T. H. North

Joint formation in dissimilar Al alloy/steel and Mg alloy/steel friction stir spot welds

Abstract The microstructural features and overlap shear strength properties of friction stir spot welds made between Al 6111 and low carbon steel, and between Mg alloy AM60 and DP600 dual phase steel, are investigated. When Al 6111 is the upper sheet in the dissimilar sandwich, completed spot welds show evidence of intermetallic layer formation and cracking. Increasing tool pin penetration into the lower sheet provided increased mechanical interlocking of the sheets due to clinching. However, increasing penetration also promoted intermetallic formation and cracking in completed welds. However, dissimilar AM60/DP600 steel friction stir spot welds produced with AM60 as the upper sheet in the dissimilar sandwich do not show evidence of intermetallic formation and cracking may be avoided by removing the zinc coating on the DP600 steel before the friction stir spot welding operation.

Peak temperatures and microstructures in aluminium and magnesium alloy friction stir spot welds

Abstract The peak temperatures during friction stir spot welding of similar and dissimilar aluminium and magnesium alloys are investigated. The peak temperatures attained during friction stir spot welding of Al 6111, Al 2024, and AZ91 are within 6% of their solidus temperatures. In dissimilar AZ91/Al 6111 spot welds the peak temperature corresponds with the α-Mg solid solution and Mg17Al12 eutectic temperature of 437°C. An a-Mg plus Mg17Al12 eutectic microstructure is produced in dissimilar friction stir spot welds when material displaced during pin penetration into the lower sheet material contacts the upper sheet material at the eutectic temperature.

Energy utilisation and generation during friction stir spot welding

Abstract Energy utilisation during spot welding is investigated using a combination of calorimetry, peak temperature measurement and plunge testing. When a steel tool, clamp and anvil support is used, only 12·6% of the energy generated during the spot welding is transferred into the welded Al 6111 sheets. In contrast, when a mica clamp and anvil support are used, 50% of the energy generated during spot welding transfers into the welded Al 6111 sheets. Only a small percentage of the energy generated during the friction stir spot welding operation is required for stir zone formation. During plunge testing of 6·3 mm thick Al 6061-T6 material, less than 4·03% of the energy which is generated during friction stir spot welding is required for stir zone formation. The remainder of the energy generated dissipates into the tool assembly, clamp, anvil support and the aluminium sheets which are being welded. The rotating pin produces around 70% of the energy generated during spot welding of 6·3 mm thick Al-6061 material, with the remainder being contributed by the tool shoulder.

Improved mechanical properties in dissimilar Ti-AISI 304L joints

The effect of low temperature post weld heat treatment on the tensile strength and bend test properties of dissimilar friction welds between titanium and AISI 304L stainless steel joints is investigated. Post weld heat treatment at temperatures less than 873 K has no effect on joint tensile strength properties, but markedly improves bend test properties. The highest bend angle is produced using a post-weld heat treatment at 773 K for 1 h (the Larson-Miller parameter corresponding to this treatment is 15.5×103 K h−1). Low temperature heat treatment improves bend ductility, because stress relaxation occurs with minimal increase in the transition region width at the bondline region. Dissimilar joint bend testing properties decrease markedly when the width of the transition region exceeds 1–2 μm. An explanation for the detrimental effect of thick transition regions at the joint interface region on the mechanical properties of dissimilar joints is proposed. It is suggested that the development of significant triaxial stress due to the constraint imposed by large, needle-shaped intermetallic particles promotes premature joint failure in joints containing thick transition regions.

Counteraction of particulate segregation during transient liquid-phase bonding of aluminium-based MMC material

Particle segregation during transient liquid-phase bonding of aluminium-based metal matrix composite material using copper filler metal was investigated. Segregation was promoted by the slow movement of the solid-liquid interface during isothermal solidification and alumina particles with diameters less than 30 Μm were segregated when the copper foil thickness exceeded 5 and 15 Μm for the base metals examined. When bonding at 853 K, the liquid widths produced using these copper foil thiciknesses were almost identical to the median inter-particle spacing in the base metals investigated. When the amount of liquid formed at the bonding temperature decreased below a critical level, the test specimens broke apart immediately following the joining operation. The minimum film thickness of copper for satisfactory joint strength increased from 0.6 Μm to 2.4 Μm, when the heating rate to the bonding temperature decreased from 1 Ks−1 to 0.01 Ks−1.

