Showing papers on "Spot welding published in 2006"

Stir zone microstructure and strain rate during Al 7075-T6 friction stir spot welding

Abstract: The factors determining the temperature, heating rate, microstructure, and strain rate in Al 7075-T6 friction stir spot welds are investigated. Stir zone microstructure was examined using a combination of transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) microscopy, while the strain rate during spot welding was calculated by incorporating measured temperatures and the average subgrain dimensions in the Zener-Hollomon relation. The highest temperature during friction stir spot welding (527 °C) was observed in spot welds made using a tool rotational speed of 3000 rpm. The stir zone regions comprised fine-grained, equiaxed, fully recrystallized microstructures. The calculated strain rate in Al 7075-T6 spot welds decreased from 650 to about 20 s−1 when the tool rotational speed increased from 1000 to 3000 rpm. It is suggested that the decrease in strain rate results when tool slippage occurs when the welding parameter settings facilitate transient local melting during the spot welding operation. Transient local melting and tool slippage are produced when the welding parameters produce sufficiently high heating rates and temperatures during spot welding. However, transient local melting and tool slippage is not produced in Al 7075-T6 spot welds made using a rotational speed of 1000 rpm since the peak temperature is always less than 475 °C.

Structure–properties relations in spot friction welded (also known as friction stir spot welded) 6111 aluminum

Abstract: This work details some aspects of the microstructure–properties–processing relations in spot friction welded (also known as friction stir spot welded) 6111 Al sheets, joined in a lap configuration. We have shown that the tool pin penetration depth has a strong effect on the failure mode of the joined samples and a lesser effect on the joint shear strength. With increasing tool pin penetration depth, and consequently with increasing depth of the tool shoulder pressing into the top sample, the failure mode in a lap-shear test changes from brittle and concentrated near the pin hole, to ductile and away from the weld towards the base metal. The sheet interface under the tool shoulder consists four regions; a region where there is no contact at all between the two surfaces, a region where only a mechanical bond (“kissing bond”) exists, a region where there is partial metallurgical bond, and a region with full metallurgical bonding. There is evidence that during welding the Fe–Si–Mn–Cu inclusions present in the as-received material are swept towards the joint interface, thus degrading the joint quality. The aluminum in the partially metallurgically bonded region, in the fully metallurgically bonded region and under the tool pin is fully recrystallized. In these regions, the presence of low angle grain boundaries indicates that additional deformation has occurred after recrystallized grains were formed. It is thus likely that recrystallization has occurred dynamically during the welding process. The material under the shoulder (a portion of the non-contacting region, the “kissing bond”, the partial metallurgical and the full metallurgical regions) has a significantly larger grain size than under the pin, as well as a different texture.

Tensile-Shear Forces and Fracture Modes in Single and Multiple Weld Specimens in Dual-Phase Steels

Abstract: In this article, weld fracture criteria based upon low strain rate (i.e., e ∼ 10 -3 -10 -2 s -1 ) tensile-shear tests of spot welds in dual-phase (DP) steels DP600, DP780, and DP980 are developed. Three empirical equations are inferred from least-squares root-fitting analyses of tensile-shear testing data. Building upon existing results in the literature, the first equation relates the tensile-shear force to the weld diameter. The second and third equations relate, respectively, a critical weld diameter and a critical tensile-shear force for interfacial fracture to the sheet thickness and hardness extrema in the heat-affected zone. These idealized equations can serve as the basis for further development of fracture criteria resembling material flow laws that account for higher strain rates and more complicated deformation paths. The effect of spot-weld placement in specific patterns or arrays on deformation and fracture behavior was also investigated to explore underlying effects from deformation field interactions between adjacent spot welds.

