Showing papers in "Science and Technology of Welding and Joining in 1998"

Investigation into properties of laser welded similar and dissimilar steel joints

Abstract: Laser beam welding is currently used in the welding of steels, aluminium alloys, thin sheets, and dissimilar materials. This high power density welding process has unique advantages of cost effectiveness, deep penetration, narrow bead and heat affected zone (HAZ) widths, and low distortion compared to other conventional welding processes. However, the metallurgical and mechanical properties of laser welds and the response of conventional materials to this new process are not yet fully established. The welding process may lead to drastic changes in the microstructure with accompanying effects on the mechanical properties and, hence, on the performance of the joint. The thermal cycles associated with laser beam welding are generally much faster than those involved in the conventional arc welding processes. This leads to the formation of a rather small weld zone that exhibits locally a high hardness in the case of C–Mn structural steels owing to the formation of martensite. It is currently difficult ...

Investigation of boundaries and structures in dissimilar metal welds

Abstract: Cracking, or disbonding, along the fusion boundary in dissimilar metal welds has been a persistent problem, particularly in applications where austenitic alloys are clad on to structural steels for corrosion protection. Many failures in dissimilar metal welds occur as a result of cracking along a boundary that runs parallel to the fusion boundary in the adjacent weld metal. A preliminary investigation was undertaken to determine the nature and evolution of boundaries and structure in dissimilar metal welds using a simple ternary system composed of a pure iron substrate and a 70Ni–30Cu (Monel) filler metal. Changes in base metal dilution were found to alter the evolution of boundaries and structures near the fusion boundary dramatically. Optical metallography and electron microanalysis reveal that the resulting weld microstructures and boundaries are similar to those observed in engineering materials used for cladding and corrosion resistant overlay. Transmission electron diffraction analysis revea...

Mechanisms of electrode degradation when spot welding coated steels

Abstract: The present work compares the behaviour of precipitation hardened and dispersion strengthened electrodes during the manufacture of resistance spot welds in coated steel sheet. The primary mechanism causing the failure of spot welding electrodes was growth of the electrode tip. Under normal welding conditions electrode tip growth was primarily dependent on local alloying morphology so that deterioration was mainly a function of the type of coating present on the steel. However, welding with high currents or use of current stepping programmes could extend electrode performance such that softening became a more dominant electrode degradation mechanism. Under these conditions, use of dispersion strengthened material could extend electrode campaign life.

Progress in joining of advanced materials: Part 1: Solid state joining, fusion joining, and joining of intermetallics

Abstract: Advanced materials generally require novel joining techniques. Developments in new materials research should be conducted hand in hand with work on weldability and joining capacity aspects. Sound joint quality for any new material has always been considered a milestone in a research and development scheme for a new material, particularly in terms of widespread applications. Better understanding of the microstructure–mechanical properties relationships of the bonded or welded joints will feed back to the materials development activities both in conventional and new materials areas. The two joining processes diffusion bonding and laser welding are considered in this literature review, since these processes are capable of joining a wide range of materials of interest in the aerospace industry, as well as in many other industrial applications, and offer remarkable advantages over conventional fusion welding processes. Of particular interest is the ability to join the more difficult aerospace alloys wi...

Progress in joining of advanced materials Part 2: Joining of metal matrix composites and joining of other advanced materials

Abstract: Advanced materials generally require novel joining techniques. Developments in new materials research should be conducted hand in hand with work on weldability and joining capacity aspects. Sound joint quality for any new material has always been considered a milestone in a research and development scheme for a new material, particularly in terms of widespread applications. Better understanding of the microstructure–mechanical properties relationships of the bonded or welded joints will feed back to the materials development activities both in conventional and new materials areas. The two joining processes diffusion bonding and laser welding are considered in this literature review, since these processes are capable of joining a wide range of materials of interest in the aerospace industry, as well as in many other industrial applications, and offer remarkable advantages over conventional fusion welding processes. Of particular interest is the ability to join the more difficult aerospace alloys wi...

