Showing papers on "Spot welding published in 2011"

On the feasibility of friction spot joining in magnesium/fiber-reinforced polymer composite hybrid structures

Abstract: In the present study, the feasibility of the friction spot joining technique on magnesium AZ31–O/glass fiber and carbon fiber reinforced poly(phenylene sulfide) joints is addressed. The thermo-mechanical phenomena associated with the friction spot joining process promoted metallurgical and polymer physical–chemical transformations. These effects resulted in grain refinement by dynamic recrystallization and changes in local (microhardness) and global strength (lap shear). Friction spot lap joints with elevated mechanical performance (20–28 MPa) were produced without surface pre-treatment. This preliminary investigation has successfully shown that friction spot joining is an alternative technology for producing hybrid polymer–metal structures.

Failure mode transition in AHSS resistance spot welds. Part I. Controlling factors

Abstract: Failure mode of resistance spot welds is a qualitative indicator of weld performance. Two major types of spot weld failure are pull-out and interfacial fracture. Interfacial failure, which typically results in reduced energy absorption capability, is considered unsatisfactory and industry standards are often designed to avoid this occurrence. Advanced High Strength Steel (AHSS) spot welds exhibit high tendency to fail in interfacial failure mode. Sizing of spot welds based on the conventional recommendation of 4t0.5 (t is sheet thickness) does not guarantee the pullout failure mode in many cases of AHSS spot welds. Therefore, a new weld quality criterion should be found for AHSS resistance spot welds to guarantee pull-out failure. The aim of this paper is to investigate and analyze the transition between interfacial and pull-out failure modes in AHSS resistance spot welds during the tensile–shear test by the use of analytical approach. In this work, in the light of failure mechanism, a simple analytical model is presented for estimating the critical fusion zone size to prevent interfacial fracture. According to this model, the hardness ratio of fusion zone to pull-out failure location and the volume fraction of voids in fusion zone are the key metallurgical factors governing type of failure mode of AHSS spot welds during the tensile–shear test. Low hardness ratio and high susceptibility to form shrinkage voids in the case of AHSS spot welds appear to be the two primary causes for their high tendency to fail in interfacial mode.

Material interactions in a novel pinless tool approach to friction stir spot welding thin aluminum sheet

Abstract: The requirement for a probe, or pin, in friction stir spot welding (FSSW) leads to an undesirable keyhole and “hooking,” which can influence the fracture path and weld strength. Furthermore, the full weld cycle for FSSW is typically longer than ideal for the automotive industry, being 2 to 5 seconds. Here, it is shown that using a novel pinless tool design it is possible to achieve high lap shear strength (~3.4 kN) in thin aluminum sheet (~1 mm thick), with short weld cycle times (<1 second). Several techniques have been exploited to study the material flow and mechanisms of weld formation in pinless FSSW, including high-resolution X-ray tomography, to understand the role of the tool design and weld parameters. Despite the “simple” nature of a pinless tool, material flow in the weld zone was found to be surprisingly complex and strongly influenced by surface features on the tool, which greatly increased the penetration of the plastic zone into the bottom sheet. Because of the rapid thermal cycle and high level of grain refinement, the weld zone was found to develop a higher strength than the parent material with little evidence of a heat affected zone (HAZ) after postweld natural aging.

Ultrasonic spot welding of aluminium to steel for automotive applications—microstructure and optimisation

Abstract: Energy efficient methods for joining aluminium to steel have potential for major applications in the automobile industry. Results are reported where 1 mm gauge 6111 aluminium and DC04 steel automot...

Failure mode transition in AHSS resistance spot welds. Part II: Experimental investigation and model validation

Abstract: The objective of this paper is to investigate and analyze the transition criteria from interfacial to pullout failure mode in AHSS resistance spot welds during the tensile-shear test by the use of both experimental and analytical approaches. Spot welds were made on three dual phase steel grades including DP600, DP780 and DP980. A low strength drawing quality special killed (DQSK) steel and AISI 304 austenitic stainless steel were also tested as a baseline for comparison. The microstructure and mechanical strength of the welds were characterized using metallographic techniques and the tensile-shear testing. Correlations among critical fusion zone (FZ) size required to ensure the pullout failure mode, weld microstructure and weld hardness characteristics were developed. It was found that critical FZ size increases in the order of DQSK, DP600, DP980, DP780 and AISI304. No direct relationship was found between the tensile strength of the base metal and the critical FZ size. It was concluded that low hardness ratio of FZ to pullout failure location and high susceptibility to form shrinkage voids are two primary reasons for high tendency of AHSS to fail in interfacial mode. HAZ softening can improve RSW mechanical performance in terms of load bearing capacity and energy absorption capability. This phenomenon promotes PF mode at smaller FZ sizes. This fact can explain smaller critical FZ size measured for DP980 in comparison with DP780. The results obtained from the model were compared to the experimental results and the literature and a reasonable agreement was obtained.

