Shixiong Lv

New technique of filling friction stir welding

Abstract A new technique of filling friction stir welding (FFSW) relying on a semiconsumable joining tool has been developed to repair the keyhole left at the end of friction stir welding (FSW) seam. The conventional non-consumable tool of FSW was transformed, and a semiconsumable joining tool consisting of alloy steel shoulder and aluminium alloy joining bit was designed to create a solid state joint. Using the combined plastic deformation and flow of the consumable joining bit and the wall of the keyhole, the FFSW process is able to repair the keyhole with both metallurgical and mechanical bonding characteristics, and the FSW seam can be achieved without keyhole or other defects. The relative tensile strength and elongation of the FFSW joint are 84·3 and 98·9% of the base weld without defects respectively.

Interface behaviours and mechanical properties of filling friction stir weld joining AA 2219

Abstract In the present work, 7·8 mm thick AA 2219 rolled plates were successfully joined without keyholes using semiconsumable tools by filling friction stir welding (FFSW). The shoulder further effect was performed to enhance mechanical stir, and mechanical properties were enhanced effectively. The results showed that the bonding interface of AA 2219 bit and keyhole is defect free. The softened region on the advanced side is the weakest part of FFSW joint rather than the bonding interface of the keyhole. The average ultimate tensile strength and elongation are 172 MPa and 11·2%, equal to 90 and 82% of the base weld without defects respectively. Excellent bonding interface and mechanical properties of FFSW joints have been exhibited.

Investigation on TIG arc welding-brazing of Ti/Al dissimilar alloys with Al based fillers

Abstract Dissimilar alloys of Ti–6Al–4V and 5A06 Al were butt joined by Al based fillers using a novel TIG welding process, referred to as keyhole arc welding–brazing. The flow behaviour of weld pool was introduced, which was characterised by the formation of a keyhole under the tungsten electrode. It was found that porosity tended to be produced in the middle of the fusion line, while adding elements prevented its formation. At brazing interface, interfacial reaction at root face was enhanced, and a uniform serrated layer, identified as TiAl3, was obtained by pure Al fillers. When Al–Cu–La fillers were used, block Ti2Al20La phases appeared at the interface between the TiAl3 layer and the brazed seam. Compared to joints brazed by pure Al fillers, the formation of Ti2Al20La reduced the hardness of the interfacial layer by more than half, while Al2Cu increased that of the brazed seam by ∼50%. The tensile strength of Ti/Al joints reached 270 MPa.

Investigation on TIG cladding of copper alloy on steel plate

Abstract Tungsten inert gas cladding of steel substrate with copper alloy under different processing conditions was investigated with the particular emphasis on the influence of cladding current on the concentration and morphology of Fe solute in the cladding layer and the hardness of the Cu/Fe interface. The application of hot wire was helpful to some extent in improving the spreadability of the filler metal but tended to cause intensive transfer of Fe into the copper layer when the current was higher than 240 A. The increase in Fe concentration in the copper layer followed an exponential relation with the growth of welding current. The morphology of Fe in the copper layer depended on its concentration. The hardness of copper layer increased with the welding current and intensively depended on its Fe content. At a welding rate of 3000 mm min–1, the optimum cladding currents were 210–270 A for the use of cold wire and 210–240 A for the use of hot wire.

Novel design of tool for joining hollow extrusion by friction stir welding

Abstract In the present work, we design a novel tool called self-support FSW (SSFSW) with adjustable dip angle to join the aluminium hollow extrusion. This tool consists of a big concave upper shoulder and a small convex lower shoulder. Compared with conventional FSW, it eliminates root flaws and does not need backing bars during operation. In particular, the SSFSW can adapt to variation of thickness and does not need predrilled pilot holes, resulting in simplifying the welding process compared with the bobbin tool FSW tool. Significant changes are observed at the cross-section of the SSFSW joint, which looks like a ‘waist’. The average tensile strength and elongation of 6005A aluminium alloy joint with a thickness of 5 mm is 190 MPa and 6·86% respectively. Furthermore, the failure of specimen presents a ductile fracture in the typical joint.

