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.     

Hybrid laser/arc welding process for controlling bead profile

Abstract

A laser hybrid welding process in which a defocused laser beam is applied beside a gas metal arc weld (GMAW) pool to modify the bead shape was studied. The present paper aims to produce welds with improved toe geometry and better fatigue life than those made with GMAW alone and to apply a numerical simulation to help configure the hybrid process. First, stationary hybrid welds were made to validate weld bead shape predictions and to characterise the spreading of the arc weld deposit to the laser heated spot. Next, the travelling hybrid process was configured with the aid of simulations and fatigue test specimens were welded. Proper application of the laser heat input induced molten metal to spread to the laser heated area, increasing the fillet weld leg length. This produced a larger weld toe angle that decreased the stress concentration and increased the fatigue life of the welds relative to standard mean values.     

Experimental study on welding characteristics of CO2 laser TIG hybrid welding process

Abstract

The experiments of CO₂ laser TIG paraxial hybrid welding with 4 mm thick AISI 321 stainless steel sheet have been performed. The arc images and welding characteristics have been investigated with different energy ratios between laser and arc. The experimental results indicate that the hybrid welding is similar to laser welding and has also two welding mechanisms: deep penetration welding and heat conduction welding. Because of the effect of keyhole induced by laser, the arc root can be stabilised and compressed, and the current density and the penetration depth are all increased significantly, which show the characteristics of deep penetration welding. However, when the current is increased to a critical value, the laser induced keyhole disappears and the arc expands obviously, which decreases the penetration depth, so that the welding mechanism has been changed from deep penetration welding to heat conduction welding. Furthermore, the effects of distance between laser beam and electrode, pulsed laser and hybrid manners on hybrid welding characteristics have also been studied.     

Mechanical properties and microstructures of hybrid laser MIG welded dissimilar Mg–Al–Zn alloys

Abstract

Bead shape, microstructure changes and mechanical properties of laser metal inert gas (MIG) welded dissimilar Mg–Al–Zn alloys (from AZ31B to AZ61) are studied. The results show that heat ratio of arc to laser (HRAL) and welding speed are dominant parameters for achieving good tensile strength efficiency and elongation property. From AZ31B to AZ61, microstructure changes are observed as cellular dendrites to equiaxed dendrites and fish bone dendrites in the upper part of hybrid weld. Besides, at weld centreline, the solidification structure of lower part is finer than that of upper part. In this study, the maximum tensile strength efficiency and elongation reached 97·6 and 7% respectively. When the HRAL matches welding speed well, the joint achieves higher tensile strength with 45° shearing fracture at heat affected zone because of fewer defects. However, when utilising too low HRAL or fast welding speed, the joints show lower tensile strengths with nearly vertical fracture at fusion zone.     

Analysis of electrical characteristics for hybrid pulsed micro-tungsten inert gas welding arc

Abstract

The present paper experimentally investigates and analyses in detail the electrical characteristics of the arc for the hybrid pulsed micro-tungsten inert gas process. In particular, an average voltage–current (V–I) graph plotting approach is proposed to describe the dynamic characteristics of the hybrid arcs using the filtered voltage and current of the arc. It is shown that the average voltage and electrical resistance of the heavily hybrid pulsating (HHP) arc are lower than those of the slightly hybrid pulsating (SHP) arc, particularly during the background current stage. Furthermore, the analyses reveal that the average V–I characteristic graph for the arc forms a hysteresis loop, primarily because of the thermal inertia of the arc, during a pulse period. This closed loop is larger for the SHP arc than for the HHP arc, and shrinks longitudinally with increasing frequency and width of the current pulse. These results thus demonstrate that the HHP arc has a greater thermal inertia, particularly at higher current and pulse frequency. Experimental results show also that the mean voltage of the arc and the features of the hysteresis loop vary markedly with weld penetration in addition to pulsating frequency and pattern, and thus suggest the possibility of monitoring the hybrid pulsed weld penetration via arc sensing.     