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 alum...

Preliminary Investigation of the Microstructure and Mechanical Behaviour of 2024 Aluminium Alloy Friction Spot Welds

Abstract: Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Materials Mechanics, Solid State Joining Processes, Max-Planck-St. 1, D-21502 Geesthacht, Germany Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Materials Mechanics, Advanced Polymer-Metal Hybrid Structures, Max-Planck-St. 1, D-21502 Geesthacht, Germany Federal University of Sao Carlos (UFSCar), Departamento de Engenharia de Materials (DEMa), Rodovia Washington Luiz km 235, 13565-905 São Carlos-SP, Brazil

Characterization of Intermetallic Compounds in Dissimilar Material Resistance Spot Welded Joint of High Strength Steel and Aluminum Alloy

Abstract: Dissimilar materials of H220 Zn-coated high strength steel and 6008 aluminum alloy were welded by median frequency resistance spot welding. Interfacial characteristics and kinetics of growth of intermetallic compound layer at steel/aluminum interface in the welded joint were investigated. The intermetallic compound layer was mainly made up of η-Fe2Al5 and θ-FeAl3 phases, and its morphology and thickness varied with positions along the interface. The growth behavior of the intermetallic compound layer was dominated by η-Fe2Al5, which exhibited parabolic characteristic. The growth coefficient of η-Fe2Al5 could be expressed as k = k0 exp(–Q/RT) with k0 of 132 m2/s and Q of 239 kJ/mol. The kinetics of growth of the intermetallic compound layer indicated that its formation and growth were mainly driven by reactive diffusion between Fe and Al atoms, and hence the thickness and morphology of the layer were dependant on interaction time between liquid aluminum alloy and solid steel, and also interfacial temperature history during welding. The brittle intermetallic compound layer at the steel/aluminum interface was the weak zone where cracks inclined to derive and propagate during tensile shear testing. The fracture surfaces of the welded joint displayed mixed fracture morphology with both brittle and ductile features.

Friction stir welding of dissimilar metals

Abstract: When a friction stir weld tool penetrates the interface of two workpieces of dissimilar metal alloy materials, the resultant weld of the different alloy materials may produce a weak weld joint. Such weak joints are often experienced, for example, when attempting to form spot welds or other friction stir welds between a magnesium alloy sheet or strip and an aluminum alloy sheet or strip. It is discovered that suitable coating compositions including an adhesive placed at the interface of assembled workpieces can alter the composition of the friction stir weld material and strengthen the resulting bond. In the example of friction stir welds between magnesium alloy and aluminum alloy workpieces, it is found that combinations of an adhesive with copper, tin, zinc, and/or other powders can strengthen the magnesium-containing and aluminum-containing friction stir weld material.

Ultrasonic metal welding of AA 6022‐T4 lap joints: Part I – Technological characterisation and static mechanical behaviour

Abstract: High power ultrasonic technology can currently count on a number of industrial applications. Ultrasonic welding, which is a standard joining technique in many applications on plastics, has few but well established metal applications, such as copper wires, pipes and connectors welding, or, considering spot welding of aluminium thin sheets, is attractive for the automotive industry field, where it could represent a possible cost effective alternative to resistance spot welding, clinching or self‐pierce riveting. The present experimental study is addressed to this kind of application in order to evidence the effects of welding parameters and, most of all, their interactions on the tensile strength of tensile–shear spot welded lap joints. Relevant results have been achieved by dedicated and non‐conventional instrumentations applied to the welder for measuring and controlling the process parameters. The best static performance has been taken as an input for the second part of the study regarding the fa...

