On formability of tailor laser welded blanks of DP/TRIP steel sheets

Abstract

This paper aims to evaluate the formability of tailor welded blanks of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets. In this work, bead on plate butt joints of 2·5 mm DP600 and 1·2 mm TRIP700 steel sheets were performed using CO₂ laser beam welding. Microhardness measurements and transverse tensile testing were carried out to characterise the welds. The formability of base metals and welds were investigated by standard Erichsen test. In a perpendicular tensile test to the weld line, all specimens were fractured at the TRIP base metal, and the strengths were somewhat higher than those of base metal. There was a significant reduction in formability caused by welding of the DP600/TRIP700 steel sheets, and the formability increased with increasing welding speed.     

Microstructural characterisation of stir zone containing residual ferrite in friction stir welded 304 austenitic stainless steel

Abstract

Friction stir welding was applied to a 2 mm thick 304 austenitic stainless steel plate. The microstructural evolution and hardness distribution in the weld were investigated. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively, which are typically observed in friction stir welds in aluminium alloys. The hardness of the SZ was higher than that of the base material and the maximum hardness was observed at the TMAZ. The higher hardness at the TMAZ was attributed to high densities of dislocations and subboundaries. Microstructural observations revealed that the ferrite was formed along grain boundaries of the austenite matrix in the advancing side of the SZ. It is suggested that the frictional heat due to stirring resulted in the phase transformation of austenite to ferrite and that upon rapid cooling the ferrite was retained in the SZ.     

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.     

Study of friction stir joining of thin aluminium sheets in lap joint configuration

Abstract

Thinning in friction stir lap joints and its relation with the process variables was investigated. Friction stir welds were made on 1 mm thick AA6111 aluminium alloy sheets in order to study the effects of rotation rate, traverse speed, plunge depth, tilt angle and pin height on faying surface defects. Sheet thinning on the advancing and retreating sides was quantified and the lap shear strength of the joints was evaluated. A decrease in the pitch of the runs at constant rotation rate increased the sheet thinning and reduced the joint strength in a linear manner. Process pitch and pin height were found to be the most critical factors in determining the faying surface lift up. A pin of the same height as the sheet thickness resulted in maximum lap shear strength. Tool tilt did not show a significant effect on the sheet thinning.     

Residual stresses simulation for friction stir welded joint

Abstract

As a solid state joining technique, friction stir welding (FSW) can produce high strength, low distortion joints efficiently. Compared to fusion welding, residual stresses in FSW joints are expected to be low due to a relatively low heat input. However, apart from the heat input, the force from the tool also plays an important role in the development of welding stresses. In the present paper, a semicoupled thermomechanical finite element model containing both thermal load and mechanical load was established to simulate the development of welding stresses during FSW process; an autoadapting heat source model was employed in the thermal analysis; the fixture was also included in the mechanical analysis model. The simulation results showed that due to the effect of the tool force, the longitudinal residual tensile stresses became smaller and were asymmetrically distributed at different sides of the weld centre; the peak of the tensile residual stresses at the retreating side was lower than that at the advancing side. Calculated and experimental results were compared.     

Numerical modelling of 3D plastic flow and heat transfer during friction stir welding of stainless steel

Abstract

Three-dimensional (3D) viscoplastic flow and temperature field during friction stir welding (FSW) of 304 austenitic stainless steel were mathematically modelled. The equations of conservation of mass, momentum and energy were solved in three dimensions using spatially variable thermophysical properties using a methodology adapted from well established previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering strain rate and temperature dependent flow stress. The computed profiles of strain rate and viscosity were examined in light of the existing literature on thermomechanical processing of alloys. The computed results showed significant viscoplastic flow near the tool surface, and convective transport of heat was found to be an important mechanism of heat transfer. The computed temperature and velocity fields demonstrated strongly 3D nature of the transport of heat and mass indicating the need for 3D calculations. The computed temperature profiles agreed well with the corresponding experimentally measured values. The non-Newtonian viscosity for FSW of stainless steel was found to be of the same order of magnitude as that for the FSW of aluminium. Like FSW of aluminium, the viscosity was found to be a strong function of both strain rate and temperature, while strain rate was found to be the most dominant factor. A small region of recirculating plasticised material was found to be present near the tool pin. The size of this region was larger near the shoulder and smaller further away from it. Streamlines around the pin were influenced by the presence of the rotating shoulder, especially at higher elevations. Stream lines indicated that material was transported mainly around the pin in the retreating side.     

