Science and Technology of Welding and Joining

Cracking during dissimilar friction stir spot welding of thixomoulded AM60 and AZ91 sheet materials is investigated. Liquation cracking occurs in the thermomechanically affected zone (TMAZ) region during friction stir spot welding when AZ91 material is the lower sheet in the dissimilar AM60/AZ91 sandwich. A TMAZ microstructure comprising elongated α-Mg grains and aligned Mg₁₇Al₁₂ is created during the tool penetration stage in spot welding and melted eutectic film formation is facilitated since the temperature in the stir zone (504°C) is much higher than the α-Mg+Mg₁₇Al₁₂ eutectic temperature (437°C).

There is no evidence of liquation cracking in the stir zone or the TMAZ region of AM60 sheet when it is the lower sheet in the dissimilar sandwich. However, liquation cracking is observed in the upper sheet material (AZ91) in the location beneath the tool shoulder close to its periphery.

The martensite start and bainite start temperatures have been determined in reheated, high strength weld deposits, with each result associated with 95% coincidence limits. This helped define isothermal transformation temperatures where bainite can be obtained without the risk of unintentional transformation to martensite. It has been demonstrated, therefore, that coalesced bainite, which is detrimental to mechanical properties, can be generated isothermally without the possibility of confusion with autotempered martensite.

Equiaxed zone (EQZ) formation in Al–Zn–Mg alloy welds as affected by base metal, filler metal chemistry and weld techniques is studied. Filler metal chemistry and welding techniques have great influence on the formation of EQZ microstructure as base metal composition has. In an effort to characterise the equiaxed grain zone formation in Al–Zn–Mg alloy welds two commercial Al alloys AA7018 and RDE40 were selected. Gas tungsten arc welding in continuous current, pulsed current and arc oscillation mode were applied to weld the base materials. The influence of Sc containing fillers have been studied and compared with the commercial filler material. Mechanical and metallurgical characterisation were carried out in the EQZ. Intergranular corrosion in EQZ was studied according to ASTM G 110-92. Results reveals that RDE40 with low solute contents showed wider EQZ but relatively better corrosion and mechanical properties compared to AA7018 EQZ. Gas tungsten arc welding in pulsed and arc oscillation mode fusion boundary region exhibits better corrosion and mechanical properties compared to continuous current mode welds. Addition of Sc to the AA5556 filler combined with pulsed mode resulted in elimination of EQZ, better corrosion and mechanical properties compared to welds made with conventional AA5556 filler and also the presence of Sc within the EQZ so called unmixed zone has been observed.

A feasibility investigation has been carried out of the formation of surface composite by uniformly distributing SiC particles 1˙25 µm in size into a surface layer of an A 1050-H24 Al plate through friction stir processing (FSP). The SiC particle was filled into a groove cut on the Al plate, covered by an Al sheet 2 mm thick, and a rotating tool was penetrated from the cover sheet so that the probe tip reached a depth beyond the groove bottom. The effects of process parameters (rotation speed and travelling speed) and applying multiple passes on the distribution of SiC particle in the nugget zone were investigated. The effects of groove size and its position relative to the tool probe were also investigated. Applying multiple passes had a great effect on the homogeneity of the SiC particle distribution. At rotation speeds of 2000–3000 rev min⁻¹, the SiC particles tended to cluster in some places in the nugget zone. By decreasing the rotation speed to 1000–1500 rev min⁻¹, the SiC particle was distributed in almost all the nugget zone area when the groove was 2–3 mm wide and 1˙5 mm deep. On the other hand, the stirring action of FSP was insufficient to distribute homogenously the SiC particles when the groove size was increased to 3×2 mm. By shifting the groove position towards the advancing side of the tool probe, the distribution of the SiC particles in the nugget zone became better. The defect free nugget zone with homogenously distributed SiC particles was obtained in a sample produced by FSP at rotation speeds of 1500 rev min⁻¹ for the first pass and 1250 rev min⁻¹ for the second and third passes. Microhardness of the nugget zone was increased to a level as high as 55 HV when the groove size was 3×1˙5 mm. The effect of the rotation speed on the particles dispersion was discussed with particular reference to the vertical material flow in the nugget zone.

Aluminium austenitic stainless steel joints find application in cryogenic engines, spacecrafts and automobiles. This dissimilar material combination is unweldable by conventional fusion welding due to a tendency for brittle intermetallic formation. Solid state welding processes such as friction welding are reported to be employed in such situations. This paper deals with continuous drive friction welding of AA6061 Al alloy to AISI 304 austenitic stainless steel. Direct welding of this combination resulted in brittle joints with 0° bend angle due to the formation of Fe₂Al₅. To alleviate this problem welding was carried out by incorporating Cu, Ni and Ag as diffusion barrier interlayers. The interlayer was incorporated by electroplating. Welds with Cu and Ni interlayer were also brittle due to the presence of CuAl₂ and NiAl₃. Ag acted as an effective diffusion barrier for Fe avoiding the formation of Fe₂Al₅. Therefore welds with Ag interlayer were stronger and ductile and could be bent to an angle of 100°.

