance and was first introduced in automotive applications in 1943 [2]. Since then, it has become a key material in the manufacture of aerospace components including fuselages, wing spurs, landing gear components and many others. Its strength and light weight  Manuscript received on March 24, 2015; revised on April 7, 2015. This work was supported in part by the Department of Mechanical Engineering Science at the University of Johannesburg. Daniel M. Madyira is a Lecturer in the Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park, Johannesburg, South Africa, 2006. Phone: +2711-559-4030; e-mail: dmadyira@uj.ac.za).  has also attracted application in such areas as rock climbing equipment and bicycle components. As a high performance material, new ways are always being developed to produce 7075-T6511 components more cost effectively. This includes power metallurgy and special castings [3] to name just a few. As a result fatigue and fracture [4] tests are always required to assess the mechanical performance of materials and components in service. One way of reducing the time and cost of conducting fracture toughness tests is to use wire electrical discharge machining (WEDM) to induce a crack into the compact tension (CT) specimen. The recommended pre-cracking method according to ASTM E-1820-11 guidelines is to use fatigue cycling [5]. Fatigue cycling is a time consuming and expensive procedure. The aim of this work is therefore to investigate the feasibility of using WEDM to pre-crack CT specimen during fracture toughness testing of 7075-T6511 aluminium alloy. WEDM is a variation of the electrical discharge machining (EDM) process [6], [7], [8]. It uses an electrical discharge between an electrode and an anode in the presence of a dielectric (typically deionised water) to erode a material. Fig. 1 shows a schematic of the process.  Fig.  1: Schematic of the Wire EDM process  The high temperatures involved in EDM and WEDM may alter the surface and sub-surface integrity of the material and hence affect the fracture performance of the material. II. EXPERIMENTAL DESCRIPTION A. Aim of the Experiments The aim for the experiment was to determine the fracture toughness of 7075-T6511 aluminium alloy which had been pre-cracked using WEDM. In addition, the effect of the WEDM process on the microstructure of the material is also evaluated. Effect of Wire EDM on Microstructure and Fracture Toughness of 7075-T6511 Aluminum Alloy Daniel M. Madyira, Member, IAENG Afeed rate wire speed work piece Dielectric Proceedings of the World Congress on Engineering 2015 Vol II WCE 2015, July 1 - 3, 2015, London, U.K.ISBN: 978-988-14047-0-1 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2015 B. Materials Aluminium 7075-T6511 alloy in round bar form was supplied by DEM Manufacturing (Pvt) Ltd. [9], Pretoria, South Africa. The chemical composition of the material as presented in the manufacturer’s certificate is shown in Table I. TABLE I COMPOSITION OF 7075 T6511 AS SUPPLIED [9] Elem. Wt. % Elem. Wt. % Elem. Wt. % Al 87.1 - 91.4 Mg 2.1 - 2.9 Si Max 0.4 Cr 0.18 - 0.28 Mn Max 0.3 Ti Max 0.2 Cu 1.2 - 2 Other, each Max 0.05 Zn 5.1 - 6.1 Fe Max 0.5 Other, each Max 0.15 - -  The mechanical properties of the same material as supplied by the manufacturer are given in Table II.  TABLE II MECHANICAL PROPERTIES OF 7075 T6511 AS SUPPLIED [9] Property Value Comment Ultimate Tensile Strength 572 MPa AA; Typical Tensile Yield Strength 503 MPa AA; Typical Fatigue Strength 159 MPa AA; 500,000,000 cycles completely reversed stress; RR Moore machine / specimen Fracture Toughness 20 MPa-m½ K(IC) in S-L Direction Fracture Toughness 25 MPa-m½ K(IC) in T-L Direction Fracture Toughness 29 MPa-m½ K(IC) in L-T Direction C. Specimen Design and Manufacture The specimen geometry was designed in accordance to ASTM E-1820-11 standard [5]. The critical thickness was calculated to be 10 mm, therefore any thickness above this can be used to measure plane strain fracture toughness. A thickness of 20 mm was selected based on the clevis opening available on the testing machine. The width and other dimensions of the specimen are given in Fig. 2.  Fig.  2: Compact tension specimen geometry [5]  The specimen were manufactured by GEM Manufacturing (Pvt) Ltd. using a Xenon Actspark WEDM machine (see Fig. 3). Deionized water was used as the dielectric. The specimen outline was cut using a 250 μm tungsten coated brass wire while the pre-crack was cut using a 100 μm tungsten coated brass wire.  (a) (b) Fig.  3: (a) Xenon Actspark WEDM (b) Clamping table  The outline of the compact tension specimen produced after the WEDM using a 250 μm wire is shown in Fig. 4. The block is then WEDM pre-cracked using a 100 μm wire followed by slicing into CT specimen size.  (a) (b) Fig.  4: (a) Outline of brass wire cut, (b) Specimen after outline cut D. Equipment Fracture tests were conducted using a 100 kN Instron 1195 tensile testing machine that was controlled using Bluehill 2 software. Fig. 5 shows the Instron machine and the clevis used to mount the specimen.  (a) (b) Fig.  5: (a) Instron 1195 tensile testing machine (b) specimen mounting clevis  Fracture surface and microstructure examination was conducted using an optical microscope. E. Testing Procedure Prior to mounting onto the testing machine clevis, extension brackets are mounted to each specimen to provide extensometer grips. The specimens are then mounted onto the machine and an extensometer attached. The specimen is then loaded under displacement control at a rate of 0.4 mm/min until the maximum load is reached or when pop-in Proceedings of the World Congress on Engineering 2015 Vol II WCE 2015, July 1 - 3, 2015, London, U.K.ISBN: 978-988-14047-0-1 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2015 is heard. During loading, the displacement and load are monitored and recorded. The specimen is then un-mounted, heat tinted if possible, soaked in liquid nitrogen and remounted on the machine and loaded to complete fracture. The final configuration of the specimen showing the extension brackets is shown in Fig. 6(a). Fig. 6(b) shows the specimen mounted in the machine clevis together with the extensometer.   (a) (b) Fig.  6: (a) Prepared CT specimen (b) Mounted in clevis with extensometer  III. RESULTS A. Load Displacement Response The load displacement response for three of the specimens is shown in Fig. 7. One of the specimens underwent premature failure and hence is not included in the presented results. The specimen load displacement responses follow similar behavior albeit with significantly different maximum loads. This behavior was unexpected.  Fig.  7. Load-displacement responses for the tested CT specimens  The actual fracture loads experienced by each specimen are then extracted for computing the fracture toughness in accordance to the ASTM E-1820-11 procedure. These forces are shown in Table II together with the average and theoretically expected value.  TABLE III MAXIMUM FRACTURE LOADS Test Specimen Fracture Load, PMAX [kN] Specimen 1 10.27 Specimen 2 6.61 Specimen 3 4.67 Average 7.19 Theoretical 21.52  For closer analysis of the response behavior, the conditional plane strain fracture load is determined by drawing a 95% secant line on the force versus displacement graph, this is illustrated in Fig. 8 for specimen 1. The point where the two lines intersect is determined and expressed as P5. A conditional fracture load PQ is determined from the force versus displacement graph. PQ depends on which principal type the response satisfies. In this case type III is satisfied, hence PQ is equal to PMAX, the maximum fracture load recorded. This conditional fracture load is used to calculate the plane strain fracture toughness. The details of this procedure are discussed in ASTM E-1820-11 [5].  Fig.  8: Determination of the conditional fracture load for specimen 1 B. Fracture Toughness Results The resulting fracture toughness values are compared in Fig. 9. Also shown in Fig. 9 are the average fracture toughness value of 15.99 MPa-m0.5 and the expected theoretical value (according to literature) of 24.5 MPa-m0.5.  There is therefore a significant discrepancy (35%) between the value reported in literature and that obtained in this work.  Fig.  9: Fracture toughness results for the tested specimens C. Crack Tip Opening Displacement (CTOD) A comparison is also made between the physically computed crack tip opening displacement (CTOD) (by similar triangles from measured displacements) and that obtained through the computed J-integral. CTOD is an indicator of the level of plastic activity in the crack tip region. Low CTOP reflects predominant elastic behavior while large CTOD is accompanied by significant plastic deformation in the crack tip zone. The procedure for determining the J-integral and crack tip opening displacement is also detailed in ASTM E-1820-11. The results are presented in Fig. 10. The close relationship between the physical CTOD and that obtained from J-integral for specimen 1 may be an indication of the predominance of plastic deformation. This might explain the high toughness value recorded for specimen 1 when compared with the other specimens. This Proceedings of the World Congress on Engineering 2015 Vol II WCE 2015, July 1 - 3, 2015, London, U.K.ISBN: 978-988-14047-0-1 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2015 was also in closer agreement with the CTOD determined from the J-integral reported in literature. The rest of the specimens exhibit CTOD values of almost 50% of the theoretical values.   