Showing papers on "Heat-affected zone published in 1999"

Finite element model of pulsed laser welding

Abstract: A three-dimensional finite element model of the heat flow during pulsed laser beam welding is presented. The heat transfer and parametric design capabilities of the finite element code ANSYS were employed for this purpose. The model calculates transient temperature profiles and the dimensions of fusion and heat-affected zones (HAZ). Temperature-dependent thermophysical properties and experimentally measured beam profiles are incorporated. Model calculations are compared and then calibrated with experimental results of pulsed laser welds. The results suggest that temperature profiles and weld dimensions are strong functions of the absorptivity and energy distribution of the laser beam. For this reason, it is essential to incorporate an accurate description of the heat source.

Porosity, underfill and magnesium loss during continuous wave Nd:YAG laser welding of thin plates of aluminum alloys 5182 and 5754

Abstract: The influence of various welding parameters on porosity and underfill formation and magnesium loss during continuous wave Nd:YAG laser beam welding of thin plates of aluminum-magnesium Alloys 5182 and 5754 was investigated. The porosity within the welds was characterized by radiography, optical microscopy and SEM. The compositional change in the welds was measured by electron microprobe analysis. The experimental results showed that the instability of the keyhole was the dominant cause of macro-porosity formation during laser welding of thin plates of aluminum Alloys 5182 and 5754. Hydrogen did not play a significant role in porosity formation. Although underfill was commonly observed at the root of full-penetration welds, sharp or deep notches, which are harmful to the mechanical properties of the welds, were not present. Reduction in magnesium concentration was more pronounced during conduction mode welding. Welding in keyhole mode resulted in much larger weld pool and less pronounced composition change. The extent of defocusing of the laser beam greatly affected the stability of the keyhole, weld pool geometry, pore formation and composition change.

Effect of Nd-YAG laser welding on the functional properties of the Ni–49.6at.%Ti

Abstract: Laser welding technology is applied successfully for joining several metallic compounds. Using a Nd-YAG source the process is suitable for precision welding of low thickness metallic sheets. In this work a Nd:YAG laser was used for welding 0.54 mm thick tapes of Ni–49.6at.%Ti shape memory alloy (SMA). Two different microstructural states of the material were chosen (fully recrystallized and partial annealed after 30% cold working) to investigate pseudoelastic and shape recovery behaviours. For the two different metallurgical conditions the mechanical properties of both reference and laser welded samples were tested at several temperatures by stress–strain measurements. A systematic comparison of the results was carried out. Moreover, differential scanning calorimeter (DSC) investigations on samples taken from the heat affected zone and hardness measurements allowed further clarification that the modification was induced by the welding procedure. The results obtained on the fully recrystallized material indicate that the stress-induced martensite and the recovery mechanisms are weakly modified by the presence of the welding zone while the pseudoelastic properties of the partial annealed material are strongly affected.

Prediction of nugget development during resistance spot welding using coupled thermal–electrical–mechanical model

Abstract: An axisymmetric finite element model employing coupled thermal–electrical–mechanical analysis of resistance spot welding is presented. The welding parameters considered include: heat generation at the faying surface and the workpiece–electrode surface; Joule heating at the workpiece and the electrode; and the thermal contact conductance between the electrode and the workpiece. The latent heat of phase change due to melting is accounted for. The effect of friction coefficient on the workpiece interface is also studied. The computed results agree well with the experimental data. Heat generation at the faying surface in the initial stages of welding dominates the nugget development, and Joule heating at long times governs the weld nugget growth. A parametric study is carried out for the nugget growth with specific consideration of resistance spot welding of Al alloys. Process control and modelling of resistance spot welding of Al alloys is more difficult than that for steel because of their high elec...

Intergranular Corrosion Following Friction Stir Welding of Aluminum Alloy 7075-T651

Abstract: Friction stir welding (FSW), a relatively new solid-state joining process, is used to join Al alloys of all compositions, including alloys essentially considered unweldable. This study foc...

Modeling macro- and microstructures of gas-metal-arc welded HSLA-100 steel

Abstract: Fluid flow and heat transfer during gas-metal-arc welding (GMAW) of HSLA-100 steel were studied using a transient, three-dimensional, turbulent heat transfer and fluid flow model. The temperature and velocity fields, cooling rates, and shape and size of the fusion and heat-affected zones (HAZs) were calculated. A continuous-cooling-transformation (CCT) diagram was computed to aid in the understanding of the observed weld metal microstructure. The computed results demonstrate that the dissipation of heat and momentum in the weld pool is significantly aided by turbulence, thus suggesting that previous modeling results based on laminar flow need to be re-examined. A comparison of the calculated fusion and HAZ geometries with their corresponding measured values showed good agreement. Furthermore, “finger” penetration, a unique geometric characteristic of gas-metal-arc weld pools, could be satisfactorily predicted from the model. The ability to predict these geometric variables and the agreement between the calculated and the measured cooling rates indicate the appropriateness of using a turbulence model for accurate calculations. The microstructure of the weld metal consisted mainly of acicular ferrite with small amounts of bainite. At high heat inputs, small amounts of allotriomorphic and Widmanstatten ferrite were also observed. The observed microstructures are consistent with those expected from the computed CCT diagram and the cooling rates. The results presented here demonstrate significant promise for understanding both macro-and microstructures of steel welds from the combination of the fundamental principles from both transport phenomena and phase transformation theory.

Hybrid welding with arc and laser beam

Abstract: This paper deals with a combination of two different welding processes, i.e. the synergic action of two different heat sources for fusion welding. The major part of the paper is focused on the combined action of a welding arc and a laser beam. The main advantage of the use of both heat sources is more efficient use of the energy supplied. With certain parameters, the quantity of molten material increases by 100% compared with the sum of the individual quantities of molten material in the individual processes. The paper further presents several practical applications of the combined welding process. Eventually, development of arc augmented laser welding may progress in two directions; the first being the synergic effect of a laser beam and plasma welding arc and the second the combination of tandem laser welding and consumable electrode welding. This paper presents two such cases schematically.

Determination of mechanical and fracture properties of laser beam welded steel joints

Abstract: Laser beam (LB) welding is increasingly being used in welding of structural steels. The thermal cycles associated with laser beam welding are generally much faster than those involved in conventