Science and Technology of Welding and Joining

The main goal of this investigation is to study the flow pattern and mixing which take place during dissimilar friction stir welding (FSW). Aspects such as the origin of onion rings and development of welding defects are considered. An evaluation of the impact of welding parameters (rotational speed) on material mixing of AA 7075-T6 and AA 2024-T3 (3 mm thickness) dissimilar butt welds was performed. The ‘stop action’ technique has been employed in order to acquire a close snap shot of the flow and mixing in one of the samples which showed limited mixing but optimal mechanical performance and surface condition. A non-stable rotational flow inside the threads has been identified in this sample due to the formation of a cavity on the rear of the pin. This fact gave rise to characteristic bands in the longitudinal section but it was not enough for developing an onion ring-like mixing pattern.

The aim of this investigation was to study material flow during dissimilar friction stir welding of AA 5083-H111 to deoxidised high phosphorus copper plates of 1 mm thickness. The welds were performed using different tool geometries and welding parameters. The positions of the copper and aluminium plates, relative to the advancing and retreating sides of the tool, were also changed. It was found that the tool geometry and relative position of the plates deeply influence the morphology of the aluminium and copper flow interaction zones, influencing the distribution of both materials in the weld and the formation of intermetallic compounds. The material accumulated under the tool during welding was found as another important aspect determining weld morphology.

This paper reports on a new method based on the friction stir welding process to join dissimilar metals in butt joint configuration. Two different systems were considered: AA1050 H16 aluminium/ASTM A284 steel and AA1050 H16/UNS C12200 H01 copper. The unthreaded steel tool pin was positioned in the aluminium plate so that it was tangential to the opposing metal. Bonding was accompanied by interfacial chemical reactions with no significant mechanical mixing. This new solid state welding process is called friction stir diffusion bonding. Room temperature cross-weld tensile strengths up to 82 MPa were obtained for both metal combinations. Microstructure characterisation suggested that higher joint strengths were associated with thinner, <1 μm thick intermetallic reaction layers at joint interfaces.

Material flow during friction stir spot welding is investigated by experimental approaches in this study. Different kinds of fine wires and WC powder are used as trace material and disposed in the interface between upper material and lower material. After spot welding, detailed microscopic observations are carried out in the various sections. Continuous distributions of tracer materials are investigated during friction stir spot welding. Three-dimensional material flow model is proposed by the combination of the results, which are obtained in the present study and previous studies.

Effect of microstructures on liquation cracking is investigated. Melted eutectic formation and cracking are observed in the stir zone region of friction stir spot welds made in squeeze cast and in incompletely solutionised AZ91 sections containing Mg₁₇Al₁₂ particles. Crack susceptibility in the stir zone region is dramatically decreased in AZ91 base material which has been forged and solution treated, and is completely free of Mg₁₇Al₁₂ particles. It is suggested that rapid heating during the tool penetration stage in friction stir spot welding creates undissolved Mg₁₇Al₁₂ particles, which facilitate eutectic melting when the temperature in the stir zone reaches 437°C, the (α-Mg+Mg₁₇Al₁₂) eutectic temperature in the binary Mg–Al equilibrium phase diagram.

In this paper, the weldability of AA 5083-H111 (non-heat treatable) and AA 6082-T6 (heat treatable) aluminium alloys, which are widely used in welding fabrication, is compared by analysing the welds obtained from both materials under a large range of welding conditions (varying tool dimensions, rotation and traverse speeds, axial loads and tilt angles) chosen to ensure high welding speeds. The differences in friction stir weldability, assessed by weld defect analysis and weld strength characterisation, will be related to the markedly different plastic behaviours of both base materials. Based on the experimental results, a methodology for determining suitable friction stir welding parameters is proposed.

