Interface properties and thermodynamic analysis of laser–arc hybrid welded Al/steel joint

Abstract

Fibre laser–cold metal transfer hybrid welding was introduced to join AA 6061 aluminium alloy with AISI 304 stainless steel using Al–12Si filler wire. Interface properties and microstructure of welded joints were observed by optical microscope, scanning electron microscope, energy dispersive spectrometry and X-ray diffraction techniques. A serrated intermetallic compound (IMC) layer was found at the interface between fusion zone and stainless steel. The morphology of IMC layer was uniform from the top to the bottom, and its average thickness was 3 μm. The IMC layer consisted of two layers: Al₈(Fe,Cr)₂Si layer close to fusion zone and (Al,Si)₁₃Fe₄ layer close to stainless steel. The joint fractured at the IMC layer and presented a tensile strength of 165 MPa. The formation of the IMC layer was closely related with the thermodynamic and kinetic behaviours of the interface and fast cooling rate of hybrid welding.     

Microstructure evolution and mechanical properties of Cu–Al joints by ultrasonic welding

Dissimilar joints of copper to aluminium were produced by high power ultrasonic welding (USW). The interfacial reaction between copper and 6061 aluminium alloy as a function of welding time was studied. The intermetallic compound (IMC) layer is mainly composed of CuAl₂ and Cu₉Al₄. The thickness of the IMC layer increases with the welding time. For a relatively long welding time (0·7 s) in USW, the dendritic solidification microstructure was observed in local regions, owing to the occurrence of the eutectic reaction, α-Al+θ→L, in the welding process. The lap shear load (or strength) of the joints first increases and then decreases with increasing welding time, and the failure of the joints occurred dominantly at the interface. This is mainly attributed to the development of IMC layer at the interface.     

Effects of resistance spot welding parameters on microstructures and mechanical properties of dissimilar material joints of galvanised high strength steel and aluminium alloy

Abstract

Intermediate frequency resistance spot welding has been adopted to join dissimilar materials of H220YD galvanised high strength steel and 6008 aluminium alloy. The effects of welding current and welding time on microstructures and mechanical properties of the welded joints were investigated. A thin intermetallic compound layer composed of Fe₂Al₅ phase and Fe₄Al₁₃ phase formed at the steel/aluminium interface. The interfacial intermetallic compound layer has higher nanohardness compared with the aluminium alloy nugget and galvanised steel. With increasing welding current (4–11?kA) and welding time (50–300?ms), the nugget diameter increased, the interfacial layer structure became coarser and the tensile shear load of the welded joints had an increased tendency. The maximum tensile shear load reached 3309?N at 9?kA for 250?ms. Crack initiated at the interfacial intermetallic compound layer of the tensile shear specimens, then propagated through the interfacial layer principally, and meantime through the aluminium alloy fusion zone near the interface partially.     

Resistance spot welding of steel and aluminium sheet using insert metal sheet

Abstract

The present paper reports the resistance spot welding of steel and aluminium sheets using aluminium clad steel sheets as insert metals. Intermetallic compound layers were formed in the weld zones in direct spot welding of steel sheets to aluminium sheets. Thus, the strength of these joints was lower than that of aluminium to aluminium joints. Intermetallic compound layers were also formed at the steel/aluminium interfaces of the insert metal in welding of steel to aluminium using an insert metal sheet. However, the strength of these joints was of the same order as that of the aluminium joints. The fracture mode of these joints varied with the welding current. The suitable welding current for steel to aluminium joints varied between the values suitable for steel to steel and aluminium to aluminium joints. The fatigue strength of joints using insert metals was somewhat lower than that of the aluminium joints.     

