Interfacial phenomena of plasma arc welding of mild steel and aluminium

The interfacial microstructures and intermetallic compounds produced by plasma arc butt fusion welding of aluminium to mild steel have been investigated. An intermetallic compound alloy layer formed at the interface region between mild steel and aluminium was determined using quantitative metallography and the mechanism of the intermetallic layer formation and growth was elucidated. The melt width and the alloy layer thickness decrease with increasing transfer-speed of the plasma torch. The intermetallic layers formed at the interface region between mild steel and aluminium are predominantly-phase (Fe₂Al₅) and-phase (FeAl₃). The-phase layer with columnar crystal grows rapidly as tongue-like structures in the direction of the mild steel substrate and the-phase layer with granular crystals projects slowly to the aluminium side in the course of solidification. As a result, many vacancies are produced in the-phase layer. The structures of the melted zone and the fusion boundary of the mild steel change into grain refinement, whereas the melted zone of the aluminium has a eutectic structure of aluminium and-phase.     

Evolution of microstructures of galvanised and galvannealed coatings formed in 0·2 wt-% aluminium–zinc bath

Abstract

The present investigation examines the evolution of the microstructures of galvanised steel during the galvannealing process with a special reference to the formation and breakdown behaviour of aluminium rich inhibition layer at the substrate/coating interface. The interstitial free steel was galvanised in a molten zinc bath with 0·2 wt-% aluminium content. The bath temperature and strip entry temperatures were 460 and 480°C respectively. Aluminium of 0·2 wt-% was chosen to retard the formation of iron–zinc intermetallic compounds in the coating. The as dipped galvanised interstitial free steel was annealed above the melting point of zinc in a salt bath at 480°C for a period ranging from 1 to 60 s. After annealing, the as galvanised coatings exhibited microstructural changes as a function of test time. The present work highlights the possible mechanisms of nucleation of iron–zinc intermetallic compounds during galvannealing treatment of steel galvanised in aluminium containing zinc bath.     

Friction welding process of 5052 aluminium alloy to 304 stainless steel

Abstract

Type 5052 aluminium alloy was joined to type 304 austenitic stainless steel via a continuous drive friction welding process. The joint strength increased, and then decreased after reaching a maximum value, with increasing friction time. Joint strength depended on the size and shape of the tensile testpiece. Friction weldability could be estimated by electrical resistmetry. The process of friction welding between the aluminium alloy and the stainless steel is proposed to evolve as follows: welding progresses from the outer to the inner region; an unbonded region is retained at the centre of the weld interface with shorter friction time; longer friction time causes the formation of an intermetallic reaction layer at the weld interface; and the reaction layer grows as the friction time increases. When the thickness of the reaction layer increased above a critical value, the joint was brittle and fractured at the weld interface. The joint was sound when there was no unbonded region and a thin reaction layer formed along the entire weld interface.     

Study of the Interface between Steel Insert and Aluminum Casting in EPC

The effective surface treatment method for steel insert composited with Al base metal by expendable pattern casting （EPC） process and the bonding interface between steel insert and Al base metal were investigated.It was found that Zn plating on steel insert was effective on improving the bonding property between steel insert and Al base metal in EPC process.Zn is thought to promote the formation of diffusion layer.But almost none content of Zn was observed in the boundary which had been plated on the steel insert.A diffusion layer consisting of Al,Si and Fe was formed at the insert/alloy interface and its hardness was higher than the steel insert as matter of course Al base metal.This layer turned out to be intermetallic compounds of Al-Si-Fe system.Higher pouring temperature promoted the diffusion of Fe into Al alloy,so Fe content in intermetallic layers increased at higher pouring temperature.The layer nearest to steel disappeared due to applied pressure.     

The characterisation of the interfacial chemistry of adhesion of rigid polyurethane foam to aluminium

The interfacial interactions between a rigid polyurethane foam (RPUF) and aluminium have been studied to understand the mechanisms of adhesion. Three different blowing systems are used in the production of the foam: chemical blowing, physical blowing and a mix of chemical and physical blowing systems. In addition an unfoamed system has been examined for comparison of the catalysts behaviour with and without blowing agents and the surfactant. Peeled failure surfaces have been examined by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF–SIMS). To examine the intact interfacial regions of the RPUFs cured against aluminium, samples have been sectioned by microtomy. The failure surfaces of the aluminium sides exhibit relatively clean aluminium surfaces with RPUF residues observed for all three foamed systems; such thin RPUF layers (ca. 1 nm) indicate good adhesion (and a cohesive failure) between foam and substrate and that the interfacial adhesion is higher than the cohesive strength of the foam. The unfoamed system behaves in a similar manner but has a higher peel strength. A fragment indicative of covalent bond formation between isocyanate and aluminium (nominal mass at 102 u: AlCHNO₃ ⁻) is observed on the failure surface of aluminium side, where RPUF/aluminium interface region is present, for all foams. The catalyst used in these formulations, pentamethyldiethylenetriamine (PMDETA), is concentrated at the interface area. Whilst examination of the sectioned specimens shows that the silicone surfactant is concentrated within the cell area fulfilling its role on cell formation and stabilisation, and is not segregated at the RPUF/aluminium interface.     

