Effect of dynamic contact resistances on advanced high strength steels under medium frequency DC spot welding conditions

Abstract

The effect of dynamic contact resistance (DCR) during MFDC spot welding of dual phase and martensitic steels was evaluated. A comparative analysis of DP590 to DP590 with DP780 to DP780 steel welds, and DP780 to DP780 with M1200 to M1200 steels welds was carried out. The DCR of DP780 steel is higher than DP590 steel during the initial stages of weld time, but is reversed later. The bulk resistance component, which is higher in DP780 steel, is dominant and generates more energy early in the process and controls melting. Although the total energy input is almost same, the higher β-peak and its early occurrence ensures better heat utilisation resulting in larger nugget size. Contrarily, in martensitic steel the interface resistance component remains high throughout the entire welding process and compensates for the lower bulk resistance effect. Even with relatively lower energy input the nuggets produced in M1200 steel are comparable to DP780 steel.     

Numerical simulation of spot welding for galvanised sheet steels

Abstract

Galvanised sheet steels are now widely used to be the substrate for body in white (BIW) construction in the automotive industry. Weldability of galvanised sheet steels much worsened compared to spot welding of low carbon steels. The present paper develops a 2D axisymmetric model and employs an incremental coupled thermal–electrical–mechanical analysis to predict the nugget development during resistance spot welding (RSW) of galvanised sheet steels. Temperature dependent contact resistance for faying surfaces was determined to take into account of the influence of zinc coat for spot welding galvanised sheet steels. The effect of dynamic contact radii on temperature distribution was studied and compared with results under constant contact area assumption. The predicted nugget shape and size agreed well with the experimental data. Higher current and longer welding time should be applied for galvanised sheet steels compared to low carbon steel spot welding. The proposed model can be applied to predict weld quality and choose optimal welding conditions for spot welding galvanised sheet steels.     

Effect of variable electrode force on weld quality in resistance spot welding

Abstract

Resistance spot weldability is defined as the acceptable welding current ranges as determined by the weld lobe in resistance spot welding. Nowadays many studies have focused on the effect of welding current and welding time under constant electrode force on the weld quality and weldability. There is little research on the influence of variable electrode force on the weld quality and weldability because of the difficulty in controlling variable electrode force using pneumatic gun. In the present study, first, the influence of three stages of electrode force, including squeeze force, welding force and forging force, on the quality of welds is analysed. Then a design of experiment approach is applied to analyse the influence of the three stages of electrode force on welding quality and thus to obtain optimum parameter of variable electrode force by controlling the electrode force with servo gun. The comparisons of tensile shear strength, nugget size, weld lobe width and wear rate of electrode tip between variable force and constant force are carried out. The results show that the weld quality and weldability can be increased evidently using optimum parameter of variable electrode force without accelerating the electrode wear rate.     

Mechanical and fatigue behaviour of laser and resistance spot welds in advanced high strength steels

Abstract

A study was carried out on laser and resistance spot welds in overlapped sheets of dual phase advanced high strength steel (DP780) and deep drawing steel (DC04) of 2˙0 mm in thickness. The aim of the study was to investigate the fatigue performance of these joints under tensile shear loading as well as the monotonic performance for applications in the automotive industry. The mechanical properties, failure behaviour and fatigue life analyses of spot welds in similar and dissimilar joints were investigated by experimental and numerical methods. The structural stress concept was used to describe the fatigue lives of spot welded specimens. The results revealed different failure types with different fatigue behaviours for laser and resistance spot welds under the application of cyclic loads at 'high load' and 'low load' levels.     

Influence of welding current shape on expulsion and weld strength of resistance spot welds

Abstract

The present paper presents the influence of welding current shape on weld strength of resistance spot welds of zinc coated mild steel sheets. The influence is analysed at different levels of the electrode wear. Welding currents with different peak values and different RMS (root mean square) values were used in the experiment. The results show that welding current with high peak values implies higher weld strength.     