Seal spot welding of steel and aluminium alloy by resistance spot welding: dissimilar metal joining of steel and aluminium alloy by Zn insertion

This research concerns a dissimilar metal joining of steel and aluminium (Al) alloys by means of zinc (Zn) insertion. The authors propose a joining concept for achieving strong bonded joints between Zn-coated steel and Al alloys. A eutectic reaction between Zn in the Zn coating and uniform Al–Fe intermetallic compound (IMC) layer at the joint interface, leading to a strong bonded joint. The ultimate aim of this research was to apply this joining concept in the resistance spot welding process for manufacturing vehicle bodies. As a practical issue characteristic to joints of dissimilar metals, anticorrosion measures against electrochemical corrosion must be undertaken. If there is moisture near a joint interface of dissimilar metals, electrochemical erosion will progress. Therefore, a sealing function that could prevent moisture intrusion is required. By applying the above-mentioned welding process to a set of metals with thermosetting resin spread in between, we realized seal spot welding, which not only prevented moisture intrusion but also retained high tensile strength. In this research, first, a cyclic corrosion test was performed on the seal spot-welded joint of galvanized (GI) steel, a steel grade widely distributed in Japan, and Al alloy was bonded by seal spot welding, and the following topics are discussed. Complete removal of sealant from the joint interface is the key to realizing the high tensile stress joint, because remaining sealant will lead to reduction in tensile strength. Therefore, heat generation at the interface was monitored by measuring electrical current and potential difference between the two electrodes, and a precise temperature control was performed. Moreover, the bonding process was clarified by stepwise analysis of the joint interface using optical microscopy, and a guideline for producing strong joints was proposed. And finally, a TEM observation also confirmed that the interface structure of the seal spot-welded joint was the same as joints without the resin; a thin and uniform Al–Fe IMC layer was formed and a strong metallurgical bonding was achieved.     

Mechanical properties of laser-pressure-welded joint between dissimilar galvannealed steel and pure aluminium

Dissimilar metal joints of Zn-coated Galvannealed steel (GA steel) and commercially available pure aluminium (A1050) sheets were produced by changing the laser power and the roller pressure by the laser pressure welding method. By this method, the YAG laser beam was irradiated into a flare groove made by these dissimilar metal sheets. In addition, the laser beam was scanned at various frequencies and patterns through the fθ lens using two-dimensional scanning mirrors. Then the sheets were pressed by the pressure rolls to be joined.

The compound layers in the weld interface were observed by an optical microscope and the layer thicknesses were measured. The thicknesses ranged from 7 to 20 μm. The mechanical properties of the welded joints were evaluated by the tensile-shear test and peel test. In the tensile-shear test, the strengths of the joints produced under the most welding conditions were so high that the fracture occurred through the base aluminium sheet. In the peel test of the specimens subjected to a laser beam of 1200–1400 W power under roller pressure of 2.94 kN, the specimen fracture took place in the base aluminium sheet. Even if the compound layer was thick, high joint strength was obtained. On the other hand, the specimen fractured in the weld interface at a laser power of 1500 W. The results of X-ray diffraction on the peel test specimen surface identified that the intermetallic compound on the GA steel side was Fe₂Al₅Zn_0.4. Moreover, the aluminium parts adhering to the GA steel side were confirmed. These results suggest that the fracture in the peel test occurred between the compound layer and A1050 and partly in the base aluminium. A micro-Vickers hardness test was performed to examine the hardness distribution in the compound layer. The hardness values near A1050 and GA steel were about 100 and 470 Hv, respectively, which suggests that the compound layer should not necessarily consist of brittle intermetallic compounds. It is therefore concluded that laser pressure welding could produce high strength joints of GA steel and A1050 dissimilar materials.     

Mechanisms of joining aluminium A6061-T6 and titanium Ti–6Al–4V alloys by cold metal transfer technology

Abstract

Cold metal transfer (CMT) welding–brazing joining of Ti6Al4V and Al A6061-T6 was carried out using AlSi₅ wire. The joining mechanisms and mechanical properties of the joints were identified and characterised by scanning electron microscope, energy dispersive spectroscopy and tensile–shear tests. Desired CMT joints with satisfied weld appearances and mechanical properties were achieved by overlapping Ti on the top of Al. The joints had dual characteristics of a welding joint on the aluminium side and a brazing joint on the titanium side. Three brazing interfaces were formed for the joint, which increased the strength of the joint. An intermetallic compound layer was formed at the brazing interface, which included Ti₃Al, TiAl and TiAl₃. Two different fracture modes were also observed: one fractured at the welding/brazing interface and weld metal and the other at the Al heat affected zone (HAZ). Clearly, the joints fractured at the Al HAZ had higher tensile strength than those fractured at the welding/brazing interface and weld metal.     

