Effect of build orientation on fracture and tensile behavior of A357 Al alloy processed by Selective Laser Melting

Abstract

Selective Laser Melting (SLM) is an additive manufacturing technology that allows producing components through local melting of a metallic powder bed using high-power laser beam in accordance with 3D digital CAD models. Cast Al-Si-Mg alloys are broadly used due to their good processability by SLM and mechanical properties. This work investigates the mechanical behavior of A357 aluminum alloy produced by SLM in the as built and artificially aged conditions. In particular, the tensile properties and fracture toughness of the alloy were investigated according to different specimen orientations on the build plate in the first case and different crack propagation planes in the second. In addition, microstructural and fractographic analyses were performed in order to investigate crack propagation paths and fracture mechanisms. The results showed that building orientation has a strong influence on material response, mainly with regards to the fracture toughness. The melt pool boundaries act as preferential paths for crack propagation and lead to a marked drop in fracture toughness of samples with cracks perpendicular to the building direction.

Introduction

Selective laser melting (SLM) is an Additive Manufacturing (AM) technology that allows producing parts with complex shape according to 3D digital models by layer-wise addition of material. SLM is based on local melting of a metal powder bed by high-power laser beam. Almost fully-dense functional and structural components are nowadays produced by SLM for service in industrial, aeronautical and medical applications using different gas-atomized alloys [[1], [2], [3]].

Among Al alloys, the Al-Si casting grades are the most used, since they are relatively easy to be processed, leading to parts without unacceptable microstructural defects. A357 is a heat treatable hypoeutectic Al-Si-Mg alloy showing good mechanical and corrosion performance, broadly used in casting of structural parts. The A357 alloy is mainly strengthened by Mg₄Al₃Si₄ coherent precipitates, known as β’’, which form upon thermal aging from the aluminum supersaturated solid solution [[4], [5], [6]]. A recent work [7] revealed that A357 powder can be processed by SLM to produce high strength dense parts and that as built condition can be considered as fully appropriate for an effective dispersion strengthening process by artificial aging. In many alloys, indeed, the extremely high cooling rate involved in SLM leads to the formation of supersaturated solid solutions that can be exploited to form strengthening dispersoids by tailored heat treatments [[7], [8], [9], [10], [11]]. In particular, in A357 alloy processed by SLM, the highest yield and ultimate tensile strengths are exhibited when the material is aged at 160 °C for 4 h starting from the as built condition [7].

In this context, the present work investigates the fracture and tensile behaviors of the A357 aluminum alloy processed by SLM, in the as built and artificially aged condition. Particular attention has been given to the anisotropic behavior of the material. Tensile properties have been evaluated along the direction orthogonal and parallel to building direction, whereas fracture toughness tests were performed on specimens with three different crack plane orientations. Tensile mechanical properties were determined from engineering stress vs. strain curves, whereas fracture toughness was obtained through crack growth resistance curves (J-R curves) using the unloading compliance method. In order to investigate the crack path in the different crack plane orientations, microstructural and fractographic analyses were carried out using optical and electron microscopes.