Chill zone formation in Al–Li alloy welds

Abstract: The presence of a fine equiaxed zone (also known as the 'chill zone') along the fusion boundaries in Al–Li alloy welds has recently been reported. Preferential failure along the chill zone during tensile testing has also been observed. Such zones have not been reported in other aluminium alloy welds. The chill zone was considered to form as a result of high undercooling during solidification of Al–Li alloys. In another study, Al3(Li,Zr) aided nucleation was proposed to be the mechanism of nucleation of the chill grains. The present work demonstrates that the chill zone forms only under conditions when the base alloy contains Al3Zr particles before welding. The presence of Al3Zr particles within the chill grains has been shown for the first time. The grain structure of the base metal does not have any role in the formation of the chill zone. A chill zone is also observed in a non-Li bearing Al alloy which contains Zr. It is further shown that in pulsed current and oscillating arc welds, the chill ...

Effect of nitrogen on corrosion of duplex stainless steel weld metal

Abstract: A type 329Jl duplex stainless steel was gas tungsten arc welded without filler material in an Ar–N2 gas mixture atmosphere with the aim of changing only the nitrogen content in the weld metal. The effect of nitrogen on the microstructure and corrosion properties of the weld metal was examined. An increase in nitrogen partial pressure increased the nitrogen content of the weld metal and brought reductions in the ferrite content and the quantity of Cr2N nitride precipitates. Three corrosion parameters, namely, critical pitting temperature (CPT), pitting potential, and corrosion rate, were measured for weld metals having different nitrogen contents. The CPT and pitting potential increased and corrosion rate decreased with increasing nitrogen content of the weld metal. The corrosion behaviour was explained in terms of changes in microstructure and pitting index depending on the nitrogen content of the weld metal.

Application of thermal modelling to laser beam welding of aluminium alloys

Abstract: A thermal model has been developed for laser welding which describes the heat input in terms of point and line sources. The model was used to generate weld profiles which closely matched those found by experiment. Outputs of the model (the thermal gradient GL and the growth rate R) were used to describe the macroscopic grain structure found along the weld centreline. Columnar structures were predicted at low welding speeds (high GL/R ratio) and equiaxed structures at high welding speeds (low GL/R ratio). Using the thermal model, cooling rates of ∼1500 K s–1 were estimated for the lowest welding speed, which increased by an order of magnitude for the highest welding speed considered. There was excellent agreement between the dendrite secondary arm spacings measured by experiment and those predicted using the thermal model.

Finite element analysis of electrode wear mechanisms: face extrusion and pitting effects

Abstract: Resistance spot welding is one of the major Joining methods widely used in automotive body fabrication and assembly. Electrode wear has been a major concern in resistance spot welding of galvanised steel and in this paper advanced finite element techniques were used to simulate the thermomechanical interactions between the electrode and the workpiece during welding. First, the coupled electric–thermal–mechanical process associated with nugget formation was studied. The cumulative effects on electrode face deformation were then simulated over a large number of welds. Electrode face pitting effects were also examined. It was found that welding process parameters, such as holding time and pressure trace, played a key role in electrode face extrusion. Any pitting on the electrode tended to accelerate the face extrusion process.

Fast method for measuring power density distribution of non-circular and irregular electron beams

Abstract: A fast method for measuring the power density distribution of electron beams has been developed. The method employs a refractory metal disc containing regularly spaced radial slits and a Faraday cup to measure electron beam profiles as the beam is oscillated in a circular pattern over the disc. This beam profiling method can be used for real time focusing along a given beam sweep orientation by monitoring one of the beam profiles. When the focus setting is satisfactory, all of the beam profiles can then be stored for computed tomography (CT) reconstruction. The CT reconstruction renders an image of the powder density distribution and provides beam statistics as a permanent file for process control records. The process (taking ∼1 min to perform in its entirety using a conventional PC) was used to investigate the influence of focus setting on the power density distribution of 5 mA electron beams operating at accelerating voltages of 80 and 140 kv. Results of these tests show the effects of beam asti...

