Heat treatment of aluminium alloys produced by laser powder bed fusion: A review

Abstract Laser powder bed fusion (LPBF) is the most widely used additive manufacturing technique and has received increasing attention owing to the high design freedom it offers. The production of aluminium alloys by LPBF has attracted considerable interest in several fields due to the low density of the produced alloys. The peculiar solidification conditions experienced by molten metal during the SLM process and its layer-by-layer nature causes a variety of microstructural peculiarities including the formation of metastable phases and supersaturated solid solutions, extreme microstructural refinement, and generation of residual stresses. Therefore, post-build heat treatments, which are commonly applied to conventionally produced aluminium alloys, may need to be modified in order to be adapted to the peculiar metallurgy of aluminium alloys manufactured using LPBF and address the specific issues resulting from the process itself. A number of studies have investigated this topic in recent years, proposing different approaches and dealing with various alloying systems. This paper reviews scientific research results in the field of heat treatment of selective laser melted aluminium alloys; it aims at providing a comprehensive understanding of the relationship between the induced microstructure and the resulting mechanical behaviour, as a function of the various treatment strategies

Selective laser melting of high-strength primary AlSi9Cu3 alloy: Processability, microstructure, and mechanical properties

Abstract The present work explores the possibility of employing the selective laser melting technique to produce parts in AlSi9Cu3 alloy. This alloy, currently prepared by high-pressure dye casting and intended for automotive application, may benefit from the refined microstructure commonly induced by additive manufacturing techniques. The process parameters were systematically varied to achieve full density, and the resulting defects were studied. Thereafter, microstructural features were analyzed, revealing that the high cooling rate, induced by the process, caused a large supersaturation of the aluminum matrix and the refinement of the eutectic structure. Again, the precipitation of the reinforcing θ phase provided numerous nucleation sites. These features were found to be related to the mechanical behavior of the SLMed AlSi9Cu3 alloy, which outperformed the conventional casted alloy in terms of elongation to failure and strain hardening rate both in the as-built and heat treated conditions

Fibre Laser Cutting and Chemical Etching of AZ31 for Manufacturing Biodegradable Stents

The use of magnesium-alloy stents shows promise as a less intrusive solution for the treatment of cardiovascular pathologies as a result of the high biocompatibility of the material and its intrinsic dissolution in body fluids. However, in addition to requiring innovative solutions in material choice and design, these stents also require a greater understanding of the manufacturing process to achieve the desired quality with improved productivity. The present study demonstrates the manufacturing steps for the realisation of biodegradable stents in AZ31 magnesium alloy. These steps include laser microcutting with a Q-switched fibre laser for the generation of the stent mesh and subsequent chemical etching for the cleaning of kerf and surface finish. Specifically, for the laser microcutting step, inert and reactive gas cutting conditions were compared. The effect of chemical etching on the reduction in material thickness, as well as on spatter removal, was also evaluated. Prototype stents were produced, and the material composition and surface quality were characterised. The potentialities of combining nanosecond laser microcutting and chemical etching are shown and discussed

Fiber laser welding of copper based open cell foams

"Porous metallic materials with cellular structures are well known to combine many physical and mechanical properties. This mix of different properties makes these systems very attractive for both structural and functional applications, depending on pore size, methodology of production and material characteristics. Because of their porous structure, unconventional machining and more in general unconventional processing is becoming more and more important nowadays for promoting the industrial applications of such a kind of materials. In this work a study on the fiber laser welding process, performed using a 1 kW continuous wave fiber laser, on Cu based foams is reported. The foams, whose the mean size of the pore is approximately 3.5 mm, were produced by means of infiltration of leachable space holders inside the metal in liquid state. After preliminary welding test in a bead on plate configuration performed only on the surface of the foams, samples in lap joint configuration were realized for evaluating the cross section of the welded bead. The effect of the process speed on the geometrical characteristic features of the joints was studied. The extent of the heat affected zone was evaluated directly by optical microscopy and indirectly by executing micro-hardness test. Then the heat affected zone extension was corrected to the process speed. Besides, electron scanning microscopy, coupled with electron dispersive spectroscopy, was adopted for the compositional analysis of the welded beads. It was shown that the laser joints could be achieved in lap joint configuration, allowing high reflectivity porous alloys with complex structures and average pore size of the order of millimeters to be connected.

Effect of Al Addition on Martensitic Transformation Stability and Microstructural and Mechanical Properties of CuZr Based Shape Memory Alloys

In this work, the effect of the Al content (x = 5, 10, and 15 at. %) on the martensitic transformation (MT) and microstructure and mechanical properties of Cu(50−x)Zr50Alx alloys was studied. The microstructure of the alloys was characterized at room temperature by means of scanning electron microscopy and X-ray diffraction. An increase in Al content reduces the amount of transforming CuZr phase, and consequently the secondary phase formation is favored. The evolution of the MT upon thermal cycling was investigated as a function of the Al content by differential scanning calorimetry. MT temperatures and enthalpies were found to be decreased when increasing the Al content. Al addition can induce a sudden, stable MT below 0 °C, while the binary alloy requires ten complete thermal cycles to stabilize. Finally, the mechanical properties were investigated through microhardness and compression testing. No linear dependence was found with composition. Hardness lowering effect was observed for 5–10 at. % of Al content, while the hardness was increased only for 15 at. % Al addition with respect to the binary alloy. Similarly, compressive response of the alloys showed behavior dependent on the Al content. Up to 10 at. % Al addition, the alloys indicate a superelastic response at room temperature, while higher Al content induced untimely failure

