Synergetic effects of hybrid laser/arc welding

Abstract

The present study reports the results of a study examining the synergetic effects of hybrid laser/arc welding. Experiments were carried out with a 500 W Nd:YAG laser in combination with standard gas tungsten arc welding equipment and attention was focused on two aspects: the heat transfer efficiency and the melting efficiency. The heat transfer efficiency was determined by calorimetric measurements, whereas the melting efficiency was obtained from the transverse cross-sections of welds produced under various conditions. In addition, analytic calculations of the melting efficiency were performed on the basis of a modified form of the Rosenthal equation. The results show that the interaction of the laser and the arc does not lead to a noticeable change in the heat transfer efficiency, but results in a significant increase in the melting efficiency. The observed synergic melting effect is caused by addition of the two heat sources (laser and arc) and the contraction of the arc by the laser beam.     

Effects of welding parameters on melting energy of CO2 laser–GMA hybrid welding

Abstract

A series of CO₂ laser–gas metal arc (GMA) hybrid welding experiments were carried out on the mild steel workpiece to investigate the effects of the welding parameters, such as laser power, arc current and the distance between laser and arc D _LA, on the melting energy. A dimensionless parameter psi was introduced to indicate the change in the melting energy of hybrid welding. The results showed that with different welding parameters, the melting energy of hybrid welding was changed by the two heat sources (laser and arc) interaction. With an optimal combination welding parameters, psi can be increased up to 23%. Finally, the role of the two different mechanisms in the heat sources interaction was quantitatively discussed in terms of psi. It can be concluded that when D _LA<4 mm, the interaction between the laser induced plasma and the arc plasma dominates the heat sources interaction, therefore the changes of melting energy, whereas the heat sources interaction is only dominated by the preheating mechanism when D _LA≥4 mm.     

Convection driven melting of anisotropic metals

Abstract

This paper numerically explores melting of a pure substance with the thermal conductivity of the solid phase, assumed to be anisotropic. A two-phase test case for such situations is deduced from the standard one-phase Gobin–Le Quéré melting benchmark. The solution is presented for Prandtl number 0·02, Stefan number 0·01 and Rayleigh number 2·5 × 10⁴ which are specific for metals. Three cases are compared in terms of the terminal interface boundary position and average liquid fraction as a function of time for isotropic case and two distinctly oriented principal directions of the thermal conductivity tensor. The calculations have been performed by using the one-domain enthalpy formulation with artificial melting interval and the recently developed explicit local radial basis function collocation method (LRBFCM) which belongs to the entirely new generation of meshless methods. The results are not sensitive to the increased thermal conductivity of the solid phase in the direction parallel with the heated boundary but sensitive with the increase of the thermal conductivity of the solid phase in the direction perpendicular to the heated boundary.     

Effect of nickel with chromium on microstructure and toughness of heat affected zone in low carbon steel

Abstract

In the present work, the effects of nickel with chromium and of varying heat input on the microstructure and toughness of the grain coarsened heat affected zone (GCHAZ) of a low carbon steel were investigated. In the welding experiments, low carbon steel specimens having five different combinations of nickel and chromium content (0·9Ni–0·3Cr, 1·9Ni–0·8Cr, 2·8Ni–1·3Cr, 3·8 Ni–1·7Cr, and 4·9Ni–2·1Cr, all wt%) were welded using a submerged arc welding process with heat inputs of 0·5, 1, and 2 kJ mm^-1. Following welding, the microstructure, hardness, and toughness of the GCHAZs were investigated. From the results, attempts were made to establish a relationship between heat input, nickel and chromium contents, microstructure, hardness, and toughness of the GCHAZ. Charpy impact testing and microstructural observation showed that, for a heat input of 0·5 kJ mm^-1, nickel plus chromium contents in the range 1·9Ni–0·8Cr to 4·9Ni–2·1Cr promoted the formation of martensite, thereby producing lower toughness values. It was subsequently found that, taking into consideration the microstructure, hardness, and toughness of the GCHAZ, an intermediate heat input (1 kJ mm^-1) gave higher toughness values for all nickel and chromium contents. However, it was observed that satisfactory toughness values could not be obtained by varying the heat input for the 3·8Ni–1·7Cr and 4·9Ni–2·1Cr steels.     