HAZ Microstructures and Local Mechanical Properties of High Strength Steels Resistance Spot Welds

Abstract: The heat affected zones (HAZ) of two Dual Phase steels spot welds, DP450 and DP980, were investigated experimentally with a Gleeble 3500 thermomechanical simulator. The thermal cycles experienced locally were identified by finite element analysis of the resistance spot welding process and their evolution with an increasing sheet thickness was highlighted within a usual range of [1.0–3.0] mm. The cooling rates are significantly lower in the case of thick sheets and this promotes the occurrence of diffusional phase transformations. HAZ microstructures and constitutive behaviours could be characterized with the Gleeble specimens. Experimental simulations were run in the subcritical and the coarse grain temperature ranges (700°C and 1200°C respectively) in order to address the main transformations for both steels. DP450 shows little sensitivity to the subcritical thermal cycles while DP980 exhibits significant softening. This is in accordance with their respective base metal martensite content and the occurrence of martensite tempering. On the contrary, DP450 microstructures and mechanical properties are strongly sensitive to the investigated range of coarse grained HAZ thermal cycles while the evolution is less pronounced in the case of DP980, which is related to their respective hardenability.

Applying spatial augmented reality to facilitate in-situ support for automotive spot welding inspection

Abstract: In automotive manufacturing, the quality of spot welding on car bodies needs to be inspected frequently. Operators often only check different subsets of spots on different car bodies with a predetermined sequence. Currently, spot welding inspections rely on a printed drawing of the testing body, with the inspection points marked on this drawing. Operators have to locate the matching spot on the drawing and the body manually to perform the inspection. The manual inspection process suffers from inefficiencies and potential mistakes. This paper describes a system that projects visual data onto arbitrary surfaces for providing just-in-time information to a user in-situ within a physical work-cell. Spatial Augmented Reality (SAR) is the key technology utilized in our system. SAR facilitates presentation of projected digital Augmented Reality (AR) information on surfaces of car bodies. Four types of digital AR information are projected onto the surfaces of car body parts in structured work environments: 1) Location of spot welds; 2) Inspection methods; 3) Operation Description Sheet (ODS) information; 4) Visualization of weld locating methods. Various visualization methods are used to indicate the position of spot welds and the method used for spot welding inspection. Dynamical visualizations are used to assist operators to locate spot welds more easily. The SAR approach does not require additional special models in finding spot welds, but only needs knowledge of location of spot welds on the part. Our system allows operators becoming more effective and efficient to in performing proper inspections, by providing them the required information at the required time without the need to refer to paper-based manuals or computer terminals.

Welding and Joining of Magnesium Alloys

Abstract: Welding and joining of magnesium alloys exert a profound effect on magnesium application expansion, especially in ground and air transportations where large-size, complex components are required. This applies to joints between different grades of cast and wrought magnesium alloys and to dissimilar joints with other materials, most frequently with aluminum and steel. Due to specific physical properties of magnesium, its welding requires low and well controlled power input. Moreover, very high affinity of magnesium alloys to oxygen requires shielding gases which protect the liquid weld from an environment. To magnify complexity, also solid state reaction with oxygen, which forms a thermodynamically stable natural oxide layer on magnesium surface, is an inherent deficiency of joining (Czerwinski, 2008). Both the conventional and novel welding techniques were adapted to satisfy these requirements, including arc welding, resistance spot welding, electromagnetic welding, friction stir welding, electron beam and laser welding. Since fusion welding has a tendency to generate porosities and part distortion, many alternative joining practices were implemented. These include soldering, brazing, adhesive bonding and mechanical fastening. However, also the latter techniques have disadvantages associated, for example, with stress induced by drilling holes during mechanical fastening, preheating during clinching or extensive surface preparation in adhesive bonding. Hence, experiments are in progress with completely novel ideas of magnesium joining. An application of magnesium is often in multi-material structures, requiring dissimilar joints, involving magnesium alloys as one side where on another end there are alloys with drastically different properties. How to weld dissimilar materials is one of the most difficult problems in welding. A difference in physicochemical properties of dissimilar joint components creates challenges for mechanically bolted assemblies as well. Due to its very low electronegative potential, magnesium is susceptible to galvanic corrosion thus affecting performance of mechanical joints in conductive environments. This chapter covers key aspects of magnesium welding and joining along with engineering applications, challenges and still existing limitations. For each technique, the typical joint characteristics and possible defects are outlined with particular attention paid to weld metallurgy and its relationship with weld strength, ductility and corrosion resistance. Although fundamentals for each technique are provided, the primary focus is on recent global activities.