Fig.  10: Comparison of physical CTOD and CTOD from J-integral D. Fracture Surface and Microstructure The fracture surfaces of all specimens displayed brittle fracture as shown on Fig. 11. The pictures show the global brittle fracture exhibited by the largely flat fracture surface (Fig. 11(a)). The lack of significant shear lips proves that the fracture was largely plane strain with insignificant plane stress. This confirms the validity of the obtained fracture toughness. The chevron marks showing the crack initiation zones and the fast fracture directions are clearly seen in Fig. 11(b).    (a) (b) Fig. 11: Fractured surfaces for specimen 1(a) global (b) close up  An examination of the microstructure shows that the WEDM process had an effect on the microstructure of the 7075 T6511 as shown in Figure 12.   (a) (b) Fig. 12: Specimen 1 microstructure (a) Parent material (b) WEDM edge   The WEDM surface has a thin layer of oxide that can serve as a source of crack initiation. In addition, grains on the WEDM surface appear smaller than the parent material. This is evidence of the existence of a heat affected zone (HAZ). This would suggest a significant change of material properties that may affect the fracture behavior of the material.  IV. CONCLUSIONS The fracture toughness of aluminum 7075-T6511 was experimentally determined using CT specimen manufactured using WEDM. WEDM was also used to create the pre-crack of the specimens. This was a deviation from the ASTM E-1820-11 guidelines. The recommended pre-cracking procedure is to use fatigue pre-cracking. Based on the results obtained, the following conclusions can be made: 1. The average KIC value obtained during the experiment was 15.99 MPa /√m, which is 35% lower than that reported in literature. 2. The WEDM pre-cracking had an effect on the fracture behavior of 7075-T6511 aluminum alloy. This differs from the findings of Madyira and Akinlabi [10] who found no effect of WEDM on the fracture behavior of Ti6Al4V. 3. WEDM with wire size of 100 μm cannot be used to pre-crack CT specimen in fracture toughness testing of 7075-T6511. 4. The average J-Integral/ strain energy release rate obtained was 3186.96 J/m2. This value is more than half of the critical J-Integral value. Thus a much lower strain energy release rate was needed to propagate the pre-crack. 5. The average CTOD obtained during the experiment was 0.00540 mm, at a CTOA of 40.63°. 6. The macroscopic analysis of the fractured surfaces indicated plane strain and brittle fracture failure modes. This was in line with the thickness of the specimen that was twice the computed critical thickness. 7. WEDM introduced a HAZ on the surface of the material which was more pronounced in the crack tip region.  It is therefore recommended that future work be conducted using smaller wire diameter during the pre-cracking of the specimen using WEDM. More investigations are also required to understand the effect of the WEDM process parameters such as current and wire feed rate on the surface condition of the WEDM’ed surface. REFERENCES [1] Key to Metals, "Aircraft and Aerospace Applications: Part Two," Key to Metals, 2010. [Online]. Available: http://www.keytometals.com/. [Accessed 23 March 2015]. [2] Key to Metals, "Aircraft and Aerospace Applications: Part One," Total Materia, 2004. [Online]. Available: http://www.keytometals.com/. [Accessed 2015 March 2015]. [3] S. Mohan Kumar, R. Pramod, M. E. Shashi Kumar and H. K. Govindaraju, "Evaluation of Fracture Toughness and Mechanical Properties of Aluminum Alloy 7075, T6 with Nickel Coating," Procedia Engineeering, vol. 97, p. 178–185, 2014. [4] M. F. Kanninen and C. H. Popelar, Advanced Fracture Mechanics, New York: Oxford University Press, 1985. [5] ASTM, ASTM E1820-11: Standard Test Method for Proceedings of the World Congress on Engineering 2015 Vol II WCE 2015, July 1 - 3, 2015, London, U.K.ISBN: 978-988-14047-0-1 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2015 Measurement of Fracture Toughness, ASTM International, 2011.  [6]  Z. B. Yu, T. Jun and K. Masanori, "Dry eletrical discharge machining of cemented carbide," Journal of Materials Processing and Technology, vol. 149, pp. 353-357, 2004. [7]  F. Nourbakhsh, K. P. Rajurkar, A. P. Malshe and J. Cao, "Wire Eletro-discharge machining of titanium alloy," in The first CIRP Conference on Biomanufacturing, Proceedings of CIRP 5, 2013, pp. 13-18. [8]  M. S. Hewidy, T. A. El-Taweel, and M. F. El-Safty,  “Modelling the machining parameters of wire electrical  discharge machining of Inconel 601 using RSM,”  Journal of Materials Processing Technology, vol. 169,  no. 2, pp. 328–336, 2005.  [9]  GEM Manufacturing (Pvt) Ltd, "Datasheet 7075-T651,  7075-T6511," 2013. [10] D. M. Madyira and E. T. Akinlabi, "Effects of Wire  Electrical Discharge Machining on Fracture Toughness  of Grade 5 Titanium Alloy," in Lecture Notes in  Engineering and Computer Science: Proceedings of  The World Congress on Engineering 2014, WCE 2014,  London, U.K., 2-4 July 2014, pp1393-1398. Proceedings of the World Congress on Engineering 2015 Vol II WCE 2015, July 1 - 3, 2015, London, U.K.ISBN: 978-988-14047-0-1 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2015