Wire melting phenomena in gas metal arc welding

Abstract

The wire melting rate in gas metal arc welding using constant voltage power supplies is well described by a parabolic model that considers the effects of electrical resistance and arc heating. However, tests performed with constant current power supplies indicate that the melting rate can deviate from this model when welding current is close to the globular–spray transition value. Although the causes of this anomaly have not yet been well established, most authors consider them to be related to variations in liquid metal temperature close to the metal transfer mode transition current. The present work evaluates the effect of welding parameters on the wire melting rate when a constant current power supply is used. A numerical model that considers the contribution of both Joule and arc heating was developed to calculate the temperature distribution in the wire and its melting rate. The model was used to assess the changes in either arc heating or metal vaporisation that might explain the melting irregularities. High speed cinematography was used to analyse metal transfer in the current range associated with the melting irregularities.     

Novel rotation arc system for narrow gap MAG welding

Abstract

The present paper develops a hollow axis motor driven high speed rotation arc system for narrow gap welding (NGW), and introduces the features of this system. Some welding experiments were then carried out to investigate the characteristics of welding wire melting and weld formation for this new process. Experimental results show that the melting rate of wire increases and the residual melting ball diameter of wire tip decreases respectively with an increase in rotation speed, and this melting rate is higher in pulsed welding and NGW respectively than in dc and flat plate welding. Furthermore, the arc rotation can obviously improve the penetration into NGW groove sidewalls and bead shape, and thus the system has been used to weld practical NGW joints successfully.     

Arc power and efficiency in gas tungsten arc welding of aluminium

Abstract

A calorimetric study of gas tungsten arc welding of aluminium is described. The present study comprised experiments in which autogenous welding runs were each made on a block of electrical conductor grade aluminium. The blocks were all approximately cubic in shape which, when combined with the high thermal conductivity of aluminium, ensured that their temperature equalised soon after the completion of a run. Each sample was immersed in insulating material before welding so that heat losses to the surroundings were minimised. Thermocouples were attached to the block in each experiment and the bulk temperature rise was related to the energy input associated with the welding run. The effects of arc polarity, alternating current balance, shielding gas composition, arc length and welding current on the arc power and arc efficiency were investigated. The results obtained with alternating current are compared to those for direct current, and the differences are explained.     

Effect of friction welding conditions on mechanical properties of A5052 aluminium alloy friction welded joint

Abstract

The present paper describes the mechanical properties of Al–Mg aluminium alloy (A5052) friction welded joints. Two types of A5052 with different tensile properties were used, namely, H112 base metal with 188 MPa tensile strength and H34 with 259 MPa tensile strength. Similar metal specimens were joined using a continuous drive friction welding machine with an electromagnetic clutch to prevent braking deformation. That is, the joints were welded using the 'low heat input' friction welding method developed by the present authors, in which the heat input is lower than in the conventional method. An A5052–H112 joint produced using a friction speed of 27·5 s^?1, friction pressure of 30 MPa, friction time of 2·0 s (just after the initial peak torque), and forge pressure of 60 MPa had approximately 95% joint efficiency. It fractured at the welded interface and in the A5052–H112 base metal. To improve the joint efficiency, an A5052–H112 joint was produced at a forge pressure of 75 MPa, which was the same as the yield strength of the A5052–H112 base metal. It had 100% joint efficiency and fractured in the A5052–H112 base metal. In contrast, an A5052–H34 joint was made using a friction speed of 27·5 s^?1, friction pressure of 90 MPa, friction time of 0·3 s (just after the initial peak torque), and forge pressure of 180 MPa. It had approximately 93% joint efficiency and fractured in the A5052–H34 base metal. This joint also had a softened region at the welded interface and in the adjacent region. To improve the joint efficiency, an A5052–H34 joint was made at a forge pressure of 260 MPa, which was the same as the ultimate tensile strength of the A5052–H34 base metal. Although this joint had a slightly softened region at its periphery, it had approximately 93% joint efficiency. The failure of the A5052–H34 joint to achieve 100% joint efficiency is due to a slight softening at the periphery and the difference in the anisotropic properties of the A5052–H34 base metal between the longitudinal and radial directions.     