Microstructural Evolution and Mechanical Behavior of Friction-Stir-Welded DP1180 Advanced Ultrahigh Strength Steel

Friction stir lap welding of a DP1180 advanced ultrahigh strength steel was successfully carried out by using three welding tools with different pin lengths. The effects of the welding heat input and material flow on the microstructure evolution of the joints were analyzed in detail. The relationship between pin length and mechanical properties of lap joints was studied. The results showed that the peak temperatures of all joints exceeded A c3, and martensite phases with similar morphologies were formed in the stir zones. These martensite retained good toughness due to the self-tempering effect. The formation of ferrite and tempered martensite was the main reason for the hardness reduction in heat-affected zone. The mechanical properties of the lap joints were determined by loading mode, features of lap interface and the joint defects. When the stir pin was inserted into the lower sheet with a depth of 0.4 mm, the lap joint exhibited the maximum tensile strength of 12.4 kN.     

Analysis of process parameters effects on dissimilar friction stir welding of AA1100 and A441 AISI steel

Abstract

A prominent benefit of friction stir welding process is to join plates with dissimilar material. In this study, an attempt is made to find effects of tool offset, plunge depth, welding traverse speed and tool rotational speed on tensile strength, microhardness and material flow in dissimilar friction stir welding of AA1100 aluminium alloy and A441 AISI steel plates. Here, one factor at a time experimental design was utilised for conducting the experiments. Results indicated the strongest joint obtained at 1·3?mm tool offset and 0·2?mm plunge depth when the tool rotational speed and linear speed were 800?rev min^??1 and 63?mm min^??1 respectively. The maximum tensile strength of welded joints with mentioned optimal parameters was 90% aluminium base metal. Fracture locations in tensile test at all samples were in aluminium sides. Owing to the formation of intermetallic compounds at high tool rotational speed, the microhardness of joint interface goes beyond that of A441 AISI steel.     

Formation of interfacial microstructure in a friction stir welded lap joint between aluminium alloy and stainless steel

The interfacial microstructure produced through tool transit of a friction stir welded lap joint between an aluminium alloy and stainless steel was studied by transmission electron microscopy in order to clarify its early stages of formation. Transmission electron microscopy studies of the bottom surface of the exit hole revealed the presence of several mixed layers of an ultrafine intermetallic compound (IMC) and stainless steel. The joining between dissimilar materials was achieved through a continuous flow of the stirred aluminium alloy into the mixed layers and the resultant growth of the ultrafine IMCs due to the heat induced by the friction between the tool and the specimen. The continuous thin reaction layer finally produced at the interface was found to be stronger than the base aluminium alloy.     

Effects of pin diameter on microstructures and mechanical properties of friction stir spot welded AZ31B magnesium alloy joints

Abstract

The effects of pin diameter on the microstructure and mechanical properties of friction stir spot welded AZ31B magnesium alloy joints were investigated using microstructural observations, tensile tests and microhardness tests. The results showed that with an increase in the pin diameter, the height and width of the curved interface in the friction stir spot welded AZ31B magnesium alloy joints increased because of the strong effect of stirring and high temperature obtained when large sized pins were adopted. An increase in the pin diameter led to the coarsening of α-Mg grains in the stir zone, thermomechanical affected zone and heat affected zone because of heat generation, resulting in the decrease in microhardness of stir zone, thermomechanical affected zone and heat affected zone. The tensile shear force of the friction stir spot welded AZ31B magnesium alloy joints increased with the increase in pin diameter because the height and width of the curved interface dominated the failure of the specimens.     

Dissimilar metal joining of aluminium to zinc-coated steel by ultrasonic plus resistance spot welding – microstructure and mechanical properties

Vehicle body structures are increasingly utilising multi-materials designs with advanced high strength steels (AHSS) and aluminium alloys. A robust process for joining aluminium alloys to AHSS based on resistance spot welding (RSW) is essential to widespread application of such bi-metallic structures in fuel-efficient vehicles. In this study, ultrasonic plus RSW was applied to join AA6022 to Zn-coated dual-phase steel DP980. During solid-state ultrasonic spot welding, an interface structure comprising multilayer, Al–Zn and Zn–Fe intermetallics formed due to alloying of aluminium with steel coating. Such structure was subsequently melted into the aluminium nugget, and new Al–Fe intermetallics formed during RSW. Ultrasonic plus resistance spot-welded joints had superior fracture energy than direct resistance spot-welded joints.     

