Microstructure and mechanical properties of dissimilar Al alloy/steel joints prepared by a flat spot friction stir welding technique

The 6061-T6 Al alloy and mild steel plate with a thickness of 1 mm were successfully welded by the flat spot friction stir welding technique, which contains two steps during the entire welding process. The rotating tools with different probe lengths of 1.0, 1.3 and 1.5 mm were used in the first step, during which a conventional spot FSW was conducted above a round dent previously made on the back plate. However, sound Al/Fe welds with similar microstructure and mechanical properties can still be obtained after the second step, during which a probe-less rotating tool was used to flatten the weld surface. The sound welds have smooth surface without keyholes and other internal welding defects. No intermetallic compound layer but some areas with amorphous atomic configuration was formed along the Al/Fe joint interface due to the lower heat input. The shear tensile failure load can reach a maximum value of 3607 N and fracture through plug mode. The probe length has little effect on the weld properties, which indicates that the tool life can be significantly extended by this new spot welding technique.     

Effect of Ti on hot cracking and mechanical performance in the gas tungsten arc welds of copper thick plates

The new welding material – ERCuTi alloys filler metals were developed for gas tungsten arc welding (GTAW) of copper. The cracking susceptibility of the welds with ERCuTi and ERCu separately in GTAW of 10 mm copper thick plates was investigated. The formation causes of hot cracking was researched by using ERCu and the suppression mechanism of hot cracking when using ERCuTi alloy filler was proposed. It has been found that, when element Ti is added into the welding pool, the Ti will combine with O preferentially rather than Cu to generate TiO₂, which process can suppress the formation of Cu₂O. The hot cracking force and the hot ductility of the welds in brittle temperature range (BTR) could be improved effectively by adding Ti in filler metal compared with that of the welds without Ti. But the degree of addition of Ti (2–4 wt%) is critical when the susceptibility of cracking is to be suppressed. If the level is allowed to exceed 4 wt%, more low-melting point eutectics (β-TiCu₄ and TiCu₂) will be formed in the welds, and cracking susceptibility will be increased again. Results of mechanical properties tests show that although adding Ti increases the hardness and strength of the weld compared to the base metal, the impact ductility and the plastic properties are not decreased significantly.     

High power fiber laser arc hybrid welding of AZ31B magnesium alloy

High power fiber laser–metal inert gas arc hybrid welding of AZ31B magnesium alloy was studied. The fusion zone consisted of hexagonal dendrites, where the secondary particle of Al₈Mn₅ was found at the center of dendrite as a nucleus. Within hybrid weld, the arc zone had coarser grain size and wider partial melted zone compared with the laser zone. The tensile results showed the maximum strength efficiency of 5 mm thick welds was up to 109%, while that of 8 mm thick welds was only 88%. The fracture surface represented a ductile–brittle mixed pattern characterized by dimples and quasi-cleavages. On the fracture surface some metallurgical defects of porosity and MgO inclusions around with secondary cracks were observed. Meanwhile, a strong link between the joint strength and weld porosity were demonstrated by experimental results, whose relevant mechanism was discussed by the laser–arc interaction during hybrid welding.     

Effect of welding parameters on microstructure and mechanical properties of friction stir spot welded 5052 aluminum alloy

Friction stir spot welding (FSSW) is a newly-developed solid state joining technology. In this study, two types of FSSW, normal FSSW and walking FSSW, are applied to join the 5052-H112 aluminum alloy sheets with 1 mm thickness and then the effect of the rotational speed and dwell time on microstructure and mechanical properties is discussed. The lower sheet material underneath the hook didn’t flow into the upper sheet due to the concave surface in the shoulder and groove in the anvil. The hardness profile of the welds exhibited a W-shaped appearance and the minimum hardness was measured in the HAZ. The results of tensile/shear tests and cross-tension tests indicate that the joint strength decreases with increasing rotational speed, while it’s not affected significantly by dwell time. At the rotational speed of 1541 rpm, the tensile/shear strength and cross-tension strength reached the maximum of 2847.7 N and 902.1 N corresponding to the dwell time of 5 s and 15 s. Two different fracture modes were observed under both tensile/shear and cross-tension loadings: shear fracture and tensile/shear mixed fracture under tensile/shear loadings, and nugget debonding and pull-out under cross-tension loadings. The performance of the welds plays a predominant role in determining the type of fracture modes. In addition, the adoption of walking FSSW brings unremarkable improvements in weld strength.     