Comparison of Self-Pierce Riveting, Resistance Spot Welding and Spot Friction Joining for Aluminium Automotive Sheet

Abstract: This work compares three aluminium sheet joining processes to determine their capability, efficiency and cost for mass production applications in automotive structures and closures. The joining processes investigated are Resistance Spot Welding (RSW), SelfPierce Riveting (SPR) and Spot Friction Joining (SFJ). Quantitative comparisons have been made on the basis of tensile strength (shear and peel), process time, equipment price and running cost. RSW is the most commonly employed joining method for steel sheet in the automotive industry. Its principle benefits are high speed and low cost operation, plus the ability to weld a wide range of joint configurations with the same gun. The main process limitations for aluminium are weld consistency and electrode-life, though recent work has shown that both of these can be largely overcome with regular electrode polishing

Energy utilisation and generation during friction stir spot welding

Abstract: Energy utilisation during spot welding is investigated using a combination of calorimetry, peak temperature measurement and plunge testing. When a steel tool, clamp and anvil support is used, only 12·6% of the energy generated during the spot welding is transferred into the welded Al 6111 sheets. In contrast, when a mica clamp and anvil support are used, 50% of the energy generated during spot welding transfers into the welded Al 6111 sheets. Only a small percentage of the energy generated during the friction stir spot welding operation is required for stir zone formation. During plunge testing of 6·3 mm thick Al 6061-T6 material, less than 4·03% of the energy which is generated during friction stir spot welding is required for stir zone formation. The remainder of the energy generated dissipates into the tool assembly, clamp, anvil support and the aluminium sheets which are being welded. The rotating pin produces around 70% of the energy generated during spot welding of 6·3 mm thick Al-6061 mater...

Predictions of microstructures when welding automotive advanced high-strength steels : A combination of thermal and microstructural modeling can be used to estimate performance of welds in advanced high-strength steels

Abstract: Weld strength, formability, and impact resistance for joints on automotive steels are dependent on the underlying microstructure. A martensitic weld area is often a precursor to reduced mechanical performance. In this paper, efforts are made to predict underlying joint microstructures for a range of processing approaches, steel types, and gauges. This was done first by calculating cooling rates for some typical automotive processes [resistance spot welding (RSW), resistance mash seam welding (RMSEW), laser beam welding (LBW), and gas metal arc welding (GMAW)]. Then, critical cooling rates for martensite formation were calculated for a range of automotive steels using available thermodynamically based phase transformation models. These were then used to define combinations of process type, steel type, and gauge where welds could be formed avoiding martensite in the weld area microstructure.

Material Flow during Friction Stir Spot Welding

Abstract: Material flow during friction stir spot welding is investigated. An examination of dissimilar Al 5754/Al 6111 spot welds was conducted to allow visualisation of material flow based on their differing etching characteristics. In addition, Al 6061–T6 spot welds containing Al2O3 tracer particles were examined to highlight the movement of material in different joint regions. It has been confirmed that upper sheet material is moved downwards into the lower sheet when a layer of upper sheet material is pushed ahead at the tip of the rotating pin, when upper sheet material becomes trapped at the root of the pin thread, and when an adhering layer of upper sheet material forms at the pin periphery during spot welding using a smooth pin. Lower sheet material is displaced upwards and outwards in a spiral motion when the rotating pin forms the keyhole. Two distinct zones of material flow are produced during friction stir spot welding: an inner flow zone close to the pin periphery where upper sheet material mo...

Coating material for protecting metals, especially steel, from corrosion and/or scaling, method for coating metals and metal element

Abstract: It was surprisingly found that when a suitable binder including a suitable filler is used during the high temperature treatment of a curing process, the coating materials of the invention change in such a manner that electrically conducting reactive layers are formed that allow welding and especially spot welding together with the metal substrate even after treatment at temperatures of more than 800 °C.

Resistance spot welding of coated high-strength dual-phase steels

Abstract: New car models will be safer, lighter, and more fuel efficient as advanced high-strength steels are introduced into production.