Quantitative modelling of motion, temperature gyrations, and growth of inclusions in weld pool

Abstract: The velocity and temperature fields, the shape and size of the fusion zone, and the motion of the inclusions were calculated by the solution of equations of conservation of mass, momentum, and energy in three dimensions for various welding conditions. The loci and the temperature gyrations experienced by a large number of inclusions were calculated numerically to seek a statistically meaningful residence time distribution of the inclusions and the number of intensities of the temperature cycles experienced by these particles. Finally, both the growth and dissolution of the inclusions were considered to calculate their size distribution. The inclusions experienced considerable recirculatory motion and strong temperature gyrations along their paths in the weld pool. The temperature–time plots for most of the inclusion particles displayed several temperature peaks. However, about one-third of the particles experienced continuous cooling behaviour. The average number of the temperature peaks in the t...

Enhanced dissolution of nitrogen during gas tungsten arc welding of steels

Abstract: Although nitrogen concentrations at levels much higher than Sieverts' Law predictions during the arc welding of iron and steel are well established, there is currently no commonly accepted methodology to determine this concentration quantitatively. The nature and concentrations of various species in the plasma phase above the weld pool surface are therefore investigated in the present work using both theoretical and experimental techniques. A comprehensive thermodynamic analysis of the nitrogen containing plasma phase of a gas tungsten welding arc shows that ionised species dominate close to the electrode, whereas neutral monatomic and diatomic nitrogen are the primary species near the metal surface at plasma temperatures as low as 5000 K. When oxygen is added to a nitrogen containing plasma, the resulting nitrogen concentration in the weld metal is further enhanced. Definitive proof is provided for a mechanism in which nitrogen and oxygen species interact in the plasma phase at temperatures below...

Metallurgical investigation of laser welds in wrought Waspaloy

Abstract: Wrought Waspaloy was autogenously welded by means of Nd-YAG pulsed and CO2 continuous wave lasers and the microstructures of laser welds investigated using scanning and transmission electron microscopy. The fusion zones of as welded specimens contained fine γ′ and isolated MC carbides. The hardness profile of laser welds in wrought Waspaloy peaked at the fusion zone and heat affected zone (HAZ) boundaries, owing to fine γ′ precipitation and MC carbides in the fusion zone, and γ′ and M23C6 precipitates at the HAZ boundaries. After a full post-weld heat treatment, the hardness of the bead can be recovered to around 400 IIV but the hardness peaks at the HAZ boundaries became higher relative to the fusion zone and base material after aging at 450°C for 20 h. The microstructures in the fusion zone of post-weld heat treated welds consist of M23C6, MC, and γ′ which are quite uniform in size. The low cycle fatigue (LCF) strength of laser welds decreased to 400 MPa compared with 450 MPa for the base materi...

Effect of nature of grain boundaries on intergranular liquation during weld thermal cycling of nickel base alloy

Abstract: An Inconel 718 based alloy containing only trace amounts of C, S, and P was subjected to thermal cycling in a Gleeble 1500 system to induce microstructural features which are produced in the heat affected zone (HAZ) during welding. In the thermally cycled material many grain boundaries were observed to liquate; however, the liquation was heterogeneously distributed, that is, only some grain boundaries, and often only a few segments of the same grain boundary, liquated. Orientation imaging microscopy and electron backscatter diffraction analysis in a scanning electron microscope revealed that the liquated grain boundaries or segments of a grain boundary that liquated were always non-special geometry (or random) boundaries, whereas low reciprocal volume density Σ coincident site lattice and twin boundaries did not liquate. Therefore, it is concluded that the extent of B segregation, to which liquation of grain boundaries in the H AZ during welding has been attributed, is greater on random boundaries...

