PhD Thesis Defence – Effect of Laser Shock Peening on Residual Stress and Mechanical behaviour of Aluminium alloy AA2219 Friction Stir Weld​

Abstract:

AA2219 is a heat treatable wrought aluminium alloy and has high specific strength and fracture toughness. Large-volume propellant tanksof space launch vehicles are manufactured by joining AA2219 aluminium alloy through the Friction Stir Welding (FSW) process. Thepropellant tanks are designed optimally to improve the payload capability of aerospace vehicle. Improvement in the performance of theFSW joint will lead to enhancement in either payload or structural margins. Hence, there is a strong need to improve AA2219 FSW jointperformance. Several post-weld treatments are suggested to improve the performance of weld joints, such as post-weld heat treatment,surface treatment, etc. Laser shock peening (LSP) is one of the most promising surface treatment techniques for improving theperformance of the FSW joint. In this work, an attempt is made to improve the performance of the AA2219 T87 FSW joint through LaserShock Peening (LSP). The impact of LSP on Residual Stress (RS), microhardness, global tensile behaviour at various temperatures, localtensile behaviour, stress corrosion cracking behaviour and surface roughness was investigated.​

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The surface residual stress distribution is non-uniform across the weld, and the joint exhibits peak tensile residual stress of +123.5 MPa(longitudinal) in the TMAZ region on the top surface. The LSP process reduces the peak tensile RS to -20 MPa, -130.2 MPa and -195.5 MPa(compressive) with single, three and six layers, respectively. The longitudinal through-thickness residual stress distribution is also non-uniform across the weld and through-thickness. Tensile residual stress exists near the weld center with a peak value of 160 MPa at mid-thickness, which is 41% of the yield strength of base metal, and it changes to compressive away from the weld center. Tensile RS is due tothermal cycle and rigid clamping experienced during FSW processing. Six layers of LSP reduced peak RS to + 65 MPa (55% reduction). TheLSP process has redistributed tensile residual stress due to self-equilibration. The LSP process has an influence not only on a surface levelbut also on the sub-surface. Results from this study indicate that the LSP process can mitigate tensile RS throughout the thickness (7 mm)of the FSW joint. RS reduction is due to increased dislocation density caused by plastic deformation of the surface due to the impact ofthe high-energy laser pulse during the LSP process. ​

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AA2219 T87 FSW joint exhibits yield strength of 197 MPa and ultimate strength of 348 MPa yield and ultimate strength at ambienttemperature with elongation of 13.7%. The LSP process increased YS by 7%, 10% and 14% with single, three and six layers of LSP,respectively. In cryogenic temperatures (77K & 20K), the increase in YS is 5-6%, 7-8% and 10-12% with single, three and six layers ofpeening, respectively. The increase in YS is less at elevated temperatures (423 K) i.e. 1%, 2% and 7% with single, three and six layers ofLSP. At all investigated temperatures, LSP resulted in an increase in yield strength. Repeated layers of LSP led to a proportional increase inyield strength at all studied temperatures. The increase in the yield strength is due to the strain hardening effect caused by surfacecompressive stress induced by laser shock peening. Surface compressive stress increases dislocation density and plastic deformationresistance. However, LSP has not influenced UTS and elongation at all investigated temperatures. ​

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AA2219 T87 FSW weld has a heterogeneous microstructure with wide variation in grain size, orientation, and precipitate distribution. Theresponse of different zones of FSW joint to LSP was investigated using the digital image correlation technique. The weld nugget regionshows an increase of 7%, 8% and 16% in yield strength with single, three and six layers of peening, respectively. Meanwhile, TMAZshowed an increase of 5%, 10%, and 21%. HAZ does not exhibit a significant increase in yield strength. However, the increase in YS in theroot side is not significant (5%) compared to the crown side. This is due to higher hardness due to a lower temperature and the heat sinkeffect of the backing bar. ​

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Micro-hardness on the top (crown side) and bottom (root side) surface of the weld increases due to laser peening in all regions of theFSW joint, which agrees with the tensile properties and indicates strain-hardening behaviour. Single-layer, three-layer and six-layer laserpeening increased average hardness by 7%, 17% and 20%, respectively, in the weld nugget region of the crown side of the weld. Single-pass peening has affected < 0.5 mm depth, whereas three and six passes of peening have influenced a depth of ~ 1.0 mm and more than2 mm, respectively. LSP increases hardness both in surface and subsurface levels, and repeated layers of LSP led to higher hardness in thesub-surface. ​

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The LSP process has increased surface roughness, and the increase is substantial in the weld nugget and TMAZ regions, which are thesoftest regions of the weld joint. AA2219 T87 FSW joint exhibits good SCC resistance in as-welded and LSP conditions, i.e. SCC index > 0.9,irrespective of the number of layers of LSP. ​

Laser shock peening of the AA2219 T87 FSW joint reduces residual stress (surface and sub-surface level) and improves yield strengthwithout compromising stress corrosion cracking behaviour. The research outcome of this work will be useful in improving the safetymargins of aerospace structures and pressure vessels. ​

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Keywords: AA2219, Friction Stir Weld (FSW), Laser Shock Peening (LSP), Residual Stress (RS), Tensile properties, Microhardness.​