Hybrid laser/arc welding of advanced high strength steel to aluminum alloy by using structural transition insert

The present investigation is related to the development of the welding procedure of the hybrid laser/arc welding (HLAW) in joining thick dissimilar materials. The HLAW was applied to join aluminum alloy (AA6061) to an advanced high strength steel (AHSS) where an explosively welded transition joint, TRICLAD®, was used as an intermediate structural insert between the thick plates of the aluminum alloy and AHSS. The welds were characterized by an optical microscope, scanning electron microscope (SEM), tensile test, charged coupled device (CCD) camera, and microhardness measurement. The groove angle was optimized for the welding process based on the allowed amount of heat input along the TRICLAD® interface generated by an explosive welding. The weld was fractured in the heat affected zone of the aluminum side in the tensile test. The microhardness was shown that the temperature variation caused minor softening in the heat affected zone satisfying the requirement that the width of the softened heat affected zone in the steel side falls within 15.9 mm far away from the weld centerline. The microstructure analysis showed the presence of tempered martensite at the vicinity of the weld area, which it was a cause of softening in the heat affected zone.     

Study on arc and laser powers in the hybrid welding of AA5754 Al-alloy

In this paper a new generation of fiber laser assisted by a MIG source was used to weld AA5754-H111 aluminum alloy in 3 mm thick butt configuration. The effects of laser and arc powers on the weld geometry and properties were studied. Weld geometry and porosity were measured. The microstructure was investigated by optical microscope and Vickers micro-hardness was taken. The residual stress close to the heat affected zone was measured by the incremental hole-drilling method. Eventually, the tensile test was conducted in order to compare the mechanical properties of the weld with those of the parent metal.For the first time the sensitiveness of the hybrid welding of the 5754 aluminum alloy to the arc and laser powers was demonstrated. Higher laser power favored the stability of the process and provided good structural and geometrical properties of the weld. Further investigation can be performed in order to optimize the weld soundness and the energy efficiency of hybrid welding an aluminum alloy using a fiber laser.     

Ultrasonic spot welding of Al/Mg/Al tri-layered clad sheets

Solid-state ultrasonic spot welding (USW) was used to join Al/Mg/Al tri-layered clad sheets, aiming at exploring weldability and identifying failure mode in relation to the welding energy. It was observed that the application of a low welding energy of 100 J was able to achieve the optimal welding condition during USW at a very short welding time of 0.1 s for the tri-layered clad sheets. The optimal lap shear failure load obtained was equivalent to that of the as-received Al/Mg/Al tri-layered clad sheets. With increasing welding energy, the lap shear failure load initially increased and then decreased after reaching a maximum value. At a welding energy of 25 J, failure occurred in the mode of interfacial failure along the center Al/Al weld interface due to insufficient bonding. At a welding energy of 50 J, 75 J and 100 J, failure was also characterized by the interfacial failure mode, but it occurred along the Al/Mg clad interface rather than the center Al/Al weld interface, suggesting stronger bonding of the Al/Al weld interface than that of the Al/Mg clad interface. The overall weld strength of the Al/Mg/Al tri-layered clad sheets was thus governed by the Al/Mg clad interface strength. At a welding energy of 125 J and 150 J, thinning of weld nugget and extensive deformation at the edge of welding tip caused failure at the edge of nugget region, leading to a lower lap shear failure load.     