  
Abstract - This paper reports on an investigation conducted to 
establish the influence of Wire Electrical Discharge Machining 
(WEDM) on the fracture toughness of aluminum 7075-T6511. 
The main objective was to determine if WEDM can be used to 
introduce a pre-crack into a compact tension specimen instead 
of the ASTM E-1820-11 specified fatigue pre-crack method. 
Fracture tests were conducted on four specimens which were 
pre-cracked using the WEDM technique. The rest of the 
fracture toughness evaluation followed the ASTM E-1820-11 
guidelines. Results obtained from the experimental data were 
found to be inconsistent with the theoretical expectations. The 
fracture toughness was found to be significantly dependent on 
the effect of the WEDM on the material. The WEDM 
introduced a Heat Affected Zone (HAZ) on the surface of the 
pre-crack which modifies the fracture behavior of the material. 
It was concluded that WEDM is not a viable alternative to 
create a pre-crack in a compact tension specimen to perform 
fracture toughness testing of aluminum 7075-T6511. 
 
Key words: EDM, Compact tension specimen, Fracture 
toughness, 7075-T6511 aluminum alloy, microstructure, Wire 
EDM 
I. INTRODUCTION 
luminum alloys offer attractive strength-to-weight 
ratios especially when compared to other structural 
materials such as steels.  Aluminum is manufactured in 
many different alloys and heat treatments such as 2014-T6, 
7075-T6, and 7079-T6 [1]. They have therefore found 
extensive use in many applications ranging from automotive 
to aerospace. In addition, aluminum alloys have good 
corrosion resistance, formability and machinability. They 
can therefore be made into many components of different 
sizes ranging from screws to aircraft wing sections. 
7075 T6511 aluminum alloy is a special heat temper 
grade of the 7075 alloy. It is the workhorse alloy for high 
performance applications. Its main alloying element is zinc. 
It has good fatigue resistance and was first introduced in 
automotive applications in 1943 [2]. Since then, it has 
become a key material in the manufacture of aerospace 
components including fuselages, wing spurs, landing gear 
components and many others. Its strength and light weight 
 