Liquid metal expulsion during laser spot welding of 304 stainless steel

Abstract: During laser spot welding of many metals and alloys, the peak temperatures on the weld pool surface are very high and often exceed the boiling points of materials. In such situations, the equilibrium pressure on the weld pool surface is higher than the atmospheric pressure and the escaping vapour exerts a large recoil force on the weld pool surface. As a consequence, the molten metal may be expelled from the weld pool surface. The liquid metal expulsion has been examined both experimentally and theoretically for the laser spot welding of 304 stainless steel. The ejected metal droplets were collected on the inner surface of an open ended quartz tube which was mounted perpendicular to the sample surface and co-axial with the laser beam. The size range of the ejected particles was determined by examining the interior surface of the tube after the experiments. The temperature distribution, free surface profile of the weld pool and the initiation time for liquid metal expulsion were computed based on a three-dimensional transient heat transfer and fluid flow model. By comparing the vapour recoil force with the surface tension force at the periphery of the liquid pool, the model predicted whether liquid metal expulsion would take place under different welding conditions. Expulsion of the weld metal was also correlated with the depression of the liquid metal in the middle of the weld pool due to the recoil force of the vapourized material. Higher laser power density and longer pulse duration significantly increased liquid metal expulsion during spot welding.

Expulsion monitoring in spot welded advanced high strength automotive steels

Abstract: Although there have been a number of investigations on monitoring and controlling the resistance spot welding (RSW) of low carbon galvanised steels, those of advanced high strength steels (AHSS) are limited. A data acquisition system was designed for monitoring weld expulsion via the measurement of voltage, current, electrode force and displacement and the calculation of resistance. The dynamic resistance, electrode force and tip displacement were characterised and correlated with the phenomenon of expulsion during RSW of dual phase (DP) steel using an ac welder. Two control strategies for DP600 spot welding were proposed on the basis of the rate of change in the dynamic resistance and the electrode force.

Dissimilar metals joining of aluminum alloy and steel sheets by friction stir spot welding

Abstract: Friction stir spot welding (FSSW) was applied to dissimilar lap joint between aluminum alloy sheet and mild steel sheet without formation of brittle intermetallic compound at the weld interface. A sufficient strength of the dissimilar joint was obtained by stirring aluminum alloy near the interface without inserting a welding tool to steel surface from the aluminum alloy side. Consequently, joint strength of FSSW was higher than that of resistance spot welds, and the all of FSSW joints showed a plug fracture through aluminum alloy parent sheet. Moreover, it was suggested that joint strength was related to area of stir zone at the weld interface. An oxide film was observed partially in the stir zone by sweeping up from the interface. No brittle intermetallic compound was observed at the weld interface. But only the thin amorphous layer of several nm in thickness including oxygen was found at the weld interface.

Improving reliability of heat and fluid flow calculation during conduction mode laser spot welding by multivariable optimisation

Abstract: Several uncertain parameters affect the reliability of heat transfer and fluid flow calculations during conduction mode laser spot welding because their values cannot be prescribed from fundamental principles. These parameters include absorptivity of the laser beam, effective thermal conductivity and effective viscosity of liquid metal in the weld pool. Values of these parameters are usually adjusted by trial and error so that the computed results agree with the corresponding experimental values. Here it is shown that by integrating multivariable constrained optimisation with convective heat transfer and fluid flow calculations, the values of the uncertain parameters can be obtained from a limited volume of experimental data. The optimisation technique requires numerically calculated sensitivity values of weld dimensions with respect to absorptivity, effective thermal conductivity and effective viscosity and minimises the discrepancy between the predicted and the measured weld dimensions. The nume...