Microstructures and properties of friction stir spot welded DP590 dual phase steel sheets

DP590 steel sheets were joined by friction stir spot welding using polycrystalline cubic boron nitride tool with an objective to produce bond diameters similar to conventional spot welding nuggets. A range of spindle rotation (400–2400 rev min^?1) and plunge speeds (0·03–3·8 mm s^?1) were exercised to attain defect free welds in 1·6 mm thick sheets. A bond diameter of 4t^1/2, alike minimum nugget diameter criteria for resistance spot welds, resulted in superior mechanical properties than conventional spot welds. The heat inputs corresponding to different welding parameters influenced the weld microstructure, including grain size, phases and their morphology. The bond diameter was higher for higher heat inputs. However, low heat input welds with weld time cycles ～4 s produced more refined microstructure and exhibited similar strengths even with reduced bond size. Plug type failure was associated with larger bond diameters (～7·1 mm), while interfacial failure was observed with smaller welds (～5·4 mm).     

Static and fatigue performance of weld bonded dual phase steel sheets

Abstract

Lap joints of dual phase steel sheets of 1·0 mm were prepared by adhesive bonding, spot welding and weld bonding processes using a one component epoxy base structural adhesive. Mechanical properties of the joints were evaluated by tensile shear and fatigue tests. The size of the weld nugget for both spot weld and weld bond was measured for different welding parameters (current, time) and compared. For identical welding parameters, weld bonded nuggets exhibit higher nugget diameter. Tensile shear strength of weld bonded joints is 40 and 58% higher than spot welded joints and 15 and 39% higher than adhesive bonded joints and for DP590 and DP780 steels respectively. Considering 10⁶ cycles, the endurance limit of weld bonded joint is much higher than spot welded joint but smaller than adhesive bonded joints. Overall the performance of weld bonded joints is superior to those of resistance spot welding.     

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.     

New technological methods and designs of stir head in resistance friction stir welding

Abstract

This paper proposes a new welding method, resistance friction stir welding (RFSW), which provides a novel method to solve some of the challenges of friction stir welding high melting point of ferrous and non-ferrous metals. The RFSW is a new hybrid welding technology, which utilises the advantages of both resistance welding and friction welding. The technological principle of RFSW uses the compound effects of resistance heating and friction heating; the integration methods are discussed and analysed in this paper. It is believed that RFSW is an innovative, practical welding technique. The national patents for invention have already been applied in China.     

Process forces during friction stir welding of aluminium alloys

Abstract

Flow and consolidation of the material under the tool shoulder and subsequent nugget formation are among the least understood aspects of friction stir welding and processing (FSW/P). Welding parameters and tool profile impact the process forces acting on the tool. This work is an observational study of the process forces associated with bead on plate runs on two aluminium alloys, 6061 and F357. Polar plots of the resultant forces acting on the tool spindle are analysed and correlated to the process parameters. The dependence of the nugget's width with various heat indices is evaluated.     

Peak temperatures and microstructures in aluminium and magnesium alloy friction stir spot welds

Abstract

The peak temperatures during friction stir spot welding of similar and dissimilar aluminium and magnesium alloys are investigated. The peak temperatures attained during friction stir spot welding of Al 6111, Al 2024, and AZ91 are within 6% of their solidus temperatures. In dissimilar AZ91/Al 6111 spot welds the peak temperature corresponds with the α-Mg solid solution and Mg₁₇Al₁₂ eutectic temperature of 437°C. An a-Mg plus Mg₁₇Al₁₂ eutectic microstructure is produced in dissimilar friction stir spot welds when material displaced during pin penetration into the lower sheet material contacts the upper sheet material at the eutectic temperature.     

Influence of forge pressure on properties of friction welded pipelines using intermediate ring

Abstract

An innovative welding method for fully automatic joining of pipelines has been developed. The proposed welding procedure is a variant of the conventional friction welding process. A rotating intermediate ring is used to generate heat necessary to realise the weld. The working principles of the welding process are described. The influence of the forge pressure on the mechanical properties of the welds and the heat affected zone microstructure was experimentally investigated. It was found that the forge pressure had no influence either on the mechanical properties or on the weld microstructure, which is in contradiction with the published data in literature concerning conventional friction welding.     