Experimental investigations on welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steel

Tungsten Inert Gas (TIG) welding is considered as one of the cleanest welding methods. It is generally adopted for thinner materials with moderate weld joint strengths. Welding of sintered porous materials continues to be a challenge due to the inherent porosity of the parent metals. The present research work attempts to address some of the issues relating to the welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steels under TIG welding. Rectangular strips of size 70 mm × 15 mm × 5 mm, obtained by blending, compacting and sintering of elemental powders of iron, graphite and molybdenum, were upset forged – both hot and cold in order to obtain alloy steel strips of various porosities. Two identical alloy steel strips of equal density were then welded both along longitudinal and transverse directions, by TIG welding, employing filler metal of suitable composition. The welded strips were then subjected to tensile test, hardness test, microstructural and Scanning Electron Microscope (SEM) fractography studies. Cold/hot upsetting of the sintered alloy preforms has led to enhanced density. As a result of improved density, their tensile strength and hardness values were also found to be enhanced. The welded alloy exhibited higher tensile strength compared to the un-welded base metal, due to strengthening by residual stress. Similarly, the strength and hardness of the welded alloy strips were found to be enhanced with increase in density. The tensile strength of welded joint is found to be higher compared to that of the base metal due to alloy metals segregation, rapid cooling and formation of acicular ferrite at the weldment of welded joint. No porosity was observed in the weld metal or Heat Affected Zone (HAZ) of the weld joint. However, the base metal had numerous micro pores, though pore migration towards weldment has not been observed.     

EBSD study on microstructure and texture of friction stir welded AA5052-O and AA6061-T6 dissimilar joint

Friction stir welded AA5052-O and AA6061-T6 dissimilar joint has a more obvious impact on microstructure and texture evolution compared to single material welding due to differences in physical and chemical parameters between two aluminum alloys. Microstructure, texture evolution and grain structure of AA5052-O and AA6061-T6 dissimilar joint were investigated by means of OM,EBSD and TEM measurements. Experimental results showed that FS weld was generalized in four regions–nugget zone (NZ),thermomechanically affected zone (TMAZ),heat affected zone (HAZ) and base metals (BM), using standard nomenclatures. NZ exhibited the complex structure of the two materials with flowing shape and mainly composed of the advancing side material Subgrain boundaries in weld nugget zone gradually transformed into high angle grain boundaries by absorbing dislocation and accumulating misorientations. Grain refinement of weld nugget zone was achieved by dynamic recrystallization. In the friction stir welding process, the presence of the shear deformation in weld made {001} < 100 > C cube texture, {123} < 634 > S texture in BM gradually transformed into {111} < 1(−)12(−) > A1¹ shear texture. HABs distribution were most significant in nugget followed by RS and then by AS. In TMAZ and NZ, numerous precipitates and lots of dislocations were observed.     

Investigation of laser welding on butt joints of Al/steel dissimilar materials

Dissimilar metals of AA6013 aluminum alloy and Q235 low-carbon steel of 2.5 mm thickness were butt joined using a 10 kW fiber laser welding system with ER4043 filler metal. The study indicates that it is feasible to join aluminum alloy to steel by butt joints when zinc layer was hot-dip galvanized at the steel’s groove face in advance, and better weld appearance can be obtained at appropriate welding parameters. The joints had dual characteristics of a welding joint on the aluminum side and a brazing joint on the steel side. The smooth Fe₂Al₅ layer adjacent to the steel matrix and the serrated-shape FeAl₃ layer close to the weld metal were formed at the brazing interface. The overall thickness of Fe–Al intermetallic compounds layers produced in this experiment were varied from 1.8 μm to 6.2 μm at various welding parameters with laser power of 2.85–3.05 kW and wire feed speed of 5–7 m/min. The Al/steel butt joints were failed at the brazing interface during the tensile test and reached the maximum tensile strength of 120 MPa.     

Microstructure and mechanical properties of mild steel joints prepared by a flat friction stir spot welding technique

With the successful application of the flat spot friction stir welding technology to aluminum alloys, this technique was expanded to the spot lap welding of 1 mm thick mild steel in this study. It reveals that sound joints can be successfully obtained with smooth surfaces and without any internal welding defects. Two welding strategies based on the welding parameter can be used to obtain the welds that fracture through plug failure mode at high shear tensile strength. One way is to weld the sheet at low heat input in the first step and the second step is used to generate large stir zone and flatten the sample surface. However, the microstructure in the stir zone is not homogeneous and a coarse columnar grain structure forms at the bottom of the stir zone. Another way is to make the stir zone penetrate into the lower sheet during the first step and the second step is only aimed to flatten the sample surface. In this case, the total heat input can be reduced and the microstructure of the stir zone can be remarkably refined. The sound joints fractured along the circumstance of the stir zone and reached about 6600 N during the shear tensile tests.     