Effect of filler metal composition on weldability of Al-Li alloy 1441

Abstract: Hardness, tensile properties, and hot cracking susceptibility of Al–Li alloy 1441 were evaluated with respect to different filler alloys AA 2319, AA 4043, and AA 5356, as well as the parent alloys. The hardness in the as welded condition was 70–90 HV and improved by 20, 30, 40, and 40 HV after heat treatment with AA 4043, AA 5356, and AA 2319, and 1441 fillers respectively. Tensile strength showed similar trends as hardness in the as welded and heat treated conditions. The hot cracking tendency was the maximum for welds deposited with 1441 filler and the minimum for welds deposited with AA 5356 filler, both values being less than the cracking tendency for autogenous welds. Hot cracking tendency was correlated with grain size, segregation distance, and distribution of the low melting phases.

Fusion zone and heat affected zone cracking susceptibility of stabilised austenitic stainless steels

Abstract: Fusion zone (FZ) and heat affected zone (HAZ) cracking behaviour of three stabilised stainless steels were studied using the Varestraint hot cracking test. Materials ranged in solidification mode from fully austenitic D9 to primary austenitic 347 and primary ferritic 321. The materials exhibited high cracking susceptibility in the FZ and base metal HAZ. In the weld metal HAZ, type 347 was highly susceptible, while alloy D9 and type 321 were resistant to cracking. The FZ cracking behaviour could be related to the stabilisation ratio (Nb, Ti)/C rather than the P + S content and Creq/Nieq ratio, as in unstabilised stainless steels. In fact, analysis of cracking data using the Creq/Nieq ν. P + S content diagram did not reveal any systematic trend in cracking susceptibility. In the base metal HAZ, the high cracking in type 347 as well as in type 321 was found to be caused by the high impurity levels in conjunction with the presence of niobium and titanium. Cracking occurred much less in alloy D9.

Autogenous laser welding of austenitic stainless steel to copper alloy

Abstract: A technique for the autogenous laser welding of stainless steel to copper alloy in a butt joint has been developed, by which microfissuring in the heat affected zone of stainless steel can be eliminated. The impinge direction and the offset of the laser beam played the dominant roles in achieving this. The sound weld of AISI 316L–C18100 contained 3·0 wt-% copper in the fusion zone with dendrite structures. However, banding and spherical structures were present in the fusion zone with a high percentage of copper and microfissuring was strongly related to the distribution of copper in this region. A model is proposed for achieving a sound butt joint and the mechanism of microfissuring is also discussed.

Non-destructive measurement of residual stresses in carbon steel weld joints

Abstract: Residual stress measurements have been carried out on 8 and 12 mm thickness carbon steel single V weld joints by X -ray diffraction and ultrasonic techniques. The maximum tensile and maximum compressive stresses on the surface of the 12 mm carbon steel weld joint are higher than those on the surface of the 8 mm carbon steel weld joint. The results also indicate that the variation in the surface residual stress across the weld is different from that for the through thickness residual stresses. The effect of constraint, which depends on the thickness of the weld joint, is found to influence the surface and through thickness residual stresses. The effect of stress relief annealing heat treatment on residual stresses was also studied.

Current stepping programmes for maximising electrode campaign life when spot welding coated steels

Abstract: Improvements in electrode life are possible when resistance spot welding coated steels, through the application of current stepping techniques. The present investigation has evaluated a number of criteria for establishing optimum rates of current increase. The most significant improvements in welding performance were obtained when the current was increased as a function of electrode tip growth. It was found that electrode softening occurred at a faster rate when current stepping, compared to welding using standard conditions. This softening was caused by recrystallisation of the base electrode material and resulted in a fast tertiary stage of electrode tip growth. The practical implications of the work are discussed.