Effect of hot dip aluminising on interfacial microstructure and mechanical properties of Ti/Al joint by TIG arc welding brazing

Abstract The joint of Al 5A06 and aluminised Ti–6Al–4V dissimilar alloys was achieved by means of tungsten inert gas arc welding brazing. The effect of aluminized coating on the spreading behaviour of filler metal on Ti substrate was studied. The spreadability of liquid filler metal on the Ti substrate was enhanced obviously due to the presence of aluminised coating. The interfacial reaction layer was characterised by a uniform lamellar layer of TiAl3 intermetallic, with a thickness of 1 μm. Sound joints with well appearance were obtained, and the optimised tensile strength of the joint reached 216 MPa. The failure initiated from the interfacial layer at the root face and then propagated within the weld seam at the upper part of the joint. Capable welding parameters were broadened by the presence of aluminised coating for dissimilar metal joining of Ti/Al.

Arc heating hot wire assisted arc welding technique for low resistance welding wire

Abstract An assistant arc was used to preheat the wire for hot wire tungsten inert gas (TIG) welding which was independent on the resistance of the welding wires and avoided the drawbacks of the traditional hot wire preheating method. The new method is suitable for low resistance wire such as copper and aluminium. The experimental results showed that the wire temperature varied linearly with preheating current and hyperbolically with wire feeding speed. The temperature of wires achieved 60% of their melting points when LF6, H90, HS201 and H08Mn2Si welding wires were used at a current less than 50 A. With arc assisted hot wire, the welding deposition efficiency of the HS201 wire was increased by 96% compared with the traditional TIG welding method, while its microstructure was similar to that of the cold wire welding.

New technique of in situ rolling friction stir welding

Abstract A new technique of in situ rolling friction stir welding (IRFSW) has been developed to reduce the residual stress and distortion. It can eliminate the weld flashes and improve corrosion resistance of the FSW seam. A new kind of FSW tool consisting of rolling balls was designed to achieve IRFSW. The residual stresses across the weld were measured by ultrasonic stress measurement experimental installation. The 3·5%NaCl solution was used to corrode the surface of welded joint, and corrosion current and potential were determined by an electrochemical analyser. The results shown that the residual tensile stress was reduced, the weld flashes were eliminated, and the corrosion resistance was improved. By applying IRFSW, the reduction of distortion was ∼34·4% compared with that of conventional FSW joint for aluminium plant with 3 mm in thickness and 300 mm in length.

Friction stir welding of aluminium hollow extrusion: weld formation and mechanical properties

Abstract Novel friction stir welding (FSW) technique, characterised by big concave upper and small convex lower shoulders, for aluminium hollow extrusion was studied. Assisted with the lower shoulder, root flaws due to the lack of tool penetration have been eliminated. The tensile strength increased with increasing welding speed. As the welding speed increases from 50 to 200 mm min−1, the width of the welding nugget zone (WNZ) decreases, and the ductile fractured location occurred at WNZ instead of heat affected zone (HAZ) adjacent to thermomechanically affected zone (TMAZ). The interface between the TMAZ and HAZ exhibited the lowest microhardness. The results indicated that the novel FSW method has the potential to join tubular structures and hollow profiles widely used in transportation industries.

Microstructure and microhardness of aluminium alloy friction stir welds with heat treatment

In this study, the Al–Mg–Si aluminium alloy with a T6 temper is self-support friction stir welded, and the effect of post-weld heat treatment on joints is investigated by detailed microstructural investigations and microhardness measurements. The hardness decreases from average 59 to 52 HV with holding time ranging from 10 to 60 minutes at temperature 723 K. With increasing holding temperature, the microhardness becomes more uniform and increases. Microhardness profiles are explained by precipitate distributions and precipitation sequences. The fine grain structure undergoes abnormal grain growth, and the degree of which depends on holding temperature and intense plastic deformation, and the biggest size is more than 1 mm.