Joining phenomena and joint strength of friction welded joint between aluminium–magnesium alloy (AA5052) and low carbon steel

Abstract

The joining phenomena and the joint strength of an Al–Mg alloy (AA5052) and low carbon steel (LCS) friction welded joints were investigated. The weld interface of the LCS side at a friction time of 1·2 s had a slightly transferred AA5052, and then the entire weld interface had it at a friction time of 3·0 s or longer. The joint efficiency increased with increasing friction time, but it decreased at a friction time of 12·0 s or longer. The joint at a friction time of 3·0 s with forge pressure of 190 MPa had 100% joint efficiency and the AA5052 base metal fracture with no crack at the weld interface. The weld interface of these joints also had no intermetallic compound. On the other hand, the joint at a friction time of 8·0 s, which had ～97% joint efficiency, fractured between the AA5052 side and the weld interface because it had the intermetallic compound at the weld interface.     

Characteristics of hollow cathode arc as welding heat source: arc characteristics and melting properties

Abstract

A feasibility study has been conducted to determine whether a hollow cathode arc (HCA) can be used for a welding heat source in space, that is high vacuum and microgravity conditions. The HCA method enables the arc discharge to form under low pressure conditions by purging a small amount of gas through the centre of the hollow cathode. The characteristics of HCAs under low pressure conditions have been experimentally investigated, in particular the melting properties and voltage-current characteristics. Results show that the penetration profile and arc characteristics of the HCA method are sensitive to process parameters such as gas flowrate, arc length, and inner diameter of the electrode, and that the penetration in HCA melting is extremely deep under the conditions of low gas flowrate and long arc length.     

Microstructure characteristics in non-modified and Sr modified Al–Si–Cu–Mg 319 type alloys

Abstract

This study was carried out on 319 alloys containing low and high levels of Mg, in the non-modified and Sr modified conditions (150 ppm Sr addition). Single step, two step and triple step heat treatments were applied to identify the optimum solution heat treatment to minimise incipient melting of the copper phases Al₂Cu and Al₅Mg₈Cu₂Si₆ in these alloys in relation to the alloy properties. In Mg free alloys, no incipient melting of Al₂ Cu was observed even in samples heat treated at 520°C. Addition of Sr leads to modification of Si particles but also to an increase in area per cent porosity and pore length, especially when the solution temperature reaches 520°C. Addition of Mg results in a decrease in the Si particle aspect ratio but an increase in particle size. Magnesium was also found to increase the possibility of incipient melting resulting from the formation of the insoluble Al₅Mg₈Cu₂Si₆ phase. To some degree, Sr decreases the effect that Mg has in increasing the area per cent porosity and pore length, while Mg impairs the effects that Sr has on modifying Si particles, even though the lowest Al–Si eutectic temperature is obtained for the 319 alloy containing both Mg and Sr.     

Effects of latent heat of fusion on thermal processes in laser welding of aluminium alloys

Abstract

Based on the Green's function method, a mathematical model allowing for the latent heat of fusion and solidification is developed to describe the steady state, two-dimensional heat flow during welding of thin plates. It is demonstrated that the latent heat has a pronounced effect on shape and size of the weld pool and mushy zone. The thermal efficiency of base metal fusion by a line heat source η _t can exceed 0·4839 considerably if the latent heat is taken into account. It is shown that the known simplified approaches for considering the latent heat can introduce large errors into the estimation of η _t. The calculated and experimental weld pool shapes are compared.     