Evaluation of weldability for resistance spot welded single-lap joint between GA780DP and hot-stamped 22MnB5 steel sheets †

Abstract: Recently, in the automotive industry, Al-coated boron steel sheets (22MnB5) have been used for hot stamping, and the use of these sheets makes it possible to achieve a tensile strength of over 1,500 MPa, since a metallurgical transformation from austenite to martensite occurs during the process. In this study, resistance spot welding (RSW) experiments were performed in order to evaluate the weldability of single-lap joints between GA780DP and 22MnB5. The effect of the weld current on the nugget diameter and load-carrying capacity was evaluated by observing the nugget diameter and performing a tensile-shear test. Furthermore, the fracture behavior was evaluated by carrying out optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) observations. Ductile regions were observed on the interfacially fractured surface of the weld, and this implies that a high load-carrying capacity can be obtained even when interfacial fracture (IF) occurs. IF is caused by the stress concentration resulting from the presence of the sharp notch at the boundary of the nugget as well as by the high hardness and the brittle microstructure of the weld; the microstructure is brittle because of the high carbon equivalent (Ceq) and the penetration of Al in the weld.

Growth Manner of Intermetallic Compound Layer Produced at Welding Interface of Friction Stir Spot Welded Aluminum/Steel Lap Joint

Abstract: Lap joining of a pure aluminum plate and a low carbon steel plate was performed using friction stir spot welding. The aluminum plate was placed over the steel plate, a rotating welding tool was inserted into the aluminum plate, and the tip of the tool was dwelled above the aluminum/steel interface. Dwell time was controlled in the range of 0 to 120 seconds. The microstructure of the welding interface was examined by optical microscopy and scanning electron microscopy. Chemical composition analysis was carried out by energy dispersive X-ray spectroscopy. Welding was achieved for all dwell times. Refined grains were formed by plastic flow in the aluminum matrix close to the welding interface. Intermetallic compound layer was produced along the welding interface. Precise backscattered electron image observation and energy dispersive X-ray spectroscopy analysis revealed that the intermetallic compound layer consisted of an Al13Fe4 phase layer and an Al5Fe2 phase layer. The thickness of the layers increased in proportion to the square root of the dwell time. The parabolic coefficient K was 1.30×10−14 and 6.06×10−13 m2/s for the Al13Fe4 layer and the Al5Fe2 layer, respectively.

A magnetic flux leakage method using a magnetoresistive sensor for nondestructive evaluation of spot welds

Abstract: Spot welding is widely used for joining metal plates. However, a highly reliable monitoring method is needed to weld a robust structure. For this purpose, we developed a magnetic flux leakage (MFL) system using a magnetoresistive (MR) sensor for nondestructive spot-weld inspection. The magnetic flux is induced between two joined plates, and the magnetic flux leakage with a tangential component parallel to the plate surface is measured. A magnetic image at the spot-welding part is obtained by two-dimensional scanning. The connected diameter of the nugget and the maximum shear load are measured after the magnetic measurement to investigate their interrelationship. The results show that the nondestructive magnetic flux leakage test shows a good correlation with the destructive shear test.

Comparing Properties of Adhesive Bonding, Resistance Spot Welding, and Adhesive Weld Bonding of Coated and Uncoated DP 600 Steel

Abstract: Zinc coated dual phase 600 steel (DP 600 grade) was investigated, utilisation of which has gradually increased with each passing day in the automotive industry. The adhesive bonding (AB), resistance spot welding (RSW), and adhesive weld bonding (AWB) joints of the zinc coated DP 600 steel were investigated. Additionally, the zinc coating was removed using HCL acid in order to investigate the effect of the coating. The microstructure, tensile shear strengths, and fracture properties of adhesive bonding (AB), resistance spot welding (RSW), and adhesive weld bonding (AWB) joints of the coated and uncoated DP 600 steel were compared. In addition, a mechanical-electrical-thermal coupled model in a finite element analysis environment was utilised. The thermal profile phenomenon was calculated by simulating this process. The results of the tensile shear test indicated that the tensile load bearing capacity (TLBC) values of the coated specimens among the three welding methods were higher than those of the uncoated specimens. Additionally, the tensile strength of the AWB joints of the coated and uncoated specimens was higher than that of the AB and RSW joints. It was determined that the fracture behaviours and the deformation caused were different for the three welding methods.

Investigation on Joint Strength of Dissimilar Resistance Spot Welds of Aluminum Alloy and Low Carbon Steel

Abstract: Resistance spot welding was used to join low carbon steel and A5250 Aluminum alloy sheets. Mechanical properties and failure behavior of the spot welds in terms of peak load, failure energy and failure mode, were evaluated using tensile- shear test. Relationship between welding current and mechanical properties was investigated. It was found that the formation of brittle intermetallic compounds in the weld fusion zone is the key governing factor for mechanical properties of dissimilar Al alloy/low carbon steel resistance spot weld. Increasing welding current, increases both peak load and energy absorption due to increasing overall bond area and transition in failure mode from interfacial to pullout failure mode.