Characterization of the untreated, stress relieve and t6 heat treated aluminium alloy alsi10mg formed by selective laser melting (slm) technique

• Main Author: Chuang, Huei Deng
• Other Authors: Chua Chee Kai
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Mechanical engineering::Prototyping
• Online Access: http://hdl.handle.net/10356/61367
• Institution: Nanyang Technological University

Additive manufacturing technique has been developed to speed up the manufacturing process besides allows users to fabricate products of high complexity which is difficult to achieve using conventional manufacturing techniques such as casting. Selective Laser Melting (SLM) is one of the powder based additive manufacturing technique which involves melting of powder to form a 3D solid. Mechanical properties of the SLM fabricated parts are closely associated with the density of the fabricated part. Therefore, this project aims to fabricate high density AlSI10Mg parts using SLM under the best parameters which was later been identified to be 350 W for laser power, 1140 mm/s laser scanning speed, 0.17 mm hatch spacing and 50 μm of powder bed thickness. Next, samples to be characterized are fabricated using the best parameters. The highest density of part reaches 99.13 % and further be improved to 99.98 % with remelting process. After the fabrication process, the samples are conditioned according to three different groups namely raw, stress relieve and T6 heat treatment. The stress relieve heat treatment process requires the heating of the samples in electric furnace for two hours under 300 °C and let to cool down in air. Whereas the T6 heat treatment process involves two treatment stages. The first stage requires the heating of the samples at temperature of 530 °C for five hours then follows by quenching in cold water. Second stage of precipitation hardening involves a lower heating temperature of 160 °C for eight hours then follows by air cool. After the conditionings are done, the samples are tested for its mechanical properties such as yield strength, ultimate tensile strength, shear strength and micro-hardness. In addition to that, microstructures of the samples are also analysed using Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS). Results from the mechanical properties characterization reveals that the SLM fabricated AlSi10Mg are much better in term of its ultimate tensile strength, shear strength and micro-hardness values as compared to the casting part. This is due to the small grain size of the SLM fabricated part formed under rapid cooling of the fabrication process. Besides that, the stress relieve and T6 heat treatment had increased the grain size of the parts which causes reduction in yield strength, ultimate tensile strength and shear strength. However the stiffness is increased in term of the young’s modulus value. In addition to that, micro-hardness of the samples is tested using Vicker’s hardness test. The result shows similar pattern at which the SLM fabricated samples have much higher hardness value as compared to casted samples, though the stress relieve and T6 heat treatment do not produce significant effect on the micro-hardness value of the samples. ii Microstructure of the samples are also been analysed using Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). Before the samples are observed under SEM, the samples are first smoothened using grit paper until mirror finish. The samples are then etched using flick reagent so that the microstructure is visible under SEM. Microstructure of the samples shows the formation of dendrites which spread evenly throughout the samples. EDS result indicates that the dendrites are made up of Mg2Si which is surrounded by Al matrix. The small grain size of the grain indicates the rapid solidification of the melt pool during the fabrication process. Besides, the difference in dendrites size on melt boundaries and within melt boundaries are observed. This phenomenon is caused by variation in cooling time interval. Other than that, pores are also been observed on the Y-Z plane of the samples only which is mainly due to the process inherent and is unavoidable. Aside from that, similarity between the raw samples microstructure and stress relieved samples microstructure reveals that stress relieve heat treatment is capable of relieving the residue stress but has no effect on the microstructure of the samples. Last but not least, precipitation of the Si immersed in pool of solid solution is observed in samples underwent T6 heat treatment process which indicates the precipitation hardening is occurring in samples after T6 heat treatment. Detailed experimental procedures of how the characterizations are carried out will be described in the next section of the report. As a conclusion, the SLM is a better technique in producing AlSI10Mg parts with superior mechanical properties as compared to conventional casting method.