3D numerical study of effects of temperature field on sensitisation of Alloy 690 butt welds fabricated by gas tungsten arc welding and laser beam welding

Abstract

This study examines the effects of the temperature field on the sensitisation of Alloy 690 butt welds fabricated using the gas tungsten arc welding (GTAW) method and the laser beam welding (LBW) method respectively. The welding thermal cycles of the two welding methods are simulated using ANSYS software based upon a moving heat source model. The validity of the numerical model is confirmed by comparing the simulation results with the corresponding experimental findings. Agreement is found between the numerical results for the temperature field and the experimental temperature measurements. In addition, it is shown that the LBW weldment experiences a more rapid heating and cooling effect than the GTAW weldment, and therefore has both a smaller heat affected zone and a narrower sensitisation region. Thus, the validity and general applicability of the thermal welding model are confirmed.     

Microstructures of gas metal arc weld metal of 950 MPa class steel

Abstract

The effects of shielding gas composition on the properties and microstructure of single pass weld metals produced by GMA (gas metal arc) groove welding of 950 MPa class steel plates have been investigated. The shielding gas employed was a mixture of argon (Ar) and carbon dioxide (CO₂) (0–25%), and the weld heat input was ～3 kJ mm. With increasing CO₂ content, the hardness of the weld metal decreased from 380 HV to 280 HV, and the absorbed energy of the Charpy impact test decreased from 130 J to 90 J. The microstructures of the weld metal, consisting primarily of low carbon martensite and carbide free bainite, became more bainitic as the CO₂ content of the shielding gas was increased. It was also found that the MA constituent, embrittling microstructure, was formed in the granular bainitic area, the volume fraction of which increased with the CO₂ content of the shielding gas.     

Study of laser MIG hybrid welded AZ31 magnesium alloy

Abstract

The laser metal inert gas (MIG) hybrid welded AZ31 magnesium alloy is discussed in weld shape, microstructure characteristics and mechanical properties in comparison of single laser and arc welding. The stable MIG arc, reliable droplet transfer and regular weld that are hardly obtained in single MIG welding can be obtained in hybrid welding by laser arc synergic effects. The ultimate tensile strength and elongation of hybrid weld are far higher than those of laser weld and reach 97·8 and 87·5% of base metal respectively. Under this experimental condition, the efficiency of hybrid welding is 1·20 times faster than that of single laser welding. Between the wide upper part (arc zone) and the narrow lower part (laser zone), obvious difference is observed. Arc zone has coarser grain size and wider partial melted zone than laser zone. Finally, the porosity reduction mechanism of hybrid weld is discussed according to the weld pool shape and the acting forces on it.     

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.     

Numerical simulation of heat transfer and fluid flow in GTA/Laser hybrid welding

Abstract

In order to understand the temperature fields, cooling rates and mixing in the weld pool, a comprehensive, three-dimensional heat transfer and fluid flow model is developed and tested by comparing model predictions with two sets of experimental data. The first set of data was taken from the literature. The experiments varied the separation distance between the heat sources for three arc current levels at a constant laser power. The second set of experiments analysed the effect of varying laser power for a constant heat source separation distance. The results demonstrate that the distance between the two heat sources significantly affects the cooling rates. The calculated results showed that the hybrid weld pool was very well mixed with strong convection currents resulting from the interaction between the electromagnetic and Marangoni forces. The calculated and experimental results showed that hybrid welding increases the weld pool width and gap bridgability when compared with laser welding. The weld pool depth in hybrid welding was affected mainly by the characteristics of the laser beam. Hybrid weld pool penetration depth is maximised at an optimal distance between the arc electrode and laser beam. The cooling rate increases significantly when the heat sources are separated beyond a critical distance. At close separation between arc and laser, calculations show that the arc radius must be decreased to achieve the observed weld depths.     

