Science and Technology of Welding and Joining

The effects of inadequate shoulder contact and poor gas shielding were examined in friction stir welds of ATI 425 titanium. Welds with varying oxidation induced surface discolorations due to poor gas shielding and varying amounts of shoulder contact were studied to determine the effects of these parameters on the microstructure, microhardness, and three-point bend test properties of the welds. The effects of these parameters and their effects on the mechanical properties of the resultant weld are discussed.

The effects of welding parameters on material consolidation are examined during friction stir butt welding of 2 mm Al 5083 alloy aluminium sheet with a surface cladding of Al 3025 alloy, which was co-cast from the melt. The influence of welding parameters on joint consolidation is investigated when tool revolutions per minute, travel speed and penetration depth were varied. It was found that modifying the pin of the welding tool to have a two-flat profile improves material consolidation and avoids defect formation during welding, and optimum welding parameters involve a combination of high tool rotation speed and travel speed. Optical and electron microscopy revealed that the integrity of the surface cladding layer could be maintained during friction stir welding while avoiding defect formation within the stir zone of the weld. The tensile strength of the joint was ∼58% of the base material due to softening within the stir zone.

The aim of the present work was to investigate the possibilities of hybrid laser arc welding regarding reliable production of longitudinal welds of high strength pipe steels X80 and X120 and to evaluate achievable mechanical properties of laser hybrid welds. The study focused on weld toughness examination in low temperature range up to −60°C. Suitable filler materials were identified in the context of this task. It could be shown that metal cored electrodes guaranteed sufficient Charpy impact toughness at low temperature for both investigated materials. Modern arc welding technologies such as modified pulsed spray arc were used to promote deeper penetration of the filler material into the narrow laser welding gap. Edge preparation with a 14 mm deep root face was considered as optimum, because no penetration of the filler material could be detected beyond this depth limit, and therefore, any metallurgical influences on the weld metal properties through the welding wire could be excluded.

A cast precipitation hardened nickel based superalloy was built up by electrospark deposition and then subjected to pulsed laser welding. The resistance of electrospark deposition layer to liquation and solidification cracking in pulsed laser welding was significantly higher than that of the cast alloy. It is shown that the extreme cooling rates involved in the solidification of electrospark deposition droplets result in the prevention of formation of grain boundaries terminal solidification constituents, which are the sources of liquation cracking in the cast alloy.

Laser and laser–arc hybrid welding are employed to join Invar 36 alloy. The microstructure, coefficient of thermal expansion (CTE) and mechanical properties of laser weld (LW) and hybrid weld (HW) are analysed and compared. The tensile test shows both LW and HW fractured in the weld metal, but the HW is stronger than the LW. Especially, the yield strength of HW is 362·8 MPa, 16·2% higher than that of LW. Both the CTE and Curie temperature of LW are almost the same with those of base material. The CTE of HW is higher than that of base material because the Ni content reduces and the content of other alloying elements increases. Moreover, the CTE of HW deviates from the theoretic curves of Fe–Ni alloy apparently. According to the theory of Invar effect, the deviation is attributed to the formation of new phase, FeCr_0·29Ni_0·16C_0·06.

This article deals with temperature characterisation by infrared thermography during welding. To demonstrate the versatility of this technique, two welding processes were investigated with different materials: cold metal transfer metal inert gas welding of al alloy and resistance spot welding of galvanised steel. Two experimental methods were used to characterise temperature without requiring implementation of thermocouples. The validity of these methods is demonstrated by metallurgical characterisations. The error on the measured values of temperature is assessed regarding the initial uncertainty on the apparent surface emissivity. The results show that it is possible to obtain absolute temperature values, cooling rates and informative visualisation of temperature gradients with a reasonable uncertainty.

Weld filler alloys that exploit transformation plasticity through low austenite to martensite transformation temperatures offer an effective method of reducing residual stresses in strong steel welds. However, in multipass welds, the heat input from later weld passes may be insufficient to retransform prior welding passes, leading to the accumulation of thermally induced strains and elevated residual stresses. In this work, the residual stress distributions produced around arc welds fabricated with a martensitic weld filler alloy that transforms at a low temperature have been studied as a function of the number of passes deposited and the interpass temperature. It is found that when the interpass temperature is above the transformation temperature of the weld metal, the entire multipass weld transforms as a single entity, thus permitting the optimum exploitation of the transformation plasticity. In contrast, the deposition of new metal with a relatively low interpass temperature leads to increased residual stresses in the underlying layers, reducing or eliminating the beneficial stress states previously created.

Dual phase 980 MPa grade (DP980) steel sheets were resistance spot welded using a pulsed current, and the effects of the pulsed current on the strength properties of the joints were investigated. The pulsed current improved the mechanical properties of the joints in cross tensile tests. In situ observations during tear tests revealed that the ductility of the nugget was improved and that the propagation of cracks into the nugget was inhibited when the pulsed current was used. Microstructural observations and electron probe microanalysis (EPMA) of the nugget showed that the segregation of phosphorus at the nugget was reduced in the joint welded using the pulsed current, suggesting that the pulsed current improved the ductility of the nugget by altering their microstructures.

DP980 dual phase and high strength low alloy (HSLA) steels were welded, using fibre laser, with varying amounts of concavity to determine its effects on the tensile and fatigue properties. Higher concavity, 25 to 35%, was observed to reduce the tensile strength of the DP980 welds, while not affecting the tensile strength of HSLA welds. All welds exhibited lower fatigue resistance compared to their base metals. However, DP980 welds with higher concavity (25 to 35%) exhibited even lower fatigue resistance while HSLA welds showed similar performance regardless of changes in concavity. Concavity could be minimised by reducing welding power and increasing the welding speed.

Use of multimaterial fabrication such as aluminium to steel to reduce overall vehicular body weight has gained significant attention in the automotive industries. Since fusion welding of aluminium to steel is difficult, friction stir welding of the same is considered as an effective recourse. Quantitative studies on friction stir welding of aluminium to steel are thus important but scarce in the literature. We present here a numerical and experimental study on friction stir lap welding of AA6061 to high strength interstitial free coated steel sheets under different combinations of tool rotational speed and welding speed. The computed values of thermal cycle, torque and traverse force are found to be in good agreement with the corresponding experimentally measured values. The computed thermal cycles along the AA6061 to steel interfaces are related qualitatively with the experimentally measured trend and distribution in Fe–Al intermetallics along the weld joint interface.

Friction stir welding was employed for joining of 12Cr heat resistant ferritic steel. Microstructure and mechanical properties in the obtained welds were examined and their relationship was established. The stir zone microstructure was found to consist of quenched martensite with numerous fine dispersoids uniformly distributed in grain interiors. The formation of this microstructure remarkably strengthened the stir zone. Tempering for 3·6 ks at 923 K reduced the as welded hardness of friction stir weld to nearly base material level.

The phenomena during inclined laser irradiating on the metal surface were investigated to imitate the behaviour on the front keyhole wall in deep penetration laser welding. The results revealed that the velocity of molten layer increased when the laser power density increased. The molten layer could be torn off to form droplets owing to the high evaporating pressure induced by the high power density laser. The heating efficiency of laser induced vapour plume was lower than that of the molten metal. In reflection experiments, only when the incident angle was 88°, the absorption of reflected laser was greater than that of the first incident laser. The concaved surface of molten layer had a great effect to the reflected laser. During multiple reflections, the considerable absorption of laser energy mainly happened in the first two times. The calculated data were closer to the actual conditions when the influence of polarisation variation was avoided.