Showing papers on "Heat-affected zone published in 2009"

Effect of welding processes on tensile properties of AA6061 aluminium alloy joints

Abstract: The present investigation is aimed at to study the effect of welding processes such as GTAW, GMAW and FSW on mechanical properties of AA6061 aluminium alloy. The preferred welding processes of these alloys are frequently gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) due to their comparatively easier applicability and better economy. In this alloy, the weld fusion zones typically exhibit coarse columnar grains because of the prevailing thermal conditions during weld metal solidification. This often causes inferior weld mechanical properties and poor resistance to hot cracking. Friction stir welding (FSW) is a solid phase welding technique developed primarily for welding metals and alloys that heretofore had been difficult to weld using more traditional fusion techniques. Rolled plates of 6 mm thickness have been used as the base material for preparing single pass butt welded joints. The filler metal used for joining the plates is AA4043 (Al-5Si (wt%)) grade aluminium alloy. In the present work, tensile properties, micro hardness, microstructure and fracture surface morphology of the GMAW, GTAW and FSW joints have been evaluated, and the results are compared. From this investigation, it is found that FSW joints of AA6061 aluminium alloy showed superior mechanical properties compared with GTAW and GMAW joints, and this is mainly due to the formation of very fine, equiaxed microstructure in the weld zone.

The arc characteristics and metal transfer behaviour of cold metal transfer and its use in joining aluminium to zinc-coated steel

Abstract: Cold metal transfer (CMT) is a modified metal inert gas welding process based on short-circuiting the transfer process, characterised by low heat input and no-spatter welding. The arc characteristics and its droplet transfer process have been studied by high-speed video photography. The process was used to join aluminium to zinc-coated steel. The results shows that no-spatter welding and low heat input during the welding process can be realized by CMT, and a dissimilar metal joint with good performance can be obtained by the CMT process.

Modelling of thermal plasmas for arc welding: the role of the shielding gas properties and of metal vapour

Abstract: The methods used to model thermal plasmas, including treatments of diffusion in arcs in gas mixtures, are reviewed. The influence of thermophysical properties on the parameters of tungsten–inert-gas (TIG) welding arcs, particularly those that affect the weld pool, is investigated using a two-dimensional model in which the arc, anode and cathode are included self-consistently. The effect of changing each of six thermophysical properties on the characteristics of an argon TIG arc is assessed. The influence of the product of specific heat and mass density is found to be particularly important in determining the arc constriction. By examining the influence of the different properties on the heat flux density, current density and shear stress at the anode, it is concluded that the weld pool depth can be increased by using shielding gases with high specific heat, thermal conductivity and viscosity. The effect of metal vapour on the arc and weld pool properties is assessed. The most important effect of the metal vapour is found to be the increased electrical conductivity at low temperatures, which leads to lower heat flux density and current density at the weld pool, implying a shallower weld pool.

High quality welding of stainless steel with 10 kW high power fibre laser

Abstract: The objectives of this research are to investigate penetration characteristics, to clarify welding phenomena and to develop high quality welding procedures in bead on plate welding of type 304 austenitic stainless steel plates with a 10 kW fibre laser beam. The penetration depth reached 18 mm at the maximum at 5 mm s−1. At 50 mm s−1 or lower welding speeds, however, porosity was generated at any fibre laser spot diameter. On the other hand, at 100 mm s−1 or higher welding speeds, underfilling and humping weld beads were formed under the conventionally and tightly focused conditions respectively. The generation of spatters was influenced mainly by a strong shear force of a laser induced plume and was greatly reduced by controlling direction of the plume blowing out of a keyhole inlet. The humping formation was dependent upon several dynamic or static factors, such as melt volume above the surface, strong melt flow to the rear molten pool on the top surface, solidification rate and narrow molten poo...

Heat transfer and fluid flow during electron beam welding of 21Cr–6Ni–9Mn steel and Ti–6Al–4V alloy

Abstract: Electron beam welding (EBW) of two important engineering alloys, Ti–6Al–4V and 21Cr–6Ni–9Mn, was studied experimentally and theoretically The temperatures at several monitoring locations in the specimens were measured as a function of time during welding and the cross-sections of the welds were examined by optical microscopy The theoretical research involved numerical simulation of heat transfer and fluid flow during EBW The model output included temperature and velocity fields, fusion zone geometry and temperature versus time results The numerically computed fusion zone geometry and the temperature versus time plots were compared with the corresponding experimentally determined values for each weld Both the experimental and the modelling results were compared with the corresponding results for the keyhole mode laser beam welding (LBW)Both experimental and modelling results demonstrate that the fusion zone size in Ti–6Al–4V alloy was larger than that of the 21Cr–6Ni–9Mn stainless steel during both the electron beam and laser welding Higher boiling point and lower solid state thermal conductivity of Ti–6Al–4V contributed to higher peak temperatures in Ti–6Al–4V welds compared with 21Cr–6Ni–9Mn stainless steel welds In the EBW of both the alloys, there were significant velocities of liquid metal along the keyhole wall driven by the Marangoni convection In contrast, during LBW, the velocities along the keyhole wall were negligible Convective heat transfer was important in the transport of heat in the weld pool during both the laser and the EBW The computed keyhole wall temperatures during EBW at low pressures were lower than those during the LBW at atmospheric pressure for identical heat input

Dissimilar friction stir spot welding of low carbon steel and Al–Mg alloy by formation of IMCs

Abstract: Dissimilar lap joints of low carbon steel and Al–Mg alloy were obtained by friction stir spot welding. Mechanically mixed layer between top and bottom plates was not formed at the weld nugget due to the limited tool penetration and the pin height of welding tool lower than the thickness of Al plate laid in top side. These welding conditions made it possible to weld steel plate using welding tools made out of a general tool steel. With increasing tool penetration depth (TPD), tensile shear force of joint increased and maximum value of 3·0 kN was obtained at the TPD of 0·5 mm, but excessive tool penetration beyond 0·5 mm was caused in a deformation of Al plate of top side. In the result of interface observation, interaction layer was formed between Fe and Al alloy, which was constituted by various intermetallic compounds (IMCs). Consequently, the size of strongly bonded area containing Fe3Al and Fe4Al13 IMCs increased with TPD, which resulted in the increase in joint strength under the limited TPD.