Laser stabilisation of arc cathode spots in titanium welding

Abstract

Cathode spot formation is very pronounced during arc welding of titanium and titanium alloys. The dynamic behaviour of these spots was observed to interfere with metal transfer during welding, this interference being a fundamental cause of poor weld quality in these alloys. In the present work, stabilisation of the arc cathode spot with a focused Nd–YAG laser beam during pulsed gas metal arc welding of titanium was investigated. The laser beam was focused near the leading edge of the weld pool and the laser power and focus spot size were varied to determine the values required to confine the cathode spot to the laser focus position. The results showed that, for fixed welding conditions, the laser power required to prevent cathode spot motion varied as a function of focus spot size. The required laser power was minimised at 200 W for a spot size of 0.6 mm. The laser stabilised arcs had lower voltage but approximately the same current density as stabilised arcs. Increased welding speeds required marginally higher laser powers to stabilise the spot, but the minimum power was still attained with a 0.6 mm focus spot diameter. The laser power density required for stabilisation decreased as spot size was increased, varying from almost 10⁶ W cm^?2 at the smallest spot size to approximately 10⁴ W cm^?2 at the largest. Cathode spot stabilisation improved weld quality by reducing spatter generation and weld bead irregularity.     

Effect of activating flux and shielding gas on microstructure of TIG welds in austenitic stainless steel

Abstract

The aim of this research is to study the effect of an activating flux, two shielding gases (100%Ar and 50%Ar z 50%He) and a range of weld currents on the microstructure of autogeneous A-TIG welds on an austenitic stainless steel. Metallographic, Mössbauer, X-ray diffraction and magnetic permeability methods were used in the study to evaluate ferrite content in the welds. The increase in welding current coarsened the microstructure and increased the retained ferrite content in welds made with and without flux. The activating flux increases the ferrite content and changes the distribution of ferrite in the welds. The influence of flux on ferrite content is less significant in Ar/He than in Ar shield welds. The process of filling steel samples, currently used in the Mössbauer method, drastically changes the microstructure of the parent and melted austenitic stainless steels.     

Laser and laser assisted arc welding processes for DH 36 microalloyed steel ship plate

Abstract

A series of laser and laser assisted metal inert gas (MIG) welds was produced from a common plate. Each weld was mechanically tested, and the welds showed broadly similar properties, except for the autogenous CO₂ laser weld metal, which had poorer toughness. This was related to a harder weld metal microstructure. Toughness and hardness were related to the lath width of the ferrite, for the welds involved. The weld metal area/volume was used as an indicator of potential distortion. In this instance, the autogenous CO₂ laser weld was superior to the CO₂ laser assisted MIG weld which was better than the Nd:YAG laser assisted MIG weld. Each weld was examined using carbon extraction replicas in the TEM, and also using an SEM with an EDAX attachment. A number of inclusions and precipitates were observed, identified and sized. It was concluded that the particles observed were not detrimental in this specific case. A tentative relationship was established between parent plate inclusion size distribution and weld metal inclusion size distribution.     

Investigation into micro-tungsten inert gas arc behaviour and weld formation

Abstract

Current pulsing patterns are defined for the micro-tungsten inert gas welding apparatus developed and are described as: no pulsation (NP), high frequency pulsation (HFP), slightly hybrid pulsation (SHP), and heavily hybrid pulsation (HHP). The characteristics of the microarc behaviour and weld formation are then investigated in detail for these patterns. The parameters and pattern of the pulsating current dominate arcing power and arc stiffness at a given average current, thus affecting arc state and bead formation. The arc image analysis shows that the HFP and HHP arcs can burn more steadily even at an average current of 2 A because of the arc stabilising effect from the superimposed high frequency components. During the on time of the pulse, the state of the hybrid pulsating (HP) arc varies successively from an apple like shape to a cone with an increased taper. Furthermore, this HP arc shrinks owing to the decreases in the peak current and arcing power with increasing frequency of the base pulse, but is still much greater in size than the NP and HFP arcs. Finally, welding experimental results demonstrate that the HP welds are the widest and become narrow with an increase in the pulse frequency, and the NP and HHP beads are somewhat wider than the HFP and SHP beads respectively.     

Microstructure and mechanical properties of laser welded austenitic high manganese steels

Abstract

The microstructure of laser welded austenitic twinning induced plasticity steel sheets joints was investigated by means of optical microscopy, SEM and electron backscattering diffraction in order to differentiate the fusion zone, heat affected zone and base material, as well as to establish present phases, grain size distribution and grain misorientation distribution caused by the welding process. Measurements of EDX were taken into account to evaluate the effect of Mn segregations. Microhardness measurements and tensile tests were performed to evaluate the mechanical properties of the joints. In addition, the twinning phenomena progress was assessed by investigating the texture evolution in the base material and fusion zone of samples plastically deformed by 5, 10 and 15%. Grain refinement was found in the fusion zone affecting substantially the mechanical properties of the welding, being the most resistant and harder region.     