Microstructural and Mechanical Response of NiTi Lattice 3D Structure Produced by Selective Laser Melting

Nowadays, additive manufacturing (AM) permits to realize complex metallic structural parts, and the use of NiTi alloy, known as Nitinol, allows the integration of specific functions to the AM products. One of the most promising designs for AM is concerning the use of lattice structures that show lightweight, higher than bulk material deformability, improved damping properties, high exchange surface. Moreover, lattice structures can be realized with struts, having dimensions below 1 mm—this is very attractive for the realization of Nitinol components for biomedical devices. In this light, the present work regarded the experimental characterization of lattice structures, produced by selective laser melting (SLM), by using Ni-rich NiTi alloy. Differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD), and compression testing were carried out for analyzing microstructure, martensitic transformation (MT) evolution, and superelasticity response of the SLMed lattice samples. The lattice microstructures were compared with those of the SLMed bulk material for highlighting differences. Localized martensite was detected in the nodes zones, where the rapid solidification tends to accumulate solidification stresses. An increase of martensitic transformation temperatures was also observed in lattice NiTi

Computational Model for Delamination Growth at SMA-GFRP Interface of Hybrid Composite

AbstractA cohesive model of the new interface of the CuZnAl SMA/GFRP hybrid composite is proposed and the interfacial delamination under Mode II loading conditions, between plain CuZnAl SMA sheet insert and GFRP matrix, as well as between CuZnAl SMA sheet insert having elliptical hole pattern and GFRP matrix, are studied in detail.The results of the pull-out tests with plain sheet insert are used to calculate the interfacial parameters of the hybrid composite. With these parameters, the cohesive interaction and failure mechanism for hybrid composite with plain sheet, as well as with patterned sheet insert, is modelled. The efficacy of the laser patterned SMA sheet inserts to improve the overall interfacial strength in the new laminated SMA/GFRP hybrid composite for applications, such as light weight and high damping material under dynamic loads, is validated

Effect of laser welding on the mechanical and degradation behaviour of Fe-20Mn-0.6C bioabsorbable alloy

Abstract The present work aims at exploring the influence of laser welding on the functional behaviour of a Fe-20Mn-0.6C (wt.%) bioabsorbable alloy. At first, the selection of the most suitable process speed (40 mm/s) was done in order to obtain a full penetration joint with limited taper. Then, microstructural and mechanical analyses of welded sheets confirmed suitable performance of the joint, without porosity, thus preserving chemical composition, mechanical resistance and ductility even after welding. In particular, the base material comprised both γ austenite and e martensite, while the welded samples showed a further type of martensite, namely α'. Moreover, ultimate tensile strength (1095 MPa and 1104 MPa in base and welded material, respectively) and elongation to failure (61.3% and 60.9%, respectively) were almost not influenced by the welding process. Considering the absorbable nature of these alloys, static immersion degradation tests were carried out, and confirmed that the surface of the welded bead did not exhibit a significant variation of the material degradation rate after 14 days in modified Hanks' solution. Finally, a significant accumulation of degradation products, mainly (Fe,Mn)CO3, was observed along the joining line

Cohesive surface model for delamination and dynamic behavior of hybrid composite with SMA-GFRP interface

The interface model between CuZnAl SMA and GFRP, used in a hybrid composite, is proposed using cohesive surfaces. Using this model and derived parameters, mode-II delamination is studied between CuZnAl SMA insert and GFRP and also between laser patterned CuZnAl SMA insert and GFRP. Natural frequency and damping ratio of the hybrid composite specimen, in the shape of slender beam in a cantilever configuration, are evaluated in impulse tests. A numerical model is also presented, to calculate the aforementioned dynamic properties numerically, using Modal Strain Energy (MSE) and Modal Dynamics procedures by considering the derived interfacial parameters. Keywords: Hybrid composite, Delamination, FE analysis, Cohesive interface, Damage initiation, Modal dynamic

Tuning of Static and Dynamic Mechanical Response of Laser Powder Bed Fused AlSi10Mg Lattice Structures through Heat Treatments

The use of additive manufacturing allows the production of complex designs, including metallic lattice structures, which combine lightness and good mechanical properties. Herein, the production of AlSi10Mg lattice structures by laser powder bed fusion is explored and consistent processing parameters are selected. Thereafter, it is demonstrated that heat treatments, specifically designed on the basis of the alloy's metallurgy, can be used to selectively induce different microstructural modifications and, consequently, finely tune the static and dynamic mechanical behavior of the aluminum lattice structures. In particular, removal of residual stresses results to be the dominant factor in allowing a smooth quasistatic compressive behavior and improving the structure's ability to absorb energy during collapse. On the contrary, the increase in ductility connected to the spheroidization of the Si network is shown to be of paramount importance in improving the structure's dynamic damping ability by allowing local plasticization