Proceedings of the
9th International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN2022)
15 – 18 November 2022, Singapore

The Effect of Hot Forging on the Mechanical Properties of Additive Manufactured 7075 Aluminium Alloy Preform through Crack Closure

Fu Lun Tan, Kai Soon Fonga, Zhiguang Zhu, Fern Lan Ng and Min Hao Goh

Singapore Institute of Manufacturing Technology, 73 Nanyang Drive, Singapore 637662


Additive manufacturing (AM) has become a research hotspot since the first introduction of the layer-by-layer 3D printing technique, mainly due to the benefits of near net shape production, zero die cost, low buy-to-fly ratio, and flexibility for complex components creation. However, in the metal AM production, especially for 7000 series aluminium alloy, the mechanical properties are adversely affected by the presence of defects like porosities, cracks, oxidation, and evaporation of volatile alloying elements. In this study, we proposed a potential solution to enhance the mechanical properties of aluminium alloy, which is by sequential hybrid AM, combining the selective laser melting (SLM) and forging processes. One benefit of choosing the hybrid route is the reduction of forging steps as would have been required in the traditional forging process, in which a near net shape preform can be formed by SLM, while the poor mechanical properties of SLM samples can be simultaneously improved by forging to achieve near forged properties. To minimize elemental evaporation, the highest laser scanning speed and the lowest laser power were selected for the SLM process, but at the expense of developing severe hot cracking in the material. To close the cracks, the SLM fabricated preform is subjected to forging at the temperatures of 200 ºC and 400 ºC with solution heat treatment performed either before or after forging, followed by a final artificial aging heat treatment. Under the observation of Scanning Electron Microscopy (SEM), it was found that forging before heat treatment will lead to better crack closure and overall defect reduction. From the results of the tensile tests, forging significantly improved the mechanical properties of the alloy, resulting in better yield strength, ultimate tensile strength, and elongation of 279 MPa, 349 MPa, and 5.2 % respectively than the as-built preform. Hence, hybrid AM with subsequent forging can be considered a promising solution to enhance the mechanical properties of AM products by greatly reducing the density of cracks.

Keywords: Hybrid Additive Manufacturing, Selective Laser Melting, Hot Forging, Mechanical properties, Cracks closure

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