热循环对Sn58Bi(纳米Ti)/Cu焊点界面与性能影响

为了改善Sn-58Bi低温钎料的性能,通过在Sn-58Bi低温钎料中添加质量分数为0.1%的纳米Ti颗粒制备了Sn-58Bi-0.1Ti纳米增强复合钎料。在本文中,研究了纳米Ti颗粒的添加对-55～125 _^oC热循环过程中Sn-58Bi/Cu焊点的界面金属间化合物(IMC)生长行为的影响。研究结果表明：回流焊后,在Sn-58Bi/Cu焊点和Sn-58Bi-0.1Ti/Cu焊点的界面处都形成一层扇贝状的Cu₆Sn₅ IMC层。在热循环300次后,在Cu₆Sn₅/Cu界面处形成了一层Cu₃Sn IMC。Sn-58Bi/Cu焊点和Sn-58Bi-0.1Ti/Cu焊点的IMC层厚度均和热循环时间的平方根呈线性关系。但是,Sn-58Bi-0.1Ti/Cu焊点的IMC层厚度明显低于Sn-58B/Cu焊点,这表明纳米Ti颗粒的添加能有效抑制热循环过程中界面IMC的过度生长。另外计算了这两种焊点的IMC层扩散系数,结果发现Sn-58Bi-0.1Ti/Cu焊点的IMC层扩散系数(整体IMC、Cu₆Sn₅和Cu₃Sn IMC)明显比Sn-58Bi/Cu焊点小,这在一定程度上解释了Ti纳米颗粒对界面IMC层的抑制作用。     

Intermetallic compounds in friction stirred lap joints between AA5754/galvanised ultra-high strength steel

This paper presents results of joining of AA5754 and DP800 based on the friction stir welding process. Joints were produced by the tool made of H13 tool steel which was allowed to penetrate through the aluminium sheet until reaching the surface of steel sheet without penetrating into it. This approach is an economic and robust way to operate the dissimilar welding process without excessive tool cost. Bonding was achieved by interfacial diffusion reactions between aluminium and iron with a formation of intermetallic compounds. The formation of brittle intermetallic compounds at the interface between the materials was studied. Three intermetallic phases were found at the interface including Al₁₃Fe₄, Al₅Fe₂ and Fe₃Zn₁₀. A range of process parameters was identified with a thickness of the intermetallic layers around 2?µm. Shear fracture failure mode was observed under overlap loading. The mechanisms of formation of the joints and factors controlling the strength were discussed.     

Interface shape and microstructure controlled dissimilar friction stir lap welded steels

Abstract

When fusion welding is conducted on the dissimilar materials between a reduced activation ferritic/martensitic steel F82H steel and an austenite stainless steel SUS 316 steel, δ ferrite is generally formed and inevitably deteriorates the weld properties. In this study, dissimilar welding of F82H to SUS 316 steel was successfully achieved by friction stir lap welding technique. It revealed that the shape and microstructure of the joint interface can be controlled by controlling the welding temperature, in another word, by changing the applied load. By controlling the welding temperature at ～710°C, a sound dissimilar joint can be obtained with a smooth joint interface and no mixed microstructure, despite the relative overlapping position of the steel plates. All the dissimilar joints showed high shear tensile strength and fracture in the base metal of F82H steel plate, which has lower strength than the SUS 316 steel plate at room temperature.     

Flux modification for AC-TIG braze welding of aluminium to stainless steel

A modified flux consisting of KAlF₄ and aluminium powder was developed for AC-TIG braze welding of aluminium to stainless steel. By employing the flux, the arc instability and wetting angle was decreased significantly, and therefore a good joint shape was obtained. The wetting and spreading behaviour as well as the action mechanism of the flux were changed. Microstructure analysis suggested that the intermetallic compound (IMC) thickness increased obviously when the addition percentage of aluminium powder in the flux exceeded 40 wt-%. Results of tensile tests illustrated that a significant improvement in mechanical properties of the butt joint was obtained with the modified flux, and tensile strength increased from 125 to 170 MPa in maximum. Relationships between aluminium powder content in the flux and wetting angle, IMC thickness as well as tensile strength were established. Results indicated that the joint strength was affected both by weld shape and IMC thickness.     