Manuscript received on March 24, 2015; revised on April 7, 2015. This 
work was supported in part by the Department of Mechanical Engineering 
Science at the University of Johannesburg. 
Daniel M. Madyira is a Lecturer in the Department of Mechanical 
Engineering Science, University of Johannesburg, Auckland Park, 
Johannesburg, South Africa, 2006. Phone: +2711-559-4030; e-mail: 
dmadyira@uj.ac.za). 
 
has also attracted application in such areas as rock climbing 
equipment and bicycle components. 
As a high performance material, new ways are always 
being developed to produce 7075-T6511 components more 
cost effectively. This includes power metallurgy and special 
castings [3] to name just a few. As a result fatigue and 
fracture [4] tests are always required to assess the 
mechanical performance of materials and components in 
service. One way of reducing the time and cost of 
conducting fracture toughness tests is to use wire electrical 
discharge machining (WEDM) to induce a crack into the 
compact tension (CT) specimen. The recommended pre-
cracking method according to ASTM E-1820-11 guidelines 
is to use fatigue cycling [5]. Fatigue cycling is a time 
consuming and expensive procedure. The aim of this work 
is therefore to investigate the feasibility of using WEDM to 
pre-crack CT specimen during fracture toughness testing of 
7075-T6511 aluminium alloy. 
WEDM is a variation of the electrical discharge 
machining (EDM) process [6], [7], [8]. It uses an electrical 
discharge between an electrode and an anode in the 
presence of a dielectric (typically deionised water) to erode 
a material. Fig. 1 shows a schematic of the process. 
 Fig.  1: Schematic of the Wire EDM process 
 
The high temperatures involved in EDM and WEDM 
may alter the surface and sub-surface integrity of the 
material and hence affect the fracture performance of the 
material. 
II. EXPERIMENTAL DESCRIPTION 
A. Aim of the Experiments 
The aim for the experiment was to determine the fracture 
toughness of 7075-T6511 aluminium alloy which had been 
pre-cracked using WEDM. In addition, the effect of the 
WEDM process on the microstructure of the material is also 
evaluated. 
Effect of Wire EDM on Microstructure and 
Fracture Toughness of 7075-T6511 Aluminum 
Alloy 
Daniel M. Madyira, Member, IAENG 
A
feed rate 
wire speed 
work piece 
Dielectric 
Proceedings of the World Congress on Engineering 2015 Vol II 
WCE 2015, July 1 - 3, 2015, London, U.K.
ISBN: 978-988-14047-0-1 
ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
WCE 2015
 B. Materials 
Aluminium 7075-T6511 alloy in round bar form was 
supplied by DEM Manufacturing (Pvt) Ltd. [9], Pretoria, 
South Africa. The chemical composition of the material as 
presented in the manufacturer’s certificate is shown in Table 
I. 
TABLE I 
COMPOSITION OF 7075 T6511 AS SUPPLIED [9] 
Elem. Wt. % Elem. Wt. % Elem. Wt. % 
Al 87.1 - 91.4 Mg 2.1 - 2.9 Si Max 0.4 
Cr 0.18 - 0.28 Mn Max 0.3 Ti Max 0.2 
Cu 1.2 - 2 Other, each Max 0.05 Zn 5.1 - 6.1 
Fe Max 0.5 Other, each Max 0.15 - - 
 
The mechanical properties of the same material as 
supplied by the manufacturer are given in Table II. 
 