The effect of process parameters (cycle time, tool speed and axial force) on the specimen temperature measured 2 mm away from the weld in spot friction welding (SFW) of Al 6111-T4 is investigated. The temperatures were correlated to the lap shear load. Results revealed that, to achieve a good joint strength with the maximum lap shear load >2˙5 kN, temperatures should be greater than a threshold value, which is 350°C at a location close to the SFW joint in this study. By studying the specimen macrographs, two internal weld geometric features based on the cross-section area were identified and correlated to the shear and mixed failure modes of the lap shear tested specimens. A model was developed and validated using experimental data of the cross section area of SFW joint with either shear or mixed mode fracture. The model predicts that the SFW joint strength is maximised at the transition region between the shear and mixed mode fracture.

The monitoring of welding process is crucial for the development of a real time quality control system for the pulsed metal inert gas welding (PMIGW) process. This work introduces an intelligent system for weld joint strength prediction in a PMIGW process based on the analysis of acquired current signal by wavelet packet transform. A thirteen-dimensional array of process features, i.e. six process parameters and seven wavelet packet features, are used to describe various welding conditions. These process features obtained from a set of experiments are employed as input vectors of an artificial neural network model to predict the corresponding weld joint strengths. The results, i.e. the prediction errors, show that the use of wavelet packet features gives much accurate prediction as compared to the use of the purely time domain features.

The present paper investigates the tensile behaviour of alloy 718 sheets processed after electron beam welding (EBW). The alloy 718 sheets were pretreated in three conditions: as received, solution and precipitation pretreatments. Metallographic and fractographic examination on the specimens were conducted with optical microscope, scanning and transmission electron microscopes. Experimental results show that cracks propagate through all of the fusion zones during tensile test. The intermetallic laves phase, which precipitates along the fine columnar grain boundary in the fusion zone, offers a preferred crack propagation path, especially for the precipitation pretreated weld. The static ductility of precipitation-treated weld decreases significantly after EBW. This was attributed to high precipitation density of laves phase along the columnar grain boundary. Relatively high tensile strength and ductility of as-received and solution-pretreated welds can be obtained by using two sequential welding passes. The first pass is full penetration welding with high heat input, and the secondary pass uses cosmetic welding to reduce the surface undercut resulted from the first pass.

Four Fe–Cr–C hardfacing alloys with carbon contents of 3˙34–6˙5% were studied. The orientation of primary carbides in the microstructures of hardfacing layers produced by arc surfacing was investigated under controlled cooling conditions. Carbon content and cooling conditions were found to play an important role in determining overlayer microstructures. Increasing carbon content or decreasing Cr/C ratio increased the tendency for primary carbides to be oriented perpendicular to the surface of the overlayers, and the carbides in the microstructure became more compact. Under water cooling conditions, the primary carbides were preferentially oriented perpendicular to the surface, which would be expected to improve wear resistance. At lower cooling rates, primary carbides were oriented randomly.

Friction stir spot welding (FSSW), which was recently developed as a spot joining technique, has been applied to a lap joint of Al alloy 6061 sheets, 1 mm in thickness, to clarify its microstructural features of the joint. A nugget shaped stir zone having finer grain size was observed around the exit hole of the probe. Crystallographic texture analyses using EBSD method suggested that the material flow occurred along the rotating direction of the FSSW tool in the wide region including the stir zone. In the periphery of the nugget shaped stir zone, which was characterised by finer grain size than the stir zone interior, no inclusions or precipitates were found on the SEM scale. A softened region was formed around the joint centre, which could be explained as resulting from dissolution and/or growth of the strengthening precipitates due to thermal cycle of FSSW.

Single sided resistance spot welding (SSRSW) is considered as a feasible method to join hydroformed or closed section parts to others in vehicle productions. Unfortunately, it is difficult to guarantee the weld quality utilising conventional air gun. Because of a lack of support inside the closed section parts, the impact of electrode driven by pneumatic gun will cause large deformation of the workpieces at the welding stage and will lead to a crack around weld region after welding completion. In addition, poor weldability is another pressing problem for welding operations. Servo gun with new gun driven method has some merits such as realising the soft touch between electrode and workpieces and changing electrode force during the welding process which are greatly suitable to the sheet to tube joining. Based on the characteristics of servo gun, the present paper investigates a new method to increase the weld quality of sheet to tube joining with SSRSW method. By adjusting the electrode force during the welding process, weld strength would be increased, weld deformation be decreased and weldability lobe curve be widened. The results verify that servo guns can provide high assurance for welding quality of sheet to tube joining and have broad prospect in SSRSW.