Analysis of coating layer formedon steel strips during aluminising by hot dipping in Al-Si baths

Abstract

Aluminising of low carbon (0.19 wt-%C) steel was carried out using AI-0, 4, 8, and 12 wt-%Si melts. Different dipping times and melt superheat were used. In all cases, a coating layer was formed which is composed of an intermetallic layer and an aluminium top coat layer. The thickness of the intermetallic layer increased with bath temperature, especially in pure aluminium baths, and decreased with increasing silicon content. Addition of more than 8 wt-%Si to the bath had no detectable effect on the thickness. This thickness X increased with time following the parabolic relationship X =KTⁿ, where the growth rate constant K decreased with silicon content. Energy dispersive X-ray analysis revealed that the intermetallic layer is composed of a thick layer of AI₅Fe₂ followed by a much thinner one of Al₃Fe on the aluminium side. In the case of a silicon containing bath, different Al_xFe_ySi_z compounds were identified. The kinetics of the reaction between solid steel and liquid aluminium were studied. In AI-Si baths, the growth rate of the intermetallic layer was lower and its dissolution rate higher compared with a pure aluminium bath. Separation of parts of the layer was also found. Iron loss from the steel strip, especially in the case of AI-Si baths, was partly used in the formation of both the measured and the dissolved layer and partly dissolved in the melt. The growth rate of the layerwas evaluated and the activation energy was found to be 138.46 and 106.65 kJ mol^-1, for pure aluminium and Al-8Si baths respectively.     

泡沫铝芯三明治板的粉末冶金制备及其板/芯界面研究

采用粉末冶金发泡法制备了Fe/Al/Fe、Ti/Al/Ti泡沫铝芯三明治结构,研究了泡沫铝芯的膨胀规律,分析了面板与泡沫铝芯的冶金结合过程,提出了微观结合机制.试验发现,结合界面由扩散反应形成的金属间化合物以及冷却得到的凝固组织两部分组成,从而形成良好的冶金结合.     

Interactions under hydrostatic pressure of a mild steel with liquid aluminium alloys

Interactions of a mild steel with liquid aluminium alloys have been studied in a pressure range of 0.1 to 350 MPa, and a temperature range from solidus to 750 °C, close to the squeeze casting process conditions. After having carried out the synthesis of previous surveys at atmospheric pressure, the influence of dipping parameters, and especially pressure, along with alloying elements on the intermetallic layer formation and on the mechanical strength of samples have been considered. These results must contribute to the improvement of the processing of steel reinforced aluminium matrix composites by squeeze casting process.     

Crushing of axially compressed steel tubes filled with aluminium foam

Summary This study, with the emphasis on experiments, investigates the applicability of aluminium foam as filler material in tubes made of mild steel having square or circular cross sections, which are crushed axially at low loading velocities. In addition to the experiments finite element studies are performed to simulate the crushing behaviour of the tested square tubes, were a crushable foam material model is shown to be suitable for describing the inelastic response of aluminium foam with respect to the considered problems. The experimental results for the square tubes reveal efficiency improvements with respect to energy absorption of up to 60%, resulting from changed buckling modes of the tubes and energy dissipation during the compression of the foam material itself. The principal features as well as the changes of the crushing process due to filling can also be studied by the numerical simulations. A global failure mechanism due to a high foam density can be observed for filled circular tubes. Aluminium foam is shown to be a suitable material for filling thin-walled tubular steel structures, holding the potential of enhancing the energy absorption capacity considerably, provided the plastic buckling remains characterized by local modes.Dedicated to Prof. Dr. Dr. h. c. Franz Ziegler on the occasion of his 60th birthday     

Structures of intermetallic phases formed during immersion of H13 tool steel in an Al–11Si–3Cu die casting alloy melt

The structures of intermetallic alloy layers formed during immersion of H13 tool steel into an aluminium die casting alloy melt have been studied by X-ray diffraction. Energy dispersive spectroscopy (EDS) analysis on the intermetallic phases was also conducted. A thick composite layer away from the H13 steel substrate consisted of irregular intermetallics and solidified cast alloy. A thin intermetallic layer was present between the thick composite layer and an inner compact layer next to the steel substrate. The intermetallic phase in the composite layer was found to have a cubic structure, _bcc-(FeSiAlCrMnCu). The thin layer was identified to be structurally isomorphous with hexagonal _H-Fe₂SiAl₈. The compositional difference between _H and _bcc intermetallic phases was mainly that the latter consisted of a higher amount of Cr+Mn+Cu. This is consistent with the suggestion that chromium, manganese and copper stabilise _bcc phase at the expense of _H phase. The inner compact layer next to the steel substrate was identified to be isomorphous with orthorhombic η-Fe₂Al₅.     

