Supriyo Ganguly

Effect of high pressure rolling on weld-induced residual stresses

Abstract The formation of large residual stresses continues to be a problematic side effect of all common welding processes. In this work, localised high pressure rolling of gas metal arc welds to relieve these residual stresses has been investigated using strain gauging and neutron diffraction. Rolling was found to remove undesirable tensile stresses and even induce large compressive ones, though only when applied after rather than during welding. Strain measurements taken during combined welding and rolling operations show that this is because material at the weld line continues to yield as it cools. This erases any beneficial effect on the stress distribution of rolling at high temperature. A method of rolling using an oscillating force is also presented and found to be just as effective as the equivalent static force process.

Comparison of joining efficiency and residual stresses in laser and laser hybrid welding

Abstract Laser welding is a high energy density process, which can produce welds with less energy input and thereby lower residual stress generation compared to arc welding processes. However, the narrow beam dimension makes it extremely sensitive in terms of fit up tolerance. This causes a problem in achieving high quality welds. Laser with arc hybrid process overcomes such issues. In this paper, longitudinal residual strains were compared for autogenous laser welding and laser/TIG hybrid processes. Joining efficiency, which is defined by the penetration depth achieved per unit of energy input, was correlated with the residual strain generation. It has been shown that to achieve a specific penetration depth, there is an optimum welding condition for each of the welding processes, which will give minimum tensile residual stress generation. The results imply that for the same penetration depth, hybrid process resulted in ∼50% higher tensile longitudinal domain compared to autogenous laser.

Validation of the Contour Method of Residual Stress Measurement in a MIG 2024 Weld by Neutron and Synchrotron X-ray Diffraction

The contour method is a newly-invented destructive technique of residual stress measurement and has been receiving increasing interest. It is advisable to validate such new methods by comparison with other well-established techniques. For this purpose, contour measurements were performed on a MIG 2024-T351 aluminium alloy welded plate. The result has been compared with the results from neutron and synchrotron X-ray and shows a favourable agreement.

Investigation of dissimilar metal welds by energy-resolved neutron imaging

Energy-resolved neutron imaging is used for a nondestructive study of bulk internal microstructure, elemental composition and distribution of voids in dissimilar metal-alloy welds of ∼10 mm thickness. All these characteristics are measured simultaneously in one experiment with a few hundred micrometre spatial resolution.

Comparative Neutron and Synchrotron X-Ray Diffraction Studies to Determine Residual Stress on an As-Welded AA2024 Plate

The residual stress field in a VPPA welded AA2024 coupon sample was measured by neutron diffraction. A similar sample was measured using synchrotron X-rays to determine the inplane strain directions. The macrostrain obtained from both sources compares well in spite of the significant difference in gauge volume and diffraction geometry. The result confirms the repeatability of the weld process and also shows good correspondence between the two diffraction processes.

FULL STRESS TENSOR DETERMINATION IN A TEXTURED AEROSPACE ALUMINIUM ALLOY PLATE USING SYNCHROTRON X-RAY DIFFRACTION

Triaxial strain scanning has been performed on a 7mm thick 7150-aluminium alloy Variable Polarity Plasma Arc (VPPA) weld using synchrotron X-rays. It has been demonstrated that the use of specific advantageous (hkl) peaks that occur in this highly textured aerospace alloy plate permits fast deep measurements in reflection as well as in transmission. The sin⁡2ψ method was also utilised to monitor the variation in deviatoric stress component (σ11−σ33) across the weld in both the test-piece and a comb like reference specimen used to map the changes in stress-free lattice spacings over the weld. The results suggest that microstresses have little effect on macrostress determination, particularly in the Heat Affected Zones (HAZ). Though small in magnitude, the macrostresses still present in the reference specimen can be allowed for to correct the stress tensor in the plate. Maximum tensile longitudinal stress values remain unchanged, whereas the stress condition in some parts of the HAZ is well described as biaxial after the proposed correction.

Weld Stress Mapping Using Neutron and Synchrotron X-Ray Diffraction

Neutron and X-ray diffraction techniques have been utilized to map residual stresses in MIG welded 7150 aluminium alloy plates with a high texture. Longitudinal (parallel to the weld seam) strain measurements were made using 40 keV X-rays whilst normal (through-thickness) and transverse strains were measured using neutrons at a pulsed spallation source. The strains have been corrected for the strong chemical composition variation across the weld. The complementarity of the methods was confirmed by comparing the strain obtained in the transverse direction. The combined results of both diffraction experiments enabled the full 3D stress tensor to be mapped across the weld in an optimized manner.

Residual strain measurement for arc welding and localised high-pressure rolling using resistance strain gauges and neutron diffraction

Neutron diffraction and foil resistance strain gauges have been used to study the state of residual stress introduced by localised high-pressure rolling of structural steel plates, and compare it to that caused by gas metal arc welding. Rolling creates a region in which the residual stress state is highly compressive in the rolling direction. Furthermore, this region is sharply defined, making it potentially very suitable for cancelling out the tensile residual stresses caused by welding. It is also demonstrated that non-destructive strain measurements made during the welding and rolling processes can be used to indicate residual elastic strain and stress, and that this method shows good agreement with conventional neutron diffraction measurements. Determination of residual stresses in this way requires consideration of the effect of curvature on the values of strain measured at the surface of the object.

Characterisation of residual stress state in laser welded low carbon mild steel plates produced in keyhole and conduction mode

Abstract Characterisation of residual stress state was performed in 4 mm low carbon steel plates laser welded in keyhole and conduction mode. Residual stress characterisation was carried out at the ENGIN-X strain scanner at ISIS (Oxford, UK). It was shown that although the maximum magnitude of tensile residual stress is similar in welded specimens manufactured under different welding modes, the distribution profile is quite distinguished. The conduction welding mode resulted in a larger tensile stress domain as compared to the keyhole mode in the longitudinal direction. This also resulted in a different magnitude of balancing compressive residual stress field. Understanding of such different stress profiles is important for application of such advanced welding processes in joining of design efficient structural material.

Residual Stress Characterization and Control in the Additive Manufacture of Large Scale Metal Structures

Additive Manufacture of metals is an area of great interest to many industrial sectors. All metal additive manufacturing processes suffer with problems of residual stresses and subsequent distortion or performance issues. Wire + Arc Additive Manufacture (WAAM) is a metal additive manufacture process that is suitable for the production of large scale engineering structures. Paramount to the successful industrial application of WAAM is the understanding and control of residual stress development and their subsequent effects. Vertical inter-pass rolling can be used to reduce these residual stresses, but its potential is limited due to the absence of lateral restraint of the wall. So it deforms the wall in its transverse direction rather than reducing longitudinal tensile residual stresses, which is the main source of the distortion. The potential of a new pinch-roller concept is currently being investigated at Cranfield University with very promising preliminary results: It was possible to entirely eliminate the distortion of a Ti-6Al-4V WAAM wall. Introduction Wire + arc additive manufacturing (WAAM) is a robotic and welding equipment based highdeposition-rate additive manufacturing (AM) process, which can be used for the manufacture of large-scale aerospace parts. Fig. 1 shows a Ti-6Al-4V landing gear rib near-net-shape demonstrator part manufactured with WAAM. Compared to conventional subtractive machining, the main benefits of WAAM are the significant time, material, and cost savings.