The present study investigates the effect of joining parameters on the microstructural and mechanical characteristics of dissimilar friction stir spot welding (FSSW) between AA 1050 Al and 22MnB5 hot stamped boron steel. Mechanical performance has been evaluated by shear and microhardness testing. Optical microscopy has been used to investigate the microstructure generated in the different FSSW regions. A macrostructural examination has revealed the creation of mechanical interlocking in the Al steel connections. No volumetric defects or any other imperfection has been found in all FSSW connections. Shear failure load has increased with increasing both tool rotational speed and plunge depth for all FSSW connections. Higher plunge depth has improved the mechanical interlocking between lower and upper sheet due to the formation of a larger secondary flash. Encouraging results have been obtained using coated WC–Co tools in terms of durability and joint performance.

This paper investigates the fracture and damage of a single lap friction stir spot welding assembly formed from thin sheets of aluminium alloy 6082 T6. For fixed process parameters and tool geometry, two configurations are taken into account for the analysis of the global mechanical behaviour of the link. An experimental approach was carried out in order to analyse the sequence of damage mechanisms using acoustic emission and measurement of fields by digital image correlation techniques simultaneously. The acoustic emission technique allows the monitoring of the evolution of acoustic activities by taking into account energy of the events. The digital image correlation technique confirms the damage scenarios after the treatment of strain field at any point near the fastener and especially between the exit hole and the shoulder footprint. The coupling of those two techniques allows identifying characteristic points and a breakdown of the load displacement curve in phases.

The effect of joint line remnant (JLR) on the fatigue lifetime of friction stir welds of a 2198 Al alloy in T851 condition has been assessed experimentally. The base material, sound welds (welded in one sheet) and welds with JLR (produced via welding of two sheets with a natural oxide layer) have been investigated. A strong decrease in microhardness is found for the weak weld zone that is consistent with the reduction in tensile properties compared to the base material: 45% in yield strength and 22% in ultimate tensile strength. The fatigue strengths of sound and JLR bearing welds at 100 000 cycles (R=0·1) are reduced by 10 and 15% respectively compared to the base material. Only for one-third of the JLR bearing specimens, the fatigue crack initiated in regions close to the JLR and this without reduction in fatigue lifetime.

The friction stir welding (FSW) process is capable of joining all aluminium alloys, especially 2xxx and 7xxx aerospace alloys, with good joint properties. Successful weld quality was obtained for a dissimilar joint 2050-T3 Al–Cu–Li alloy/7449-TAF Al–Mg–Zn–Cu alloy at 20 mm thickness thanks to pulsation of the advancing speed. This FSW dissimilar joint was post-weld heat treated: 2050-T8/7449-T79. The joint ultimate tensile strength (UTS) was 336 MPa, similar to a 7449 bead on plate with the same thickness. Moreover, the fracture location takes place in the 7449 heat affected zone (HAZ), the weakest point indicated by the microhardness profile. The ‘pulsed’ process induces an improved stirring based on alternation of dark and light bands, whose chemical composition is close to 2050 and 7449 respectively.

Friction stir welding (FSW) is a low distortion, high quality solid welding. There is no melting during the welding process, which results in improved welding quality. Ductile iron has the advantages of being low cost, of excellent castability, and of being good mechanically. Therefore, it is generally used in many structural engineering parts. In this study, ferritic ductile iron and low carbon steel were used to explore the qualities of dissimilar metal welding under different conditions. The FSW process, changes in the microstructure of the welding area and the mechanical properties of joints were explored. According to the research, we found that when dissimilar metal welding is conducted at 982 rev min^–1 with a travelling speed of 72 mm min^–1, flawless welding quality can be obtained if the stir rod rotates counterclockwise with carbon steel fixed in the advancing side and with ductile iron in the retreating side. FSW successfully provided defect free welds. However, fine pearlite and martensite structures appear in the stir zone, which result in mechanical property degradation of weldments. The stir zone in the weldments is very hard due to martensitic transformation. After heat treatment, the tensile strength improves, and the fracture site appears in the base metal of the carbon steel. However, the welding nugget is not completely filled when the stir rod directs ferritic ductile iron to the advancing side in the clockwise direction and carbon steel in the retreating side, which results in defects and lower welding quality.