Effect of nitrogen on toughness and strain age embrittlement of ferritic steel weld metal

Abstract: Investigations were carried out to evaluate the influence of dissolved nitrogen in ferritic steel weld metal on its toughness and strain aging behaviour through fracture mechanics as well as conventional impact testing approaches. Manual metal arc C–Mn steel weld metals with nitrogen contents between 80 and 210 ppm (wt) were investigated under four different post-weld conditions, namely, (i) as welded, (ii) stress relieved, (iii) artificially strain aged, and (iv) artificially strain aged and stress relieved. Quantitative metallography and low load microhardness studies of microphases were an integral part of these investigations. The results demonstrate the highly detrimental effect of nitrogen on the toughness behaviour of C–Mn steel weld metal, particularly under strain aging conditions. This is substantiated by the decrease of acicular ferrite content with the accompanying increase in primary ferrite and ferrite with second phases in the microstructures. Also, there is a distinctive increase i...

Residual stress measurement in ferritic steel tube welds using X-ray diffraction

Abstract: The X-ray diffraction (XRD) technique has been used to measure the residual stresses before and after post-weld heat treatment in autogenous butt weld joints in 2·25Cr–1 Mo steel tubes. The tubes are used in the steam generator assemblies of fast breeder reactors. Measurement data show that the stress in the weld centre is ∼150 MPa maximum. The stress distribution around the weld centre is asymmetrical with larger amounts of compressive stress on the side of the tube sheet block. The stress becomes compressive beyond ∼5 mm from the weld centre reaching values of ∼200 MPa and more. Post-weld heat treatment (PWHT) at 923 K for 30 min removes most of the residual stresses and PWHT at 988 K for 2·5 h removes all the residual stresses in the weld regions.

Modelling of heat transfer in short circuiting gas metal arc welding

Abstract: This paper deals with heat transfer during short circuiting gas metal arc welding of mild steel. Three major heat sources can be distinguished: arc heating of the anode, arc heating of the cathode, and Joule heating of the electrode extension. Based on these heat sources a model is presented which describes the heat transfe; both to the electrode and to the workpiece. The validity of the model was tested by comparing predicted values of the heat transfer with experimental data. It appears that good agreement exists between model and experiment.

Algorithm based on non-linear programming method for optimum heat input control in arc welding

Abstract: A simple mathematical method for the estimation of the optimum heat inputs to make the temperature field coincide with a requirement during arc welding has been developed. The optimisation problem in the present work means the objective function which is defined as the difference between the calculated and required temperatures. An analytical solution of the heat flow equation is applied for the calculation of the temperature field in welding, and non-linear programming methods, i.e. the convex programming method and the steepest descent method, have been used to solve the objective function in the problem mathematically. It is concluded that both optimising methods are sufficient to determine the optimum heat input pattern in the case of gas tungsten arc (GTA) welding on thin plate and small sized pipe specimens. Furthermore, the applicability of the estimated optimum heat inputs is evaluated experimentally by bead on plate welding tests.

Derivation of carbon equivalent to assess hardenability of steel

Abstract: A method to derive carbon equivalent to assess hardenability is demonstrated. The assumptions used in the present method are the additivity rule and the empirical calculation of the incubation time in the time–temperature–transformation diagram. In the present study, the empirical equation for the incubation time before ferrite, pearlite, and bainite transformations proposed by Kirkaldy et al. is employed. The coefficients for silicon, manganese, copper, and nickel calculated in the present study show good agreement with experiments, whereas a considerable discrepancy is observed for the coefficients for chromium and molybdenum. Prior austenite grain size, which influences hardenability, can be incorporated in the carbon equivalent using the method presented in this study.

Laser welding of high hardness armour steel

Abstract: A feasibility study of laser welding of high hardness steel (HHS) armours has been carried out at SMC DERA. The main issues investigated were weldability using different fillers and various combinations of beam power and travel speed; heat affected zone (HAZ) size and hardness; Charpy toughness; and ballistic behaviour. The best results in terms of a combination of cracking resistance and toughness were obtained in the autogenous weld, the filler wires tested clearly not being ideally suited to laser welding. A relationship emerged between the weld hardness profiles, in respect of both weld metal hardness and variable HAZ width and softening with beam travel speed, and their ballistic performance.