A computational investigation of the effectiveness of different shielding gas mixtures for arc welding

Abstract: Tungsten?inert-gas welding arcs are modelled using a two-dimensional axisymmetric computational code. Both electrodes (the tungsten cathode and the metal anode workpiece) and the arc plasma are included self-consistently in the computational domain. The influence of adding helium, hydrogen and nitrogen to the argon shielding gas is investigated. It is found that addition of any of the gases increases the heat flow to and the current density at the anode. The shear stress and the arc pressure at the anode surface are increased by adding hydrogen or nitrogen or up to about 50?mol% helium, but decrease when more helium is added. It is predicted that the effect of adding any of the gases is to increase the depth of the weld pool, in agreement with the experimental evidence. The results are explained by referring to the thermodynamic and transport properties of the gas mixtures.

Weld pool flows during initial stages of keyhole formation in laser welding

Abstract: Weld pool transport phenomena during the transition from conduction-mode laser spot welding to keyhole laser spot welding of titanium were studied by numerical simulation. A range of laser powers were simulated and temperature dependent evaporation recoil pressure and cooling were applied as boundary conditions on the weld pool surface. Simulation results predicted a complex time-varying flow pattern during weld pool development. The surface-normal flow at the weld pool centre oscillated between upwards and downwards during the simulation time due to interaction of competing effects of evaporation recoil and surface tension pressures and laser heating and evaporation cooling. The results show that the laser weld pool flow dynamics play a key role during the transition from conduction-mode laser welding to keyhole welding.

The relation between liquation and solidification cracks in pulsed laser welding of 2024 aluminium alloy

Abstract: It is a known fact that 2024 aluminium alloy is susceptible to solidification cracking in the weld metal and liquation cracking in the base metal when welded with fusion processes. The main purpose of this study is investigating whether these two types of cracks act independently or are related with each other in terms of initiation and propagation as this can lead to enhancing the understanding of the hot cracking phenomena in these alloys. Laser welding whether continuous or pulsed has promising outlooks for welding heat treatable aluminium alloys. But the fast heating and cooling rates involved in pulsed laser welding give rise to unique successively repeating microstructural features which provides an interesting base for studying the cracks. Thus, the experimentation involved Nd:YAG pulsed laser welding of 2024 aluminium alloy. The observations indicate that liquation cracks in the partially melted zone of wrought base metal have strong association with solidification cracks in the weld metal and accordingly it is proposed that the liquation cracks act as a strong initiation sites for solidification cracks. It is also shown that healing of liquated grain boundaries through backfilling can have a significant role on resistance to liquation cracking in the partially melted zone and that in turn can affect tendency for solidification cracking in the weld metal.

Multi-Pass Friction Stir Welding in Alloy 7050-T7451: Effects on Weld Response Variables and on Weld Properties

Abstract: In some situations it may be necessary or desirable to perform a friction stir weld pass through material, which has already been friction stir welded. Examples include weld repair or crossing weld beads. In either case, one would like to know whether the control parameters used for welding require modification for passes through existing welded material and what is the effect of the multiple passes on the weld properties. To examine this issue, multiple weld passes (five) were performed in 6.4 mm thick 7050-T7451 plate at a tool rotation speed of 540 rpm and a welding speed of 6.77 mm s −1 . Results indicate that for this material and these conditions weld control parameters require no adjustment. The overall reduction from pass 1 to 5 in transverse tensile strength is 7%. In addition, weld metallurgy is only slightly changed in the heat affected zone (HAZ) due to overaging for each weld pass while there is no change in metallurgy in the nugget. Hardness in the HAZ reduces by 14 Vicker's hardness points from pass 1 to 5. The peak longitudinal residual stress is reduced with increasing number of passes.

Welding process impact on residual stress and distortion

Abstract: Residual stress and distortion continue to be important issues in shipbuilding and are still subject to large amounts of research. This paper demonstrates how the type of welding process influences the amount of distortion. Many shipyards currently use submerged arc welding (SAW) as their welding process of choice. In this manuscript, the authors compare welds made by SAW with DC gas metal arc welding, pulsed gas metal arc welding, Fronius cold metal transfer (CMT), autogenous laser and laser hybrid welding on butt welds in 4 mm thick DH36 ship plate. Laser and laser hybrid welding were found to produce the lowest distortion. Nevertheless, a considerable improvement can be achieved with the pulsed gas metal arc welding and CMT processes. The paper seeks to understand the relationship between heat input, fusion area, measured distortion and the residual stress predicted from a simple numerical model, and the residual stresses validated with experimental data.

The theory of laser materials processing : heat and mass transfer in modern technology

Abstract: Mathematics in Laser Processing.- Simulation of Laser Cutting.- Keyhole Welding: The Solid and Liquid Phases.- Laser Keyhole Welding: The Vapour Phase.- Basic Concepts of Laser Drilling.- Arc Welding and Hybrid Laser-Arc Welding.- Metallurgy of Welding and Hardening.- Laser Cladding.- Laser Forming.- Femtosecond Laser Pulse Interactions with Metals.- Comprehensive Numerical Simulation of Laser Materials Processing.