Influence of increased processing speed on the microstructure evolution and mechanical property relationship in friction stir welding of AA5182-H111 (T500)
- Authors: Bernard, Dreyer
- Date: 2016
- Subjects: Friction stir welding , Aluminum alloys -- Welding
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/7322 , vital:21319
- Description: Friction stir welding of 5xxx series aluminium is generally carried out at low travel speeds, which limits the commercial applications of this process. This manuscript discusses the dynamic eformation characteristics of AA5182-H111 (T500), which allowed a travel speed of 1500 mm/min to be realised. This was achieved by using a spindle speed of 500 rev/min; the same as that required to make a weld of 200 mm/min. The thermal cycles, microstructural evolution due to tool / work-piece interaction, and the local microstructural and mechanical properties of the completed weld are discussed for a travel speed for 200 mm/min and 1500 mm/min. At a rate of 200 mm/min, heat generated due to contact between the shoulder and work-piece surface was transferred ahead of the tool, driving dynamic recrystallization which lowered the flow stress. Hereafter the material entered the stir zone where it was extruded around the pin during a process of continuous dynamic crystallization. At a travel speed of 1500 mm/min, the rate of heat conduction ahead of the tool was not rapid enough, and the material was deformed at low temperatures. A low weld pitch rotations per unit length travelled) was required to ensure that the material was sufficiently strained, to allow the formation of geometric necessary boundaries, which resulted in grain subdivision. The number of high angle grain boundaries was increased through subdivision, thus, increasing the nucleation sites for dynamically recrystallized necklace grains to form. In the absence of heat conduction from the shoulder, the high plastic strain ahead of the tool provided the energy required to drive dynamic recrystallization. Heat was generated from the high plastic strain and during recrystallization, where the stored plastic energy was adiabatically released. Once the material entered the stir zone, the high strain rates associated with the tool rotation which drove dynamic recrystallization that allowed the material to reach high levels of strain, resulted significant grain refinement. The weld zone of the 200 mm/min displayed weld a slight increase in yield strength, with respect to the parent material, due to grain refinement to 10 μm, while the yield strength of the 1500 mm/min weld was significantly increased due to grain refinement to 5 μm. The high weld speed did, however, generate high tensile residual stresses. This work is not only significant for the field of friction stir welding, but also in terms of material processing. The dynamic deformation characteristics associated with Al-5Mg-Mn, alloys which was observed ahead of the tool, is of high value in processes where plastic deformation is applied to improve the mechanical properties of these alloys.
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- Date Issued: 2016
Increasing the gap tolerance in friction stir welded joints of AA6082-T6
- Authors: Oyedemi, Kayode
- Date: 2012
- Subjects: Friction stir welding , Aluminum alloys -- Welding , Welding
- Language: English
- Type: Thesis , Masters , MTech
- Identifier: vital:9628 , http://hdl.handle.net/10948/d1012325 , Friction stir welding , Aluminum alloys -- Welding , Welding
- Description: This research project was conducted to increase weld gap tolerance in Friction Stir Welding (FSW) of 8 mm thick aluminum alloy 6082 – T6. Investigation was done on I-STIR PDS platform and a Johnford milling machine. The research project involved tool-pin design with varying re-entrant features and varying parameters as a method of weld gap closing to produce successful welds. Direction of spindle rotation and dwell time were established as part of a preliminary study. Clockwise spindle rotation with 20 seconds dwell time allows sufficient plasticity and improved material flow which assisted in achieving welds with prior 30 percent weld gap of the plate thickness. Final welds were made using three rotational speeds and feed rates with sufficient plunging to prevent root defects. Analysis of the results were detailed which include vickers microhardness test, tensile test and metallographic observation to access the suitability of the weld structure. From the set of tool-pins designed, the flare tool-pin gave a well-defined weld nugget with improved stirring at the weld root. Also, with a concave shoulder, right hand threaded tool-pin and counterclockwise flutes undergoing a clockwise spindle rotation, plasticized material flow was upward which was beneficial in reducing the amount of plate thinning. The right hand thread counter clockwise flute with a flute machined in the foot exhibited superior tensile strength for welds containing 30 percent weld gap.
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- Date Issued: 2012
Development and analysis of a friction stir spot welding process for aluminium
- Authors: Stephen, Michael George
- Date: 2005
- Subjects: Friction welding , Electric welding , Aluminum alloys -- Welding
- Language: English
- Type: Thesis , Masters , MTech
- Identifier: vital:9631 , http://hdl.handle.net/10948/1351 , Friction welding , Electric welding , Aluminum alloys -- Welding
- Description: Friction Stir Spot Welding (FSSW) has been developed from the conventional Friction Stir Welding (FSW) process, developed at The Welding Institute (TWI). FSSWs have been done without the keyhole being eliminated. Elimination of the keyhole would result in the process being more commercially viable. This dissertation focuses on an attempt of eliminating the keyhole using a retractable pin tool as well as a comparison of the weld integrity of a FSSW to that of a conventional Resistance Spot Weld (RSW). Welds were conducted on aluminium alloy 6063 T4. Comparisons between different weld procedures were done. Further analysis of the weld integrity between FSSW and RSW were conducted, comparing tensile strengths, microstructure and hardness. For the above welding procedure to take place, the current retractable pin tool, patented by PE Technikon, was redesigned. Problems associated during the welding process and the results obtained are documented. Reasons for the keyhole not being eliminated as well as recommendations for future work in the attempt to remove the keyhole are discussed.
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- Date Issued: 2005
Analysis of material flow around a retractable pin in a friction stir weld
- Authors: Georgeou, Zacharias
- Date: 2003
- Subjects: Friction welding , Aluminum alloys -- Welding
- Language: English
- Type: Thesis , Masters , MTech (Mechanical Engineering)
- Identifier: vital:10812 , http://hdl.handle.net/10948/196 , Friction welding , Aluminum alloys -- Welding
- Description: Friction StirWelding (FSW) has been researched for a number of years since its inception in 1991. The work thus far has been based on understanding the material and thermal flow using the standard fixed pin tool. The keyhole resulting during tool extraction in a FSW weld, is a disadvantage and a current limiting factor. Eliminating this effect from a weld using a movable pin tools would make FSW more commercially viable. This dissertation focuses on the design of a novel retractable pin tool, and highlights the problems encountered during the welding of Aluminum plates, Al2024 and Al5083. Previously studied techniques of material and thermal flow were used, to investigate the effect of the tool during extraction in a FSW weld. A prototype retractable tool was designed using parametric and axiomatic design theory, and implementing a pneumatic muscle actuation system. The resulting problems in the calibration of the retractable pin tool and the resulting welds are presented, these results confirming previous studies. The movable pin produced discrepancies the heat generation around the shoulder during a FSW weld. The failure of this tool to produce a reasonable weld showed that previous ideas into the workings of a retractable pin tool requires further investigation, furthermore a fresh approach to the interpretation and understanding of the FSW weld process needs consideration.
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- Date Issued: 2003