TABLE II 
MECHANICAL PROPERTIES OF 7075 T6511 AS SUPPLIED [9] 
Property Value Comment 
Ultimate Tensile 
Strength 
572 MPa AA; Typical 
Tensile Yield 
Strength 
503 MPa AA; Typical 
Fatigue Strength 159 MPa AA; 500,000,000 cycles 
completely reversed stress; 
RR Moore machine / 
specimen 
Fracture Toughness 20 MPa-m½ K(IC) in S-L Direction 
Fracture Toughness 25 MPa-m½ K(IC) in T-L Direction 
Fracture Toughness 29 MPa-m½ K(IC) in L-T Direction 
C. Specimen Design and Manufacture 
The specimen geometry was designed in accordance to 
ASTM E-1820-11 standard [5]. The critical thickness was 
calculated to be 10 mm, therefore any thickness above this 
can be used to measure plane strain fracture toughness. A 
thickness of 20 mm was selected based on the clevis 
opening available on the testing machine. The width and 
other dimensions of the specimen are given in Fig. 2. 
 Fig.  2: Compact tension specimen geometry [5] 
 
The specimen were manufactured by GEM 
Manufacturing (Pvt) Ltd. using a Xenon Actspark WEDM 
machine (see Fig. 3). Deionized water was used as the 
dielectric. The specimen outline was cut using a 250 μm 
tungsten coated brass wire while the pre-crack was cut using 
a 100 μm tungsten coated brass wire. 
 
(a) (b) 
Fig.  3: (a) Xenon Actspark WEDM (b) Clamping table 
 
The outline of the compact tension specimen produced 
after the WEDM using a 250 μm wire is shown in Fig. 4. 
The block is then WEDM pre-cracked using a 100 μm wire 
followed by slicing into CT specimen size. 
 
(a) (b) 
Fig.  4: (a) Outline of brass wire cut, (b) Specimen after outline cut 
D. Equipment 
Fracture tests were conducted using a 100 kN Instron 
1195 tensile testing machine that was controlled using 
Bluehill 2 software. Fig. 5 shows the Instron machine and 
the clevis used to mount the specimen. 
 
(a) (b) 
Fig.  5: (a) Instron 1195 tensile testing machine (b) specimen mounting 
clevis 
 
Fracture surface and microstructure examination was 
conducted using an optical microscope. 
E. Testing Procedure 
Prior to mounting onto the testing machine clevis, 
extension brackets are mounted to each specimen to provide 
extensometer grips. The specimens are then mounted onto 
the machine and an extensometer attached. The specimen is 
then loaded under displacement control at a rate of 0.4 
mm/min until the maximum load is reached or when pop-in 
Proceedings of the World Congress on Engineering 2015 Vol II 
WCE 2015, July 1 - 3, 2015, London, U.K.
ISBN: 978-988-14047-0-1 
ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
WCE 2015
 is heard. During loading, the displacement and load are 
monitored and recorded. The specimen is then un-mounted, 
heat tinted if possible, soaked in liquid nitrogen and 
remounted on the machine and loaded to complete fracture. 
The final configuration of the specimen showing the 
extension brackets is shown in Fig. 6(a). Fig. 6(b) shows the 
specimen mounted in the machine clevis together with the 
extensometer. 
 
 (a) (b) 
Fig.  6: (a) Prepared CT specimen (b) Mounted in clevis with extensometer 
 
III. RESULTS 
A. Load Displacement Response 
The load displacement response for three of the 
specimens is shown in Fig. 7. One of the specimens 
underwent premature failure and hence is not included in 
the presented results. The specimen load displacement 
responses follow similar behavior albeit with significantly 
different maximum loads. This behavior was unexpected. 
 Fig.  7. Load-displacement responses for the tested CT specimens 
 
The actual fracture loads experienced by each specimen are 
then extracted for computing the fracture toughness in 
accordance to the ASTM E-1820-11 procedure. These 
forces are shown in Table II together with the average and 
theoretically expected value. 
 