Exploring the mechanical properties of spot welded dissimilar joints for stainless and galvanized steels

Abstract: Spot weldability of dissimilar metal joints between stainless steels and nonstainless steels was investigated. The aim was to determine the spot welding parameters for the dissimilar metal joints and to characterize the mechanical properties of the joints. Metallographical investigations, microhardness measurements, peel tests, lap shear tests, cross-tension tests, corrosion fatigue tests, and stress corrosion cracking tests were performed. It was found that in the dissimilar metal joints between stainless steel and nonstainless steel, the failure load of the cross-tension specimens was around 72-78% of that of the lap shear specimens. The weld nugget of the dissimilar metal joints was fully martensitic, but it was ductile enough so that the failure type was plug failure in both lap shear and cross-tension tests. In the case of the corrosion fatigue testing of the spot welded joints, different strength levels of the base materials did not have an effect on the corrosion fatigue strength, but the sheet thickness had a significant effect. The fatigue strength of a spot welded specimen increased with the increasing sheet thickness. Electro-coating of the test specimens did not have an effect on the corrosion fatigue properties of the spot welded joints. Stress corrosion cracking tests showed that the stainless steel EN 1.4318 and zinc-coated nonstainless steel ZStE260BH dissimilar metal joints are susceptible to hydrogen embrittlement in 3.5% sodium chloride solution at room temperature. Comparable cracking was also observed in the stainless-stainless steel joints, when they were galvanically coupled to zinc. The reason for hydrogen embrittlement of the dissimilar metal welds is that the weld nugget is fully martensitic and the corrosion potential is low due to the zinc plating.

Resistance Spot Welding Process: Experimental and Numerical Modeling of the Weld Growth Mechanisms with Consideration of Contact Conditions

Abstract: To investigate the weld growth mechanisms in resistance spot welding, a fully coupled numerical and experimental approach has been performed using the finite-element method. This numerical model dedicated to the resistance welding simulation is presented in the first part of this article. To take the contact conditions carefully into account, a finite-element formulation is detailed in the second part, where the experimental device used to measure the interface contact properties is also presented. An example is detailed to validate the approach presented in this article. The experimental shape evolution of the nugget is compared to numerical results.

Methods and apparatus for retractable pin friction stir welding and spot welding

Abstract: An apparatus for friction stir welding is described that includes a stationary assembly having a bore therethrough, the bore having an inner diameter, and a rotational assembly having a welding end. At least the welding end of the rotational assembly extends through the bore. A portion of the rotational assembly is adjacent the inner diameter of the bore. At least one of the adjacent portion of the rotational assembly and the inner diameter of the stationary assembly are configured such that rotation of the rotational assembly will cause plasticized material from a welding process that has entered an area between the adjacent portion and the inner diameter to move towards a welding zone located proximate the welding end.

Nugget growth characteristic for AZ31B magnesium alloy during resistance spot welding

Abstract: An axisymmetrical finite element model for studying the distribution of temperature for resistance spot welding (RSW) to predict weld nugget growth of AZ31B Mg alloy was developed by employing a co...

Dissimilar Metal Welding of Steel to Al-Mg Alloy by Spot Resistance Welding

Abstract: We tried to join steel to Al-Mg alloy using a resistance spot welding method. The effect of Mg in Al-Mg alloy on the strength and the interfacial microstructure of the joint was investigated. Additionally, the effect of insert metal of commercially pure aluminum, which was put into the bonding interface, on the joint strength was examined. The obtained results were as follows. The cross-tensile strength of a joint between SS400 steel and commercially pure aluminum (SS400/Al) was high and fracture occurred in the aluminum base metal. However, the strength of a joint between SS400 and Al-Mg alloy was remarkably low and less than 30% of that of the SS400/Al joint. An intermetallic compound layer developed so thickly at the bonded interface of the SS400/Al-Mg alloy joint that the joint strength decreased. The intermetallic compound layer developed more thickly as Mg content in the Al-Mg alloy increased. Using insert metal of commercially pure aluminum containing little Mg successfully improved the strength of the SS400/Al-Mg alloy joint and the strength was equivalent to that of the base metal.

