Role of Direct Aging and Solution Treatment on Hardness, Microstructure and Residual Stress of the A357 (AlSi7Mg0.6) Alloy Produced by Powder Bed Fusion

Lavinia Tonelli,Erica Liverani,Alessandro Morri,Lorella Ceschini

doi:10.1007/s11663-021-02179-6

Lavinia Tonelli, Erica Liverani + Show 2 more

Open Access

https://doi.org/10.1007/s11663-021-02179-6

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Abstract

Applying additive manufacturing (AM) technologies to the fabrication of aluminum automotive components, with an optimized design, may result in improved vehicle light weighting. However, the post-process heat treatment of such alloys has to be customized for the particular AM microstructure. The present study is aimed at investigating the effect of different heat treatments on the microstructure, hardness and residual stress of the A357 (AlSi7Mg0.6) heat-treatable alloy produced by laser-based powder bed fusion (LPBF, also known as selective laser melting). There are two major issues to be addressed: (1) relieving the internal residual stress resulting from the process and (2) strengthening the alloy with a customized heat treatment. Therefore, stress-relief annealing treatment, direct aging of the as-built alloy and a redesigned T6 treatment (consisting of a shortened high-temperature solution treatment followed by artificial aging) were examined. Comparable hardness values were reached in the LPBF alloy with optimized direct aging and T6 treatments, but complete relief of the residual stress was obtained only with T6. Microstructural analyses also suggested that, because of the supersaturated solid solution, different phenomena were involved in direct aging and T6 treatment.

Highlights

VEHICLE light weighting and engine downsizing are two of the main strategies pursued to meet strict regulations on pollutant and greenhouse gas emissions
Precipitation of Si was shown by the analysis of the lattice parameter (Figure 6): the increase in the lattice parameter found after direct aging (AA) of the as-built laser-based powder bed fusion (LPBF) alloy suggested a release of Si atoms from the solid solution
The present work was focused on assessing the role of heat treatment on the microstructure, hardness and residual stress of the A357 (AlSi7Mg0.6) alloy produced by laser powder bed fusion (LPBF)

Summary

Introduction

VEHICLE light weighting and engine downsizing are two of the main strategies pursued to meet strict regulations on pollutant and greenhouse gas emissions. For this reason, aluminum alloys are one of the most widely used structural materials in the transportation field. LPBF consists of the localized melting of subsequent layers of fine metallic powder (powder bed) by means of a computer-driven focused laser beam.[3] Due to its layer-wise approach and high accuracy, which the focused beam can guarantee, a complex and customized design can be produced and features such as inner cavities, conformal cooling channels and even lattice structures can be realized for improved weight reduction. Most of the research has been focused on LPBF of eutectic AlSi12 and near-eutectic AlSi10Mg alloys, while more limited data are available

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