TABLE III 
MAXIMUM FRACTURE LOADS 
Test Specimen Fracture Load, PMAX [kN] 
Specimen 1 10.27 
Specimen 2 6.61 
Specimen 3 4.67 
Average 7.19 
Theoretical 21.52 
 
For closer analysis of the response behavior, the 
conditional plane strain fracture load is determined by 
drawing a 95% secant line on the force versus displacement 
graph, this is illustrated in Fig. 8 for specimen 1. The point 
where the two lines intersect is determined and expressed as 
P5. A conditional fracture load PQ is determined from the 
force versus displacement graph. PQ depends on which 
principal type the response satisfies. In this case type III is 
satisfied, hence PQ is equal to PMAX, the maximum fracture 
load recorded. This conditional fracture load is used to 
calculate the plane strain fracture toughness. The details of 
this procedure are discussed in ASTM E-1820-11 [5]. 
 Fig.  8: Determination of the conditional fracture load for specimen 1 
B. Fracture Toughness Results 
The resulting fracture toughness values are compared in 
Fig. 9. Also shown in Fig. 9 are the average fracture 
toughness value of 15.99 MPa-m0.5 and the expected 
theoretical value (according to literature) of 24.5 MPa-m0.5.  
There is therefore a significant discrepancy (35%) between 
the value reported in literature and that obtained in this 
work. 
 Fig.  9: Fracture toughness results for the tested specimens 
C. Crack Tip Opening Displacement (CTOD) 
A comparison is also made between the physically 
computed crack tip opening displacement (CTOD) (by 
similar triangles from measured displacements) and that 
obtained through the computed J-integral. CTOD is an 
indicator of the level of plastic activity in the crack tip 
region. Low CTOP reflects predominant elastic behavior 
while large CTOD is accompanied by significant plastic 
deformation in the crack tip zone. The procedure for 
determining the J-integral and crack tip opening 
displacement is also detailed in ASTM E-1820-11. The 
results are presented in Fig. 10. 
The close relationship between the physical CTOD and 
that obtained from J-integral for specimen 1 may be an 
indication of the predominance of plastic deformation. This 
might explain the high toughness value recorded for 
specimen 1 when compared with the other specimens. This 
Proceedings of the World Congress on Engineering 2015 Vol II 
WCE 2015, July 1 - 3, 2015, London, U.K.
ISBN: 978-988-14047-0-1 
ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
WCE 2015
 was also in closer agreement with the CTOD determined 
from the J-integral reported in literature. The rest of the 
specimens exhibit CTOD values of almost 50% of the 
theoretical values. 
 
 Fig.  10: Comparison of physical CTOD and CTOD from J-integral 
D. Fracture Surface and Microstructure 
The fracture surfaces of all specimens displayed brittle 
fracture as shown on Fig. 11. The pictures show the global 
brittle fracture exhibited by the largely flat fracture surface 
(Fig. 11(a)). The lack of significant shear lips proves that 
the fracture was largely plane strain with insignificant plane 
stress. This confirms the validity of the obtained fracture 
toughness. The chevron marks showing the crack initiation 
zones and the fast fracture directions are clearly seen in Fig. 
11(b).  
 
 (a) (b) 
Fig. 11: Fractured surfaces for specimen 1(a) global (b) close up 
 
An examination of the microstructure shows that the 
WEDM process had an effect on the microstructure of the 
7075 T6511 as shown in Figure 12. 
 
 
(a) (b) 
Fig. 12: Specimen 1 microstructure (a) Parent material (b) WEDM edge  
 
The WEDM surface has a thin layer of oxide that can 
serve as a source of crack initiation. In addition, grains on 
the WEDM surface appear smaller than the parent material. 
This is evidence of the existence of a heat affected zone 
(HAZ). This would suggest a significant change of material 
properties that may affect the fracture behavior of the 
material.  
IV. CONCLUSIONS 
The fracture toughness of aluminum 7075-T6511 was 
experimentally determined using CT specimen 
manufactured using WEDM. WEDM was also used to 
create the pre-crack of the specimens. This was a deviation 
from the ASTM E-1820-11 guidelines. The recommended 
pre-cracking procedure is to use fatigue pre-cracking. Based 
on the results obtained, the following conclusions can be 
made: 
1. The average KIC value obtained during the experiment 
was 15.99 MPa /√m, which is 35% lower than that 
reported in literature. 
2. The WEDM pre-cracking had an effect on the fracture 
behavior of 7075-T6511 aluminum alloy. This differs 
from the findings of Madyira and Akinlabi [10] who 
found no effect of WEDM on the fracture behavior of 
Ti6Al4V. 
3. WEDM with wire size of 100 μm cannot be used to pre-
crack CT specimen in fracture toughness testing of 
7075-T6511. 
4. The average J-Integral/ strain energy release rate obtained 
was 3186.96 J/m2. This value is more than half of the 
critical J-Integral value. Thus a much lower strain 
energy release rate was needed to propagate the pre-
crack. 
5. The average CTOD obtained during the experiment was 
0.00540 mm, at a CTOA of 40.63°. 
6. The macroscopic analysis of the fractured surfaces 
indicated plane strain and brittle fracture failure modes. 
This was in line with the thickness of the specimen that 
was twice the computed critical thickness. 
7. WEDM introduced a HAZ on the surface of the material 
which was more pronounced in the crack tip region. 
 