A Novel Method for Resistance Plug Welding of 7075 Aluminum Alloy

This article investigates a novel resistance welding method – resistance plug welding – to weld 7075 aluminum alloy plates. Lap joints were made of two 1-mm thick 7075 aluminum alloy plates with circular holes punched in the center of the lap zone. A filler rod made of 5052 aluminum alloy, which has better resistance spot weldability, was inserted into the hole. The weld formation, microstructure, and mechanical properties of the joints were analyzed and compared with traditional resistance spot welding joints. The results showed that the current density was more concentrated during resistance plug welding (RPW), which led to larger nugget diameters and higher peak loads and energy absorption in a RPW joint. Defects such as hot cracking and pores, which form easily in 7075 RSW joints, were efficiently avoided in 7075 RPW joints because of the superior weldability of the 5052 aluminum alloy. Partial-thickness, pullout, and ductile fractures occurred in RPW joints, whereas interfacial fracture features and brittle fractures were observed in RSW joints.     

Fatigue lifetime and tearing resistance of AA2198 Al–Cu–Li alloy friction stir welds: Effect of defects

The fatigue strength and failure mechanisms of defect-free (“sound”) and flaw bearing friction stir butt-welds of 3.1 mm-thick AA2198-T8 Al–Li–Cu alloy have been investigated via S–N curves at R = 0.1 using cross weld specimens. The fatigue strength of sound welds is only reduced by 10–15% at the aimed lifetime of 10⁵ cycles compared to the base material. Joint Line Remnant (JLR) bearing welds have a similar fatigue strength as sound welds and the JLR is not the crack initiation site. Kissing Bond (KB) bearing welds that have undergone a weld root polishing show a reduction in fatigue strength by 17% compared to sound welds. For specimens loaded at or above yield strength of the weld nugget the crack systematically initiates from the KB during the first cycle, which is interpreted further using fracture mechanics. The strongest reduction, about 28% in fatigue strength, is found for welds with an initial gap between the parent sheets (GAP welds) along with initiation at intergranular surface microcracks. Kahn tear tests show a reduction in tearing resistance for the flaw bearing welds with a similar ranking as for the fatigue strength.     

The effect of post-weld heat treatment on the notched tensile fracture of Ti–6Al–4V to Ti–6Al–6V–2Sn dissimilar laser welds

Dissimilar welding of the Ti–6Al–4V (Ti-6-4) to Ti–6A1–6V–2Sn (Ti-6-6-2) alloys was performed by CO₂ laser in this work. The effect of post-weld heat treatment (PWHT) on the notched tensile strength (NTS) of the dissimilar weld was evaluated. Moreover, the results were also compared with the homogeneous laser welds with the same PWHT. Similar to the Ti-6-4 welds, the NTS of the FZ for dissimilar welds was less sensitive to PWHT conditions; the NTS of the FZ for distinct dissimilar welds fell within the range of 1060–1180 MPa. The results indicated a minor rise in the Mo equivalent of the titanium alloy promoted the formation of fine α + β microstructures in the form of basket weave in the welds, which resulted in high hardness accompanied with low NTS of the welds.     

Development of a process envelope for friction stir welding of DH36 steel – A step change

Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. However, the fundamental knowledge of the friction stir welding process in relation to steel remains relatively limited. A microstructure and property evaluation of friction stir welded low alloy steel grade DH36 plate, commonly used in ship and marine applications has been undertaken. In this comprehensive study, plates of 2000 × 200 × 6 mm were butt welded together at varying rotational and traverse speeds. Samples were examined microscopically and by transverse tensile tests. In addition, the work was complemented by Charpy impact testing and micro-hardness testing in various regions of the weld. The study examined a wide range of process parameters; from this, a preliminary process parameter envelope has been developed and initial process parameter sets established that produce commercially attractive excellent quality welds through a substantial increase in the conventionally recognised weld traverse speed.     