Influence of silicon in aluminium on the mechanical properties of titanium/aluminium friction joints

The influence of post-weld heat-treatment and of residual silicon in aluminium on the mechanical properties of dissimilar friction joints between titanium and aluminium was investigated. Although joint tensile strength and bend test properties were drastically reduced following post-weld heat treatment, the responses of Ti/h.p. Al and Ti/c.p. Al joints were quite different. The tensile strength and bend test properties of Ti/h.p. Al joints were markedly decreased by heat-treatments involving shorter holding times at lower temperatures.Joint failure in post-weld heat-treated joints was associated with Al3Ti formation at the bondline region. The growth rate of the Al3Ti intermetallic layer at the joint interface was much faster in post weld heat-treated Ti/h.p. joints. More than 20 at%Si segregated in the region between the titanium substrate and the Al3Ti intermetallic phase in heat-treated Ti/c.p. Al joints. It is suggested that silicon segregation retards Al3Ti formation by acting as a barrier to titanium and aluminium diffusion at the joint interface.

Mechanism of cracking in AZ91 friction stir spot welds

Abstract The mechanism of liquid penetration induced (LPI) cracking is investigated in AZ91 friction stir spot welds. Liquid penetration induced cracking results from the following sequence of events: melted eutectic film formation in the periphery of the stir zone region, engulfment of melted eutectic films when the stir zone grows in width during the dwell period, penetration of α-Mg grain boundaries in the stir zone extremity and crack propagation when torque is applied by the rotating tool. Liquid penetration induced cracking occurs early in the dwell period during AZ91 spot welding and almost the entire stir zone is removed when the rotating tool is withdrawn. However, tool withdrawal does not provide the driving force for cracking during AZ91 spot welding; the upwards axial movement of the rotating tool at the end of the spot welding operation merely separates sections which are already cracked. The cracking tendency (the amount of the stir zone material produced during spot welding, which is removed when the rotating tool is withdrawn) is determined by the competing effects resulting from dissolution of melted eutectic films at α-Mg grain boundaries and the incorporation of eutectic rich material into the stir zone.

Numerical model for steady-state flow in friction welding

Abstract The purpose of this paper is to present an approximate mathematical model for the study of non-Newtonian flow regimes within friction welds. It is shown that the complex flows of the steady-state stage during friction welding can be described by a numerical solution of the Navier-Stokes equations. The results obtained predict many features observed in the micrographs of experimental samples. Three main findings are discussed in the paper. The first relates to a heuristic method which can be used for the estimations of the parameters of a non-Newtonian viscosity model required for the analysis. The second concerns the retention of particles, and of the flow itself, within the weld zone. The third suggests a theoretical framework which may permit the evaluation of joint strength from first principles.

Cracking in dissimilar Mg alloy friction stir spot welds

Abstract 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.

Local melting and cracking in Al 7075-T6 and Al 2024-T3 friction stir spot welds

Abstract Local melting of eutectic films and cracking is found in Al 2024 and Al 7075 alloy friction stir spot welds. Dissolution of melted films removes all evidence melted film formation in spot welds made using typical welding parameter settings. For this reason friction stir spot welding is carried out at a rapid plunge rate of 10 mm s−1 and an extremely short dwell time of 0·05 s and after tool retraction, the welded samples are rapidly quenched using a mixture of methanol and liquid nitrogen at a temperature of −80°C. Eutectic films rich in Zn and Cu are formed in Al 7075 spot welds while melted Al2CuMg particles promote the formation of α-Al+Al2CuMg eutectic films in Al 2024 spot welds. Melted eutectic formation and cracking is also observed beneath the tip of the rotating pin during Al 7075 friction stir spot welding and is consistent with the occurrence of melt wear in this location.