Laser micro welding of copper and aluminum

Abstract: Aluminum combines comparably good thermal and electrical properties with a low price and a low material weight. These properties make aluminum a promising alternative to copper for a large number of electronic applications, especially when manufacturing high volume components. However, a main obstacle for a wide use of this material is the lack of a reliable joining process for the interconnection of copper and aluminum. The reasons for this are a large misalignment in the physical properties and even more a poor metallurgical affinity of both materials that cause high crack sensitivity and the formation of brittle intermetallic phases during fusion welding. This paper presents investigations on laser micro welding of copper and aluminum with the objective to eliminate brittle intermetallic phases in the welding structure. For these purposes a combination of spot welding, a proper beam offset and special filler material are applied. The effect of silver, nickel and tin filler materials in the form of thin foils and coatings in a thickness range 3-100 μm has been investigated. Use of silver and tin filler materials yields to a considerable improvement of the static and dynamic mechanical stability of welded joints. The analysis of the weld microstructure shows that an application even of small amounts of suitable filler materials helps to avoid critical, very brittle intermetallic phases on the interface between copper and solidified melt in the welded joints.

Welding teaching point correction system and calibration method

Abstract: Disclosed is a welding teaching point correction system, including: a robot; a spot welding gun comprising two welding tips provided to be opposed to each other; an imaging apparatus to image a welding point of a workpiece, the imaging apparatus being provided detachably to or exchangeably with at least one of the two welding tips; an operation control unit to control the robot and the spot welding gun in accordance with an teaching program to teach welding operation to the robot and the spot welding gun; an image processing unit to acquire positional information of the welding point of the workpiece in the image; and a program correction unit to correct an teaching point for the robot in the teaching program in a plurality of directions based on the positional information of the welding point of the workpiece in the image acquired by the image processing unit.

Surface modification of resistance welding electrodes by electro-spark deposited composite coatings Part II. Metallurgical behavior during welding

Abstract: Electrodes with monolithic TiCP/Ni coatings and multi-layer Ni/(TiCP/Ni)/Ni coatings were used to resistance spot weld Zn-coated sheet steel to investigate metallurgical behaviour of the coatings during welding. Scanning electron microscopy, energy-dispersive X-ray analysis and X-ray diffraction were employed to characterize the microstructure of coatings, reactions of electrodes with Zn-coating and alloy layer formation. The results showed that molten Zn penetrated TiCP/Ni coatings via the cracks that were present within as-coated TiCP/Ni coating, starting from the first weld. Additional cracks continually formed in the coating during welding due to action of the welding force on the low toughness coating, resulting in formation of a granular loose overlay at the outer surface which were easily detached and stuck onto the work sheet. On the contrary, cracks could be rarely found within Ni/(TiCP/Ni)/Ni coating until 100welds or more were made, and much fewer cracks formed up to 400welds, compared to the TiCP/Ni coating. With Ni/(TiCP/Ni)/Ni coating on the electrode surface, alloying between copper alloy and molten Zn as well as pitting (erosion) of the electrode tips were remarkably reduced, and hence, a slower growth rate of tip diameter was observed.

Crushing test of double hat-shaped members of dissimilar materials with adhesively bonded and self-piercing riveted joining methods

Abstract: There is a great demand for the development of lightweight vehicle to enhance fuel efficiency and dynamic performance. An effective way to reduce weight of vehicle is by using lightweight materials such as aluminum and magnesium. However, if such materials are used in vehicles, there are often instances when dissimilar materials such as aluminum and steel need to be joined to one another. The conventional joining method, namely resistance spot welding, cannot be used in joining dissimilar materials and self-piercing rivet (SPR) and adhesive bonding are good alternatives to resistance spot welding. In this study, the crushing test of double hat-shaped member made by resistance spot welding, SPR and adhesive bonding was performed. Various parameters of crashworthiness are analyzed and evaluated. Based on these results, the applicability of SPR and adhesive bonding are proposed as an alternative to resistance spot welding.