Novel technology for improving damping capacity of aluminium foam by interface layer

Abstract

A novel technology of designing interface layer using macromolecule resin element, polystyrene, to enhance the damping capacity of aluminium foam is successfully attempted. The interface layer was fabricated by coating a polystyrene film on the pore surface of the aluminium foam using a similar sol–gel technology. Results of the measurements show that the introduction of interface layer not only improves the mechanical strength, but also significantly enhances the damping capacity that can be an order increase relative to the corresponding aluminium foam matrix in the temperature range of room temperature to 80°C. The involved damping mechanisms include the intrinsic damping and interface damping of the constituents. The latter should be dominant as a result of relatively easy energy dissipation through interfacial friction.     

Effects of tool positioning on joint interface microstructure and fracture strength of friction stir lap Al-to-steel welds

Friction stir lap welding (FSLW) experiments have been conducted to study the effects of tool positioning on microstructures formed in the Al-to-steel interface region and on joint strength, defined as maximum applied force over the width (F _m/w _s) of the test sample, of the welds. Various pin positioning and speed conditions were used in the FSLW experiments followed by microstructure examination on the interface regions and tensile-shear testing on the welds, including an examination on crack propagation in mixed stir zone. It was found that when the pin was close to the bottom steel piece, Al-to-steel reaction occurred resulting in intermetallic outbursts formed along the interface. This represents the case of incomplete metallurgical joint. When the pin was lowered to just reach the steel, a thin and continued interface intermetallic layer formed. Evidences and consideration on growth kinetics have suggested that the layer could only remain thin (≤2.5 μm) during FSLW. This layer could bear a high load during tensile-shear testing and the adjacent aluminium deformed and fractured instead. The resulting F _m/w _s was high. When the pin penetrated to steel, F _m/w _s reduced due to brittle fracture being dominant inside mixed stir zone. Evidences have shown that the amount of penetration and speed condition during FSLW do not have large effects on F _m/w _s.     

Morphological features of interfacial intermetallics and interfacial reaction rate in Al-11Si-2.5Cu-(0.15/0.60)Fe cast alloy/die steel couples

Soldering reactions are commonly observed during high pressure die casting of aluminium alloys, and involve the formation and growth of interfacial intermetallics between the die and the cast alloy. It is generally believed that close to 1% Fe is necessary in the aluminium alloy to reduce soldering. However, the role of iron in the interfacial reaction has not been studied in detail. In this investigation, reaction couples were formed between H13 tool steel substrates and an Al-11Si-2.5Cu melt containing either 0.15 or 0.60% Fe. Examination revealed distinctly different intermetallic layer morphology. The overall growth and chemistry of the reaction layer and the reaction rate measured by the consumption of the substrate were compared for the two alloy melts. It was demonstrated that a higher iron content reduces the rate of interfacial reaction, consistent with an observed thicker compact (solid) intermetallic layer. Hence, the difference in reaction rate can be explained by a significant reduction in the diffusion flux due to a thicker compact layer. Finally, the mechanism of the growth of a thicker compact layer in the higher iron melt is proposed, based on the phase relations and diffusion both within and near the interfacial reaction zone.     

Effect of material inhomogeneity on fracture modes in aluminium–steel resistance spot welds

Resistance spot welding (RSW) is attractive for joining dissimilar materials, especially, aluminium to steel in automotive body. The direct joining of aluminium to steel forms an intermetallic compound (IMC) layer at their interface that dominates mechanical behaviour of the joint. A new formula was developed that considers material inhomogeneities such as the different mechanical properties in the weld such as base metal, heat affected zone (HAZ) and the weld nugget to accurately calculate the minimum weld nugget diameter required to enable pull‐out fracture. The shear strengths of weld regions such as the HAZ and IMC were directly measured and used as inputs to this new formula. The new formula was validated using experimental measurements from six combinations of aluminium–steel welds in comparison with analogous aluminium–aluminium welds. The new derivation was able to accurately predict fracture modes for all material combinations.     

