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Title:	Effect of the post weld heat treatment on microstructure and mechanical properties of friction stir welded
Authors:	Amirhossein Baghdadi (P65750)
Supervisor:	Zainuddin Sajuri, Assoc. Prof. Dr.
Keywords:	Universiti Kebangsaan Malaysia -- Dissertations Dissertations, Academic -- Malaysia Alloys Aluminum Microstructure Friction
Issue Date:	8-Mar-2019
Description:	The automotive and aerospace industries are always aiming to produce lightweight structure by joining aluminium (Al) alloys. A common problem in Al joining by conventional welding methods are hot cracking, porosity, and missing of strengthening phases at the welding zone. Friction stir welding (FSW) can be performed to produce a sound joint of Al alloys. However, the loss of strength usually occurs in the FSWed region. In the present work, post weld heat treatment (PWHT) was introduced to overcome this problem. The effect of PWHT on weldability and mechanical strength of Al-Mg-Si and Al-Mg aluminium alloys joints were analysed and compared with the base materials involving hardness, tensile, and fatigue properties at room temperature. The evaluation was made by performing a morphological, microstructural and mechanical characterization of welded joints. T6-PWHT has then carried out on friction stir welded (FSWed) samples. A similar FSW joint of Al6061(T6)-Al6061(T6) and a dissimilar FSW joint of Al5083-Al6061(T6) were produced. The FSW was performed at rotation speeds of 800 to 1400 rpm and travel speed of 100 to 400 mm/min. The result showed that no defect found in both the similar and dissimilar FSWed samples. Finer grain size was observed in the stir zone (SZ) in comparison the thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ) due to plastic deformation and dynamic recrystallisation at the same time during the welding process. Hardness profile of similar FSWed samples showed a clear reduction in the welding zone due to softening effect and led to coarsening or/and solutionising of the strengthening elements in the welding zones. The minimum hardness measured was at the advancing side with 50 HV. PWHT causes increasing of hardness in the weld zone because of a homogeneous distribution of precipitation particles. In contrast, an onion ring pattern is observed in the SZ of the dissimilar FSWed sample because of the mixing of dissimilar materials. The T6-PWHT is causing grain growth in the welding zones. The hardness profile of dissimilar FSWed was found fluctuating across the weldment. The low hardness in the HAZ of Al6061 observed was due to precipitate coarsening or dissolving of β particles that increased towards the SZ. Tensile test of both similar and dissimilar FSWed samples displayed a reduction of strength compared to the base material with welding efficiency of 60 to 70%. However, PWHT had notable development for increasing the hardness and tensile strength of the samples. The welding efficiency increased to almost 90 to 100%. In the fatigue test, FSWed samples exhibited low fatigue strength compared to the base material. However, the PWHT improves the fatigue strengths of both similar and dissimilar joint samples significantly. The fracture locations in tensile and fatigue tests of both T6-post weld heat treated samples of similar and dissimilar joints were at the HAZ which had the lowest hardness value. In conclusion, the T6-PWHT could effectively improve the welding efficiency and the mechanical strength of both the similar and dissimilar FSW joint of aluminium alloys.,Ph.D.
Pages:	203
Call Number:	TA480.A6B334 2019 3 tesis
Publisher:	UKM, Bangi
Appears in Collections:	Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

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