Research on fatigue behavior and residual stress of large-scale cruciform welding joint with groove

Fatigue fracture behavior of the 30 mm thick Q460C-Z steel cruciform welded joint with groove was investigated. The fatigue test results indicated that fatigue strength of 30 mm thick Q460C-Z steel cruciform welded joint with groove can reach fatigue level of 80 MPa (FAT80). Fatigue crack source of the failure specimen initiated from weld toe. Meanwhile, the microcrack was also found in the fusion zones of the fatigue failure specimen, which was caused by weld quality and weld metal integrity resulting from the multi-pass welds. Two-dimensional map of the longitudinal residual stress of 30 mm thick Q460C-Z steel cruciform welded joint with groove was obtained by using the contour method. The stress nephogram of Two-dimensional map indicated that longitudinal residual stress in the welding center is the largest.     

Direct joining of carbon-fiber–reinforced plastic to an aluminum alloy using friction lap joining

A carbon-fiber–reinforced thermoplastic (polyamide 6 with 20 wt.% carbon fiber addition) and an aluminum alloy (A5052) were joined using friction lap joining. The joint characteristics were evaluated to investigate the effects of A5052 surface treatments and the joining speed on the joint properties. Carbon-fiber–reinforced thermoplastic and A5052 were joined via an interfacial magnesium oxide layer. Surface grinding of the A5052 generated the aluminum hydroxide on the alloy surface and increased the tensile shear strength of the joint. The tensile shear strength increased as the joining speed increased from 100 to 1600 mm min⁻¹, and decreased thereafter.     

An experimental investigation of fatigue behavior and deformation of double spot friction welded joints

Fatigue behavior of double spot friction welded joints in aluminum alloy 7075-T6 plates is investigated by conducting monotonic tensile and fatigue tests. The spot friction welding procedures are carried out by a milling machine with a designed fixture at the best preliminary welding parameter set. The fatigue tests are performed in a constant amplitude load control servo-hydraulic fatigue testing machine with a load ratio of (R = P_min/P_max) 0.1 at room temperature. It is observed that the failure mode in cyclic loading (low-cycle and high-cycle) resembles that of the quasi-static loading conditions i.e. pure shearing. Primary fatigue crack is initiated in the vicinity of the original notch tip and then propagated along the circumference of the weld’s nugget.     

Effects of CNTs on microstructure and hardness of laser welds of the CNT-reinforced magnesium composite

A magnesium composite reinforced with 1.3 wt.% multiwall carbon nanotubes (MWCNTs) was welded using a CO₂ laser. Abundant nanoscale and submicron carbon particles formed in the laser welds, owing to the coalescence of the reinforcing CNTs as the result of conjoint effect of laser irradiation and welding thermal annealing. Dense intra- and inter-granular carbon particles and carbon particle-enveloped equiaxed dendrites in the magnesium matrix constituted the characteristic weld microstructure. Lateral-irregular cellular growth originated from the weld fusion boundary and the carbon particle-enveloped equiaxed dendrites with substructure formed in the weld center. It is inferred that the CNTs affected the weld solidification mainly via retarding solidification growth rate. The gained weld hardening is primarily attributed to the local refined weld microstructure and locally denser carbon particles and MWCNTs. Lower laser fluence facilitated finer weld structure and less laser irradiation on the CNTs hence more hardening effect.     

Effects of microstructural heterogeneity on very high cycle fatigue properties of 7050-T7451 aluminum alloy friction stir butt welds

In this study, the very high cycle fatigue (VHCF) properties of 7050-T7451 aluminum alloy and its friction stir welding (FSW) butt welds have been investigated. The results show that the failure of FSW joints still occurs at 7.0 × 10⁸ cycles. The fatigue properties of the FSW joints are superior to those of the base material, especially in the super long life regime. Most fatigue cracks initiate at the thermo-mechanically affected zone and heat affected zone on the advancing side of the FSW joints, and the susceptibility of these zones to fatigue is attributed to the metallurgical heterogeneity.