Novel heat treatment methodology for residual stress relieve of AlSi10Mg without compromise of mechanical strength

• Main Author: Lim, Chong Heng
• Other Authors: Li Hua
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Material testing and characterization; Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/151398
• Institution: Nanyang Technological University

Additive manufacturing (AM) has made the fabrication of complex parts possible which otherwise, would not have been possible with conventional subtractive manufacturing. Laser Beam Powder Bed (LBPB) Fusion, commonly known as Selective Laser Melting (SLM), is one of the methodologies in AM for metal fabrication. Many types of research have been done pertaining to fabricating functional parts via LBPB methodology, and one of the challenges faced is the residual stresses that reside in the metal parts due to repeated uneven heating and cooling process. Residual stresses can cause the LBPB process to fail by dimensional warping of the parts, and also compromise the functionality of the final product especially if there is tensional stress on the surface, introducing the risk of premature failure below its operational load. AlSi10Mg is a great strength-to-weight ratio alloy that is commonly manufactured by the LBPB process. It is an excellent material suitable for fabrication via the LBPB process, as it has proven to have superior mechanical properties as compared to that made by the conventional cast method. The current conventional method of relieving the residual stresses of LBPB manufactured AlSi10Mg involves heat treatment to 300ºC. However, AlSi10Mg manufactured by the LBPB process is much more sensitive to heat treatment as compared to conventional cast AlSi10Mg, resulting in a drastic reduction in mechanical strength after heat treatment. Performing heat treatment at a lower temperature will result in less reduction in the mechanical strength of the material, but the residual stress will not be completely relieved. Hence, the current industrial method of residual stress relief is achieved at a compromise of its mechanical property, which poses an engineering dilemma. This thesis presents a novel method of residual stress relief of LBPB manufactured AlSi10Mg without any compromise of its compressive strength, discussed in section 3.1.2. and the results verified in Chapter 4. Also, two novel phenomena were observed in the study of microstructural characterization of the material in sections 5.4 and 5.5 respectively. The novel residual stress relief methodology involves uneven air cooling after heat treatment at a relatively lower temperature, therefore inducing thermal residual stress during the cooling process, such that it cancels out the existing residual stress that remains after the heat treatment. This method successfully relieved the residual stress without compromising the compressive strength of the part. The effect of heat treatment temperature on the compressive strength of the part is justified via scanning electron microscopy (SEM) images. A novel phenomenon was observed on the trend that low angle grain boundaries (LAGB) are found to be concentrated in grain orientated towards <001> direction. Semi-transparent grain orientation map superimposed on grain boundaries map reveals this trend via visual observation. Another novel phenomenon was observed of the relationship between LAGB density and the magnitude of residual stress relief was made in which both parameters show a similar trend at different treatment temperatures. This further substantiates the nature of the new methodology of stress relief where a larger induced residual stress results in an increase in dislocation, and therefore an increase in LAGB. General observation of the grain orientation map, and precipitation distribution of intermetallic compounds are also discussed.