Corrosion and deposition of magnetite in ferritic steel steam generator tubes under acid chloride conditions

Abstract

Corrosion of a tube made up of 9Cr–1Mo, 2·25Cr–1Mo, and 2·25Cr–1Mo–Nb ferritic steels and containing artificial defects has been investigated under realistic steam generator conditions (355°C, 17·6 MPa) with acid chloride fault water chemistry (2 mg/kg HCl). Four regions of corrosion and magnetite deposition behaviour were observed on the tube surface. In non-heat flux regions, magnetite deposition was affected by mass transfer and probably also by surface potential. In low heatflux regions (<660 kW m^?2) observed increases in the rates of magnetite deposition and corrosion wereprobably due to rises in the degree of iron supersaturation and HCl concentration, brought about by boiling. Enhanced HCl concentrations in the normal heat flux region (660 kW m^?2) prevented magnetite deposition and caused an increase in corrosion of the three steels. Increases in corrosion and magnetite deposition were also observed at the weld between the 9Cr–1Mo and 2·25Cr–1Mo steels. In defects, accelerated corrosion was seen only in the 9Cr–1Mo steel and was confined to the top 0·15 mm. It is concluded that the corrosion behaviour observed in this work is determined by the residence time and concentration of solutions of HCl on tube and defect surfaces.     

Study on ac-PMIG welding of AZ31B magnesium alloy

Abstract

Welding of AZ31B magnesium alloy is carried out using alternating current pulsed metal inert gas (ac-PMIG) welding with 1·6 mm diameter of filler wire. Typical current waveform is used to make sure arc given an accurate energy input into filler wire. The arc characteristics, metal transfer forms, microstructure and mechanical property of ac-PMIG welding of AZ31B magnesium are investigated. The results show that a stable welding procedure and continuous joints can be obtained easily under a wide range of welding parameters. The most important factors for ac-PMIG welding are negative electrode (EN) ratio and pulse rework current, which give an accurate energy input into filler wire. The grain in fusion zone is much finer and more uniform, and grain size does not grow significantly in the heat affected zone compared with base metal. The average ultimate tensile strength of weld beads is 97·2% of base metal.     

Properties and improvement of super fine grained steel friction welded joint

Abstract

This paper describes friction welded joint properties of super fine grained steel (SFGS) and discusses improvements in these joint properties. The average grain size diameter of the SFGS base metal is ～0·6 μm, and its ultimate tensile strength is 660 MPa. The joint, made by a continuous drive friction welding machine (conventional method), fractured at the welded interface even though it possessed 100% joint efficiency. This was due to both the coarsening of the grain size and the softening of the welded interface with its adjacent region caused by heat input during braking times. The authors developed a joining method using a continuous drive friction welding machine that has an electromagnetic clutch to eliminate heat input during braking time, which was called the 'low heat input friction welding method' (LHI method). The joint obtained by the LHI method had the same tensile strength as the base metal at the friction time when the friction torque reached the initial peak. That is, the joint obtained 100% joint efficiency and fractured at the base metal, although the adjacent region of the welded interface softened only slightly. The grain size of this joint was smaller than that obtained by the conventional method. It was clarified that the optimum friction welded joint of the SFGS could be obtained by the LHI method in comparison with the conventional method.     

Effect of relative location of laser beam and TIG arc in different hybrid welding modes

Abstract

The laser leading hybrid welding mode [laser tungsten inert gas (laser-TIG)] and TIG leading hybrid welding mode (TIG-laser) were utilised in the magnesium alloy welding process. The effects of TIG arc current and the distance between laser beam and TIG electrode (Dla) on penetration depth and melting efficiency in different hybrid welding modes were investigated. Results reveal that there is a decided difference in variation trend of penetration depth with Dla in different welding modes. The penetration depth increases first and then decreases with the Dla increasing from 1 to 7 mm in TIG-laser hybrid welding mode while the penetration depth decreases monotonously with Dla in laser-TIG hybrid welding mode. The increase percentage of melting efficiency remains at ～20% in laser-TIG hybrid welding mode while the increase percentage fluctuates acutely and achieves the maximal value (～53·14%) at the arc current of 120 A in TIG-laser hybrid welding mode.     