Failure mode transition and mechanical properties of similar and dissimilar resistance spot welds of DP600 and low carbon steels

Abstract

The present work addresses the microstructure and mechanical properties of similar and dissimilar resistance spot welds of low carbon steel (LCS) and dual phase steel (DP600). Correlations between the critical fusion zone size required to ensure pullout failure mode, the weld microstructure and the weld hardness characteristics were developed. Dissimilar DP600/LCS spot welds exhibit the lowest tendency to fail in interfacial failure mode. Effects of weld physical attributes and weld microstructure on the peak load and energy absorption of similar and dissimilar DP600/LCS resistance spot welds are analysed.     

Online quality inspection of resistance spot welded joint based on electrode indentation using servo gun

Abstract

Resistance spot welding is one of the major joining techniques widely used for car body assembly. Weld quality may significantly influence the durability and reliability of the automobile body. Automotive manufacturers often rely on destructive testing and monitoring variables which indirectly reflected weld quality to assess the weld quality and control the welding process. However, these approaches have inherent limitations and are difficult to be implemented in plant environments. Therefore, it is imperative to develop an online inspection method to evaluate weld quality. In the present study, a method of producing a series of substantially uniform spot welds between two metal parts using a servomotor driven movable electrode and an axially opposing fixed electrode is proposed. The indentation in the workpiece surface is suitably measured by the displacement of the movable electrode as it applies an electrode force and welding current is passed through the weld site of the workpiece. The optimal indentation range is determined by peel test and metallographic examinations with respect to various sheet gages and grades. Consequently, online weld quality inspection results are achieved based on developed optimal indentation range.     

Estimating the strength of resistance spot welds based on sonic emission

Abstract

Audible sound signals detected during the resistance spot welding (RSW) of zinc coated steels were investigated in order to assess their suitability for estimating the strength of the weld. A new sonic emission indicator was introduced and compared to a commonly used emission count indicator. A new method of spot weld strength estimation based on the two indicators is presented. The advantage of the method is that it makes it possible to establish the stage when the electrode is worn out. The method enables the development of improved RSW process control algorithms.     

Influence of initial gap on weld expulsion in resistance spot welding of dual phase steel

Abstract

The weld expulsion is prone to occur and severely affects the nugget quality when the initial gap between dual phase (DP) steel sheets exist in resistance spot welding (RSW). To investigate the effect of initial gap on weld expulsion, a finite element model was developed to analyse the weld nugget formation process with different initial gaps for DP steels. An estimation method of expulsion occurrence based on the ratio of the nugget radius R_n and the contact radius R_c between sheets was proposed to get the critical initial gap without expulsion. The simulation and experimental results showed that the weld expulsion would not happen until the gap spacing reaches the critical value. The critical initial gap of DP steel is much smaller than that of low carbon steel. For both DP steel and low carbon steel, the critical initial gap would increase with the thickening of the steel sheet.     

Effects of electrical current on transport processes in resistance spot welding

Abstract

The effects of alternating current (ac) and direct current (dc) on cooling rate, solute distribution and nugget shape after solidification, which are responsible for microstructure of the fusion zone, during resistance spot welding, are realistically and extensively investigated. The computer program developed by Wang and Wei is used to predict transport variables in workpieces and electrodes during heating, melting, cooling and freezing periods. The model accounts for electromagnetic force, heat generations at the electrode/workpiece interface and faying surface between workpieces, and dynamic electrical resistance including bulk resistance and contact resistances at the faying surface and electrode/workpiece interfaces, which are functions of hardness, temperature, electrode force and surface condition. The computed results show that in contrast to dc, using ac readily produces the nugget in an ellipse shape. Deficit and excess of solute content occur in a thin layer around the boundary and interior of the nugget respectively.     

Influence of microstructure and weld size on the mechanical behaviour of dissimilar AHSS resistance spot welds

Abstract

Resistance spot welds were produced in dissimilar combinations of advanced high strength steels. A 600 MPa dual phase (DP) steel was welded to a high strength low alloy, a 780 MPa DP, and a 780 MPa transformation induced plasticity steel. The microstructure and mechanical properties were characterised using metallurgical techniques and lap shear and cross-tension testing. The results show that a pullout failure mode with improved mechanical properties is obtained when DP600 is paired with other advanced high strength steels, compared to the DP600 welded to itself, which is prone to interfacial failure and poor mechanical properties, given the same weld size. An in depth comparison of the interfacial to pullout failure transition in similar DP600 and DP780 and dissimilar DP600–DP780 welds was performed. The results show that the interfacial to pullout transition for the DP600–DP780 welds is significantly lower than with DP600 welded to itself. Increased fusion zone strength through dilution with the DP780 promotes button pullout at smaller weld sizes. Furthermore, it was observed that softening in the heat affected zone of DP780 promoted a pullout failure mode in that material.     