Analysis of arc voltage, current, and light signals in pulsed gas metal arc welding of aluminium

Abstract: The use of readily acquired weld arc data such as voltage, current, or light emissions to identify droplet transfer events or transfer mode, or for quality purposes such as seam tracking is of interest for thin section applications in arc welding of aluminium. In the present study, voltage, current, and light emission signals are acquired at high rates, and synchronised with high speed shadowgraphing. The usefulness of the various signals for identifying droplet detachment events and transfer modes during pulsed gas metal arc welding of aluminium is compared. It is found that individual droplet detachments are consistently identifiable during globular, spray, and streaming transfer, and during peak and background current detachment, using the voltage signal. Although the arc light signal also contains droplet detachment data, it is difficult to identify droplet detachments occurring during the background phase at low background current levels. Preliminary measurements show that these signals can a...

Laser welding with powder feed

Abstract: Laser welding with a powder feed has been investigated using a 3·0 kW CO2 laser system. This type of welding is a modification of autogenous laser welding. In the present investigation, a straight thin tube nozzle was used to deliver powder to the welding area by a computerised powder feeder. The delivered powder is melted and mixed by interaction with the laser beam to form a weld with the base metal. This technique can extend the capabilities of autogenous laser welding while retaining many of its advantages. A series of welding parameters was used to produce butt joints with a satisfactory weld bead profile at various joint gap sizes. The main objective of the present study was to use a power feed to accommodate joint gaps which often present problems in autogenous laser welding. Promising results have been obtained. A joint gap of up to 1·0 mm has been welded successfully in 3·0 mm thickness steel: a great improvement compared with autogenous laser welding. In the present study, weld microstru...

Diffusion bonding of Ti–25Al–10Nb alloy

Abstract: Titanium aluminide is a potential material for high temperature structural application. Recent research on Ti–25Al–10Nb indicates that the alloy possesses excellent superplasticity at high temperature and is suitable for diffusion bonding. In this study, the bonding process parameters, such as surface roughness, bonding temperature, bonding time, bonding pressure, and post-bond heat treatment (PBHT), affecting the properties of bonded joints were investigated. The American Welding Society constrained single lap shear test was applied to assess the quality of the joints. Experimental results showed that a smooth surface roughness is beneficial for the diffusion bonding of Ti–25Al–10Nb. Shear strength of joints increased with the bonding temperature, bonding pressure, and bonding time. However, in the bonding temperature range 850–950°C, shear strengths of joints were all less than 80% of that of the base metal in the single lap shear test. After PBHT at 1050°C for 120 min, the quality of joints cou...

Heterogeneity of coarsened oxide inclusions in C-Mn steel welds

Abstract: A preliminary investigation of the coarsening of oxide inclusions has shown that the heterogeneous nature of the oxide inclusion, i.e. crystalline particles in a glassy matrix, is retained in the coarsened inclusions. There is evidence that the crystalline particles may grow during the coarsening process.

Structural evolution of pulsed Nd-YAG laser welds of AISI 1006 steel

Abstract: Although correlations of welding parameters with the metallurgical features of conventional fusion welds in low carbon steels are well established, information on process–structure–property relationships associated with pulsed laser welds is more limited. This paper presents results on the characterisation of weld metal and heat affected zone (HAZ) microstructures observed in laser welded AISI 1006 steel. Pulsed Nd-YAG laser welds in the bead on plate configuration were used for this purpose, both in overlapping and non-overlapping bead configurations. As very rapid heating and cooling cycles occur during laser welding, the microstructures observed in the weld metal are the result of rapid solidification producing thin columnar austenite grains extending from the fusion boundary, which transform to martensite and bainite during fast cooling to ambient temperature. The HAZ structure in the base plate can also be rationalised in terms of the rapid thermal cycling experienced. The HAZ is narrow with ...