Effect of laser spot weld energy and duration on melting and absorption

Abstract

To facilitate pulse Nd–YAG laser spot weld development, it is common practice to adjust the pulse energy, duration, and focus spot size. An accurate understanding of the effect of these parameters on melting, weld appearance, and heat input is thus required. Calorimetric measurements of the net heat input to 304 stainless steel workpieces for laser spot welds have been completed. A pulse Nd–YAG laser was used with varying pulse energies from 1 to 5·5 J, and pulse durations of 2·2 and 7·0 ms. Measurements showed the absorption for spot welds produced using the pulsed Nd–YAG laser to vary from 38 to 67% and to be relatively insensitive to beam intensity. Analysis of the continuous point source equation for conduction heat flow in solids was used to predict the weld size for the pulse energy and duration measured in the experiment. Calculations of the weld pool volume from the weld metallography were used to determine the melting for each spot weld. Comparisons of the measured weld size with the three-dimensional model predicted size indicated that the observed weld pools are larger than is expected from the measured workpiece energy. Analysis of the experimental data and the theoretical model has revealed a substantial increase in melting for short duration pulses versus long duration pulses of the same energy. The benefit of laser spot welding parameter optimisation is hence indicated.     

Study of process/structure/property relationships in friction stir welded thin sheet Al–Cu–Li alloy

Abstract

Microstructure evolution in friction stir welds produced in artificially aged Al–4Cu–1Li–0·36Mg–0·14Zr–0·28Ag alloy over a range of process parameters was studied using transmission electron microscopy. Process parameters did not have a major effect on the weld microstructure and mechanical properties. The stir zone exhibited an appreciable decrease in hardness relative to the unaffected base metal due to dissolution of T₁ and θ′ precipitates. The heat affected zone exhibited almost complete dissolution of θ′ precipitates and partial dissolution of T₁ precipitates. The effect of process conditions on T₁ precipitate density in the heat affected zone was studied and it was found that dissolution was experienced at lower tool rotation speed to traverse rate ratios, while welds produced at higher tool rotation speed to traverse rate ratios experienced both dissolution and growth of T₁ precipitates. The results obtained on this thin sheet aluminium alloy were compared to those of friction stir welds produced in thicker sections of the same alloy.     

Measurement and calculation of arc power and heat transfer efficiency in pulsed gas metal arc welding

Abstract

This study investigates the heat transfer efficiency of the pulsed gas metal arc welding (GMAW-P) process. The arc power and heat input were calculated from arc current and voltage measurements and the heat input was also measured with a liquid nitrogen calorimeter. The measured heat transfer efficiency for GMAW-P varied slightly over a wide range of pulse parameters, with an average value of 70%, a maximum of 72% and a minimum of 68%. Welding heat transfer efficiency based on arc power calculated as the product of average current and voltage was too high (averaging 82%), while that calculated using the product of the root mean square (RMS) of the average current and voltage was too low (averaging 61%). Both also varied significantly with pulsing parameters. Mathematical analysis shows that average instantaneous power values must be used when current and voltage vary significantly with time. The experimental differences between the average instantaneous power and the other calculated values are explained by the relative phases of the pulsed current, voltage and arc resistance waveforms.     

Spot weldability of TRIP assisted steels with high carbon and aluminium contents

Abstract

We examine here the spot welding characteristics of transformation induced plasticity assisted steels, which contain δ-ferrite as a consequence of their aluminium concentrations of 3·5 or 5·6 wt-% and which also have high carbon contents of 0·3 or 0·4 wt-% when compared with conventional automotive steels. The resistance spot welds are tested in both shear and cross-tensile tests in order to determine the so called ductility ratio, which is a parameter associated with the fitness of such welds for automotive applications. With an increase in the δ-ferrite fraction from 0·19 to 0·5, the hardness variation across the weld and heat affected zone is decreased approximately from 400 to 150 HV. It seems that the presence of stable δ-ferrite is helpful in reducing hardness variations and in achieving a significant ductility ratio of 0·39.