Determination of lower bound fracture toughness of a high strength low alloy steel welded joint

Abstract

The use of high strength low alloy steels for high performance structures (e.g. pressure vessels and pipelines) requires high strength consumables to produce an overmatched welded joint. This globally overmatched multipass welded joint contains two significantly different microstructures, as-welded and reheated. In this paper, the influence of weld metal microstructure on fracture behaviour is estimated in comparison with the fracture behaviour of composite microstructures (as-welded and reheated). The lower bound of fracture toughness for different microstructures was evaluated by using the modified Weibull distribution. The results, obtained using specimens with crack front through the thickness, indicated low fracture toughness, caused by strength mismatching interaction along the crack front. In the case of through thickness specimens, at least one local brittle microstructure is incorporated in the process zone at the vicinity of the crack tip. Hence, unstable fracture occurred with small, or without, stable crack propagation. Despite the fact that the differences between the impact toughness of a weld metal and the that of base metal are insignificant, the fracture toughness of a weld metal can be significantly lower.     

Application of electrode force change in single sided resistance spot welding using servo gun

Abstract

Single sided resistance spot welding (SSRSW) is considered as a feasible method to join hydroformed or closed section parts to others in vehicle productions. Unfortunately, it is difficult to guarantee the weld quality utilising conventional air gun. Because of a lack of support inside the closed section parts, the impact of electrode driven by pneumatic gun will cause large deformation of the workpieces at the welding stage and will lead to a crack around weld region after welding completion. In addition, poor weldability is another pressing problem for welding operations. Servo gun with new gun driven method has some merits such as realising the soft touch between electrode and workpieces and changing electrode force during the welding process which are greatly suitable to the sheet to tube joining. Based on the characteristics of servo gun, the present paper investigates a new method to increase the weld quality of sheet to tube joining with SSRSW method. By adjusting the electrode force during the welding process, weld strength would be increased, weld deformation be decreased and weldability lobe curve be widened. The results verify that servo guns can provide high assurance for welding quality of sheet to tube joining and have broad prospect in SSRSW.     

Numerical analysis of heat transfer process for double sided tungsten inert gas – metal inert gas weld pool

Abstract

Double sided arc welding is a new type of technology developed in recent years. Many experiments show that this technology has great advantages over single arc welding for the joining of intermediate thickness stainless steel and aluminium alloy base metals. In the present work, a three-dimensional transient numerical model is created to reveal the heat transfer process for a double sided tungsten inert gas (TIG) - metal inert gas (MIG) weld pool from the viewpoint of heat transfer literature and hydrodynamics. Considering of the features of the model, effective calculation software using finite element technology is adopted. The temperature fields in the weld pool for double sided TIG - MIG welding are successfully calculated; in addition, the configuration of the weld pool is also calculated.Comparisons show that thecalculatedresults agree approximately with the experimentally measured results. STWJ/310     

Effect of weld nugget size on overload failure mode of resistance spot welds

Abstract

In the present paper, effects of welding current, welding time, electrode pressure and holding time on the weld nugget size were studied. A failure mechanism was proposed to describe both interfacial and pullout failure modes. This mechanism was confirmed by SEM investigations. In the light of this mechanism, the effect of welding parameters on static weld strength and failure mode was studied. Then, an analytical model was proposed to predict failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout failure mode in shear tensile test. On the contrary to existing industrial standards, in this model, critical nugget diameter is attributed to metallurgical characterisation of material (weld nugget hardness to failure location hardness ratio), in addition to sheet thickness. For a given sheet thickness, decreasing H_WN/H_FL increases interfacial failure mode tendency. The results of this model were compared with experimental data and also with the literature.     