Study on the factors in creating the IMC-free region: dissimilar metal joining of aluminium alloys to steel by a MIG-braze welding by using the advanced hot-dip aluminized steel sheet

Dissimilar metal joining of aluminium alloys to steel is generally difficult to be in practical use because of a formation of brittle intermetallic Fe–Al compound (IMC) at the interface of the joint. The authors have been researching in order to minimize the thickness of this brittle IMC in order to get excellent joint strength and have found that the formation of this brittle IMC is regionally prevented by using the advanced hot-dip aluminized steel sheet and by adopting suitable joining conditions. In particular, this paper focuses on the mechanism of creating this IMC-free region in the case of MIG-braze welding and the results obtained are as follows. (1) The creation of IMC-free region is initiated as the first process by the dissolution of the τ₅ phase (Fe–Al–Si) in the aluminized layer into the weld metal, and temperatures of more than 886 K for dissolution during MIG-braze welding and the use of filler metal for dilution of Fe and Si in τ₅ phase have significant effects. (2) In the second process, the diffusion between aluminium-alloy weld metal and base steel is restricted by AlN on the surface thin layer of the base steel which existed under 908 K temperature conditions during MIG-braze welding.     

Wettability,microstructure and properties of 6061 aluminum alloy/304 stainless steel butt joint achieved by laser-metal inert-gas hybrid welding-brazing

Laser-metal inert-gas (MIG) hybrid welding-brazing was applied to the butt joint of 6061-T6 aluminum alloy and 304 stainless steel. The microstructure and mechanical properties of the joint were studied. An excellent joint-section shape was achieved from good wettability on both sides of the stainless steel. Scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffractometry indicated an intermetallic compound (IMC) layer at the 6061-T6/304 interface. The IMC thickness was controlled to be ～2 μm, which was attributed to the advantage of the laser-MIG hybrid method. Fe₃Al dominated in the IMC layer at the interface between the stainless steel and the back reinforcement. The IMC layer in the remaining regions consisted mainly of Fe₄Al₁₃. A thinner IMC layer and better wettability on both sides of the stainless steel were obtained, because of the optimized energy distribution from a combination of a laser beam with a MIG arc. The average tensile strength of the joint with reinforcement using laser-MIG hybrid process was improved to be 174 MPa (60% of the 6061-T6 tensile strength), which was significantly higher than that of the joint by traditional MIG process.     

Diode laser brazing of aluminium alloy to steels with aluminium filler metal

Abstract

Diode laser brazing of aluminium alloy (A5052) to interstitial free steel (IF steel) or type 304 stainless steel (SUS304) was conducted using aluminium filler metal (BA4047) with Nocolock flux. The processing parameters of laser power, wire feed rate and travel speed were varied. The strength of lap joints of A5052 on steels was evaluated by tensile shear test. The joint strength of A5052/steels was increased with increasing laser power and reached the maximum strength, more than approximately 80% of the A5052 base metal strength, at a laser power of 1300 W. Voids and incomplete penetration of filler metal were observed at the A5052/braze layer interface when the laser power was below 1100 W. The Fe–Al intermetallic compounds were formed at the steel/braze layer interfaces and grew drastically when the laser power exceeded 1300 W. Superior brazability of A5052/steels was found at brazing conditions corresponding to a temperature of filler metal droplet of 1050–1250 K.     

Identification of the intermetallic compound layer formed at the interface of roll-bonded aluminum-clad steel by thermal annealing

Aluminum clad steel (ACS) is an excellent layered composite material. An intensive investigation into roll-bonded ACS was carried out to identify the intermetallic compound (IMC) formed at the interface between the aluminum (Al) and the steel. A series of analyzing methods was applied to the IMC layer formed at the interface during annealing at 540 °C for 16 h. An electron probe micro-analyzer (EPMA) measured a value of 28.5 atomic percent for the Fe in the IMC layer, which coincided with the Fe₂Al₅ phase. An analysis of the X-ray diffraction pattern for the IMC layer in the ACS materials showed a very strong peak for the (002) plane of Fe₂Al₅. Vickers microhardness testing of the IMC layer revealed a very high value (1,110 Hv). The results of EPMA, XRD and microhardness values verified that the IMC layer in the sample that had been annealed at 540 °C for 16 h was the Fe₂Al₅ phase.