It is therefore recommended that future work be 
conducted using smaller wire diameter during the pre-
cracking of the specimen using WEDM. More 
investigations are also required to understand the effect of 
the WEDM process parameters such as current and wire 
feed rate on the surface condition of the WEDM’ed surface. 
REFERENCES 
[1] Key to Metals, "Aircraft and Aerospace Applications: 
Part Two," Key to Metals, 2010. [Online]. Available: 
http://www.keytometals.com/. [Accessed 23 March 
2015]. 
[2] Key to Metals, "Aircraft and Aerospace Applications: 
Part One," Total Materia, 2004. [Online]. Available: 
http://www.keytometals.com/. [Accessed 2015 March 
2015]. 
[3] S. Mohan Kumar, R. Pramod, M. E. Shashi Kumar and 
H. K. Govindaraju, "Evaluation of Fracture Toughness 
and Mechanical Properties of Aluminum Alloy 7075, 
T6 with Nickel Coating," Procedia Engineeering, vol. 
97, p. 178–185, 2014. 
[4] M. F. Kanninen and C. H. Popelar, Advanced Fracture 
Mechanics, New York: Oxford University Press, 1985. 
[5] ASTM, ASTM E1820-11: Standard Test Method for 
Proceedings of the World Congress on Engineering 2015 Vol II 
WCE 2015, July 1 - 3, 2015, London, U.K.
ISBN: 978-988-14047-0-1 
ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
WCE 2015
 Measurement of Fracture Toughness, ASTM 
International, 2011.  
[6]  Z. B. Yu, T. Jun and K. Masanori, "Dry eletrical 
discharge machining of cemented carbide," Journal of 
Materials Processing and Technology, vol. 149, pp. 
353-357, 2004. 
[7]  F. Nourbakhsh, K. P. Rajurkar, A. P. Malshe and J. 
Cao, "Wire Eletro-discharge machining of titanium 
alloy," in The first CIRP Conference on 
Biomanufacturing, Proceedings of CIRP 5, 2013, pp. 
13-18. 
[8]  M. S. Hewidy, T. A. El-Taweel, and M. F. El-Safty, 
 “Modelling the machining parameters of wire electrical 
 discharge machining of Inconel 601 using RSM,” 
 Journal of Materials Processing Technology, vol. 169, 
 no. 2, pp. 328–336, 2005.  
[9]  GEM Manufacturing (Pvt) Ltd, "Datasheet 7075-T651, 
 7075-T6511," 2013. 
[10] D. M. Madyira and E. T. Akinlabi, "Effects of Wire 
 Electrical Discharge Machining on Fracture Toughness 
 of Grade 5 Titanium Alloy," in Lecture Notes in 
 Engineering and Computer Science: Proceedings of 
 The World Congress on Engineering 2014, WCE 2014, 
 London, U.K., 2-4 July 2014, pp1393-1398. 
Proceedings of the World Congress on Engineering 2015 Vol II 
WCE 2015, July 1 - 3, 2015, London, U.K.
ISBN: 978-988-14047-0-1 
ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
WCE 2015


Effect of wire EDM on microstructure and fracture toughness of 7075-t6511 aluminum alloy

Abstract

Similar works

Full text

Available Versions

Name not available

University of Johannesburg Institutional Repository

Name not available