Microstructural modification of cast Mg alloys by friction stir processing

Abstract

Cast Mg alloys were processed using friction stir processing (FSP) to acquire a fine grained structure and high strength. Actually, FSP is a novel grain refinement method for light metal alloys. Using FSP, a cast microstructure with coarse grain size was refined to equiaxial fine grain through dynamic recrystallisation; second phase particles were finely dispersed by FSP. Moreover, FSP is effective to eliminate cast defects such as microshrinkages or porosities. Commercial die cast Mg alloy (AZ91D) and high strength Mg–Y–Zn alloy plates were prepared for FSP. Heat input using a rotational tool during FSP closely affected the microstructure in the stirred zone. Actually, FSP with lower heat input produced a finer grain size and higher hardness. Changes in the friction stir processed microstructures affecting mechanical properties were not only grain refinement, but also second phase particle distributions. Results show that alloys with high hardness by FSP have finely dispersed second phase particles without dissolution during FSP.     

Effect of tool rotation rate on microstructure and mechanical properties of friction stir welded copper

Abstract

Defect free copper welds were achieved by friction stir welding (FSW) carried out at a constant welding speed of 100 mm min^?1. The influence of tool rotation rate on microstructure, mechanical properties and fracture location was investigated. As the tool rotation rate increased, the grains of nugget zone grew significantly, the thermomechanically affected zone became indistinct and the grain size increased, but the effect of tool rotation rate on the grain size of heat affected zone was limited. Both ultimate tensile strength (UTS) and elongation increased first and then decreased with increasing rotation rate and the UTS achieved a highest value of 282 MPa at the rotation rate of 400 rev min^?1 together with the welding speed of 100 mm min^?1, which was on the level of the base metal. The fracture occurred at the cavity defect on the advancing side of the joint when the FSW was performed at a low tool rotation rate, while it occurred on the retreating side when the tool rotation rate was relatively high.     

Modelling of 7050 aluminium alloy friction stir welding

Abstract

A thermal model combined with a microstructural and yield strength model has been developed to give a prediction of precipitate evolution and strength in the as welded and post-weld heat treated condition for friction stir welding of 7xxx aerospace aluminium alloys. This fully coupled model is applied to an overaged high strength 7050 aluminium alloy friction stir welded using a range of welding rotation and translation speeds. The evolution of the microstructure has been predicted as a function of the process parameters. The resulting microstructural evolution is shown to be a complex function of both peak temperature observed during the weld cycle and heating/cooling rates. Yield strength has been calculated from the microstructural predictions and a comparison between predicted yield strength and measured hardness has been used to test the modelling approach. Reasonably good agreement between model and experiment is found over the wide range of process parameters investigated.     

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.     

The effect of cryogenic CO2 cooling on corrosion behaviour of friction stir welded AA2024-T351

Abstract

Cryogenic cooling with CO₂ was applied during friction stir welding of AA2024-T351 in order to reduce the temperature increase during welding, and thus improve the corrosion resistance of the weld. The effect of cryogenic cooling on corrosion susceptibility was investigated with gel visualisation, immersion tests and local electrochemical measurements. The most susceptible area for both uncooled and cooled welds was in the heat-affected zone (HAZ) region, which showed intergranular attack. Cryogenic cooling had no detectable influence on the degree of anodic reactivity in the weld region. However, it did decrease the width of the reactive HAZ.     

CFD modelling of friction stir welding of thick plate 7449 aluminium alloy

Abstract

CFD modelling of friction stir welding has been conducted to understand and optimise the welding of thick, 7449 aluminium alloy for aerospace applications. The aim is to produce high strength, defect free welds that do not break the tool. The models compared different pin profiles and rotation speeds and were undertaken in two stages. The first stage involved creating a thermal model to better understand the generation and flow of heat. The second stage involved analysing the flow near the tool with a two-dimensional model. The traversing force results from the two-dimensional planar models compared favourably with experimental findings. The pressure distribution and deformation region size were compared for the different models. Novel maps of the deformation conditions experienced in each weld were produced. The analysis suggested reasons why some pin profiles and rotation speeds are preferable to others and explained the difference in the traversing force measurements.