High power laser welding of Nb microalloyed steel plates

Abstract

Laser welding studies on 12 mm thick Nb microalloyed steels were done using a 25 kW CO₂ laser at welding speeds of up to 3 m min^?1 with the aim of identifying the influences of Nb and carbon on laser weldability and weld properties. Welds were examined for solidification flaws, penetration characteristics, microstructure and mechanical properties. Solidification cracking did not appear to be influenced by carbon or Nb in the ranges studied when welding at power levels between 22 and 25 kW. The area fraction of martensite was shown to increase with decreasing energy input and increasing carbon content. Weld metal toughness was improved by reducing carbon content in the range 0·08–0·05%C but was little influenced by Nb. Weld metal and heat affected zone hardness levels were reduced, as expected, by reducing carbon content. The results show that increasing Nb content leads to higher strength material without significant loss of toughness, while lowering carbon content can improve toughness without loss of strength. Thus low carbon Nb microalloyed steels with about 0·05%C can achieve a good combination of parent plate and laser weld properties and appear to be suitable for high power laser welding situations.     

Microstructure of cobalt based alloy coatings produced by laser deposit welding

Abstract

Coatings of cobalt based alloy HMSP 2528, deposited on a nickel based superalloy using a transverse flow CO₂ laser, are investigated. Sections of the coatings have been examined to reveal their microstructure and phase composition using an Olympus PME–3 type optical microscope, scanning electron microscope, transmission electron microscope, and X-ray diffractometer. The results showed that primary phase (γ-Co) and eutectic (γ-Co + Cr₂₃C₆) were present in the coatings. Rapid directional solidification was found at the interface, where the growth direction was perpendicular to the interface. Multidirectional growth and growth parallel to the scanning direction were also found in the central region and near the top surface, respectively. At the remelted boundary between two tracks, a novel microstructure was found in which the crystallisation direction of the first track was transmitted to the second track. As more tracks were deposited, coarsening dendrites could be found at the interfaces.     

Control of nitrogen content and porosity in gas tungsten arc welding of high nitrogen steel

Abstract

In welding of high nitrogen steel (HNS), it is essential to control the nitrogen content and porosity in the weld metal. In this paper, the influence of shielding gas composition and heat input on the nitrogen content and porosity in the weld metal of HNS was investigated by gas tungsten arc welding. The experimental results indicate that the weld nitrogen content increases as N₂ in the shielding gas is increased in the same heat input of welding. The weld nitrogen content decreases with increasing the heat input for pure argon used as a shielding gas, whereas it increases with increasing the heat input for the shielding gas including some nitrogen. The nitrogen pore can be avoided when the nitrogen content in the shielding gas is <4% in the heat input range of 528–2340 J mm^–1.     

Laser welding of AA 5083 samples by high power diode laser

Abstract

Laser welding is a very attractive technique to join different alloys at the industrial level, due to its low heat input, high flexibility, high weld quality and high production rate. In this work, the weldability of the aluminium alloy AA 5083 with a high power diode laser has been tested. Concisely, samples were subjected to lineal treatments of laser radiation, with the objective of studying the properties of the bead on plate welds generated. The main objective of the present work has been to study the influence of both the processing rate and the superficial treatment of the AA 5083 samples, on the morphological, microstructural and corrosion properties of the laser weld beads. The sizes of the welds were higher as the processing rate was decreased. The weld beads were seen to have better behaviour against corrosion than the base metal due to the microstructural refinement. It was also verified that a blasting process before processing gave beads with lower size but better corrosion resistance than the application of a black layer, due to the minimisation of the magnesium evaporation in this former superficial treatment.     

Hardness changes on pass by pass basis in mild steel gas metal arc welds

Abstract

A series of welds were fabricated in mild (ASTM A36) steel. The first pass was the entire length of the plate and each subsequent pass was indented ～25 mm. This allowed us to determine the change in hardness on a pass by pass basis by mapping the hardness over each weld pass.