Interfacial microstructure and mechanical properties of aluminium–zinc-coated steel joints made by a modified metal inert gas welding–brazing process

The microstructure and properties of aluminium–zinc coated steel lap joints made by a modified metal inert gas CMT welding–brazing process was investigated. It was found that the nature and the thickness of the high-hardness intermetallic compound layer which formed at the interface between the steel and the weld metal during the welding process varied with the heat inputs. From the results of tensile tests, the welding process is shown to be capable of providing sound aluminium–zinc coated steel joints.     

Effect of friction welding parameters on mechanical and metallurgical properties of aluminium alloy 5052–A36 steel joint

Abstract

The mechanical and metallurgical properties of friction welded joints between type 5052 aluminium alloy and type A36 steel have been studied in the present work. Joint strength increased with increasing upset pressure and friction time until it reached a crictical value. The strength of the joint settled at a lower value, compared with that of the base metal, in the case of increasing friction time, caused by the formation of an intermediate phase (intermetallic compound, oxides). The microstructure of 5052 alloy was greatly deformed near the weld interface, and underwent dynamic recrystallisation owing to frictional heat and deformation resulting from the friction welding process. Therefore, a very fine and equiaxed grain structure was observed near the interface. Elongated grains were observed outside the dynamic recrystallisation region at the peripheral part, while the A36 steel side was not deformed. The hardness of the near interface was slightly softer than that of the 5052 alloy base metal, and maximum softened width was ~8 mm from the interface. In the present work, the conditions of friction time t ₁ = 0.5 s and upset pressure P ₂ = 137.5 MPa gave maximum joint strength of 202 MPa when the friction pressure, upset time and rotation speed were fixed at 70 MPa, 5 s and 2000 rev min^-1, respectively, and these were the optimum friction welding conditions for the aluminium alloy 5052-A36 steel joint.     

Deformation characteristics of metal foams

The deformation behaviour of a series of aluminium and zinc foams was investigated by uniaxial testing. Because the deformation behaviour of metal foams is expected to be anisotropic owing to the existence of a closed outer skin and with respect to the foaming direction, a series of measurements was carried out where the orientation of the outer skin and the foaming direction were varied. Stress–strain diagrams and corresponding compression strengths were determined for aluminium- and zinc-based foams. The influence of an age-hardening heat treatment was investigated. Finally, the axial deformation behaviour of aluminium tubes filled with aluminium foam was tested under uniaxial loading conditions. The results of the measurements are discussed in the context of possible applications of metal foams as energy absorbers. © 1998 Chapman & Hall     

Intermetallic compounds formed during brazing of titanium with aluminium filler metals

The effect of aluminium filler metal composition on the formation of AI-Ti intermetallic compounds was investigated in brazed aluminium-to-titanium (Al/Ti) joints and titanium-totitanium (Ti/Ti) joints. The clearance filling ability was also studied. In Ti/Ti joints, the thickness of the intermetallic compound layer was strongly dependent on the aluminium filler metal composition, whereas the clearance filling ability was independent of the composition. The maximum intermetallic compound layer thickness was observed in 99.99% highly pure aluminium filler metal; therefore all additional elements reduced the layer thickness. Above all, the addition of 0.8% Si greatly reduced the thickness. After brazing at 680° C for 3 min, the intermetallic compound formed by Al-0 to 0.8% Si filler metal was found to be of type Al₃Ti. Other compounds, of types Ti₉Al₂₃ and Ti₇Al₅Si₁₂, were also found in joints brazed by Al-3 to 10% Si filler metals. AI-0.8% Si filler metals maintained a higher joint strength than pure aluminium filler metal under brazing conditions of high temperature and long heating time. In Al/Ti joints, AI-Cu-Sn and AI-Cu-Ag filler metal mainly formed Al₃Ti, and Al-10Si-Mg filler metal mainly formed Ti₇Al₅Si₁₂ at the brazed interface of the titanium side after brazing at 600 to 620° C.     

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.     

冷金属过渡条件下AZ61镁合金在两种钢板上的润湿行为

采用动态座滴法研究冷金属过渡条件下,AZ61镁合金分别在Q235钢板和镀锌钢板表面的润湿行为及其界面微观结构。结果表明:润湿行为与焊接工艺参数中的送丝速率密切相关;无论基板采用镀锌钢还是Q235钢在界面处均观察到Al-Fe金属间化合物层,其形成符合热力学形成条件;在Q235钢表面润湿时,送丝速率增加,界面反应变得剧烈,因而润湿性变好,在镀锌钢表面润湿时,送丝速率增加,加剧锌的挥发,使裸露的表面显金属性,因而润湿性变好;当送丝速率≤10.5m·min~(-1)时,镁在Q235钢板上的润湿性要好于镀锌钢板,且后者锌的挥发将导致工艺不稳定。