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Investigation of Hot Cracking Phenomena in Fusion Welding of High-Strength Aluminum Alloys and the Effect of Friction Stir Processing (FSP) Pre-Treatment on the Weldability of 7075 Aluminum Alloy

Pirjamadi, Alireza | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55209 (07)
  4. University: Sharif University of Technolog
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid; Movahedi, Mojtaba
  7. Abstract:
  8. High-strength aluminum alloys are widely used in various industries; however, their usage is limited to solid-state welding or mechanical joints due to their low weldability. The main metallurgical challenge of alloy 7075 welding is hot cracking. Thus, the mentioned alloy is introduced as a "non-weldable alloys" category. In this study, the PMZ was investigated, and it was shown that precipitations play an essential role in intensifying the phenomenon of partial melting. On the other hand, the result of the mechanisms activated in the PMZ, called the "L" region, occurs in the grain boundaries during welding. Welded plate with FM1050 causes liquation cracking, while Welded plate with FM4043 does not cause the cracking in the PMZ. As a result, welding of alloy 7075 through FM4043 is possible. However, loss of the PMZ mechanical properties is a challenge that must either be taken into account in the calculations or must be eliminated. The amount of elongation has decreased from 11% (base metal in the T6 condition) to 5% (As well as a 51% reduction in ultimate tensile strength). It was shown that in the bead on plate welding, the hot cracking phenomenon is highly dependent on the heat input. After increasing from a critical limit, hot cracking appears throughout, containing liquation, solidification, and growing to the base metal cracks. At the lower heat inputs, the cracking phenomenon is observed as solidification microcracks (in the melted zone), reducing fracture ductility of the welded plate. It is possible to weld two sheets with a V-groove with a similar filler metal by controlling the heat input. To eliminate the negative effect of solidification microcracks, a heat treatment was proposed to develop the microstructure and mechanical properties. The mechanical properties of the weld were improved by 6 hours of solutionizing at 460° C (instead of 2 hours). The joint, which is accompanied by heat input controlling and is subjected to the proposed heat treatment, has a strength of about 500 MPa with a 9% elongation, which is an acceptable result for the 7075 non-weldable alloy. Also, FSP pre-treatment, which causes the grains to become fine-grained in the PMZ, significantly reduces the possibility of hot cracking occurrence. FSP pre-treatment eliminates the macro cracking of the alloy and turns it into micro cracks in MZ. Finally, the square-groove joint created by the FSP pre-treatment and fusion welding (with the proposed heat treatment condition) has an ultimate tensile strength of about 550 MPa. In comparison, the ultimate tensile strength of the base metal in the T6 condition is about 585 MPa.
  9. Keywords:
  10. Aluminum Alloy 7075 ; Hot Cracking ; Friction Stir Processing ; Partially Melted Zone (PMZ) ; Weldability ; High Strength Alloy ; Fusion Welding

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