Resistance and friction stir spot welding of DP600: a comparative study

Abstract

Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Friction stir spot welding (FSSW) was invented as a novel method to spot welding sheet metal and has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn coated DP600 AHSS (1·2 mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process. Results show a correlation found among microstructure, failure loads, energy requirements and bonded area for both spot welding processes.     

Finite element analysis of effect of electrode pitting in resistance spot welding of aluminium alloy

Abstract

The effect of electrode pitting on the formation of the weld nugget in resistance spot welding of an aluminium alloy was investigated using the finite element method. Pitted electrodes were simulated by assuming a pre-drilled hole of varying diameter at the centre of the electrode tip surface. The results showed that a small pitting hole would not have a detrimental influence on the nugget size. The actual contact area at the electrode/sheet interface did not change significantly when the diameter of the pitting hole was increased. However, a large pitted area at the electrode tip surface resulted in a greatly increased contact area and hence reduced current density at the sheet/sheet interface, which in turn led to the formation of an undersized weld nugget. The numerical calculation of the nugget shape and dimensions agreed well with experimental observations.     

Probing the role of instantaneous current waveform in numerical modelling of resistance spot welding process

Abstract

In numerical modelling of the resistance spot welding process, a significant input parameter is the value of the weld current and traditionally, the rms value corresponding to the actual, instantaneous current waveform is used. The rms value implies a constant weld current for the entire weld time instead of the real-time current waveforms. Although the rms value represents an effective approximation of the real-time current waveform, the influence of the peak current and of the current slopes in each half cycle on the welding process cannot be realised in modelling when the rms value is used. Mathematically, an alternating current waveform with higher peak value and lower current on-time in each half cycle may correspond to a rms value that is nearly similar corresponding to another waveform with lower peak value and larger current on-time in each half cycle. The resulting rate of heating and the subsequent size of the weld nugget may not be the same for both the current signals since the resistive heating in resistance spot welding is transient in nature. This is precisely observed in the present work through a detailed investigation using three different ac spot welding machines. A two-dimensional, axisymmetric model is used to analyse the spot welding process using both the actual current waveform and the corresponding rms value as inputs. The computed weld dimensions show better predictions with the instantaneous current waveform as input rather than the corresponding rms value.     

Joint formation in dissimilar Al alloy/steel and Mg alloy/steel friction stir spot welds

Abstract

The microstructural features and overlap shear strength properties of friction stir spot welds made between Al 6111 and low carbon steel, and between Mg alloy AM60 and DP600 dual phase steel, are investigated. When Al 6111 is the upper sheet in the dissimilar sandwich, completed spot welds show evidence of intermetallic layer formation and cracking. Increasing tool pin penetration into the lower sheet provided increased mechanical interlocking of the sheets due to clinching. However, increasing penetration also promoted intermetallic formation and cracking in completed welds. However, dissimilar AM60/DP600 steel friction stir spot welds produced with AM60 as the upper sheet in the dissimilar sandwich do not show evidence of intermetallic formation and cracking may be avoided by removing the zinc coating on the DP600 steel before the friction stir spot welding operation.     

Finite element analysis of effects of soft zones on formability of laser welded advanced high strength steels

Abstract

Applications of advanced high strength steels in tailor welded blanks are a major focus to reduce vehicle weight and manufacturing cost, and to improve part performance. In the present work, formability of two types of laser welded steels in biaxial stretch forming has been studied. Laser welded blanks of the dual phase steel showed larger reduction of formability than those of high strength low alloy steel. This was due to formation of soft zones in the outer heat affected zone of dual phase steel where strain localisation and fracture occurred. Finite element simulations of dome testing incorporating local weld zone properties were able to accurately predict experimentally observed behaviour including deformation patterns and failure locations. In particular, the simulations have shown that under matching of the softened heat affected zone leads to reduced formability by localisation of strain; and that the fracture location changes from radial to circumferential plane when the flow curve of soft zone is evenly matched with the base metal. It was also concluded that the width of the soft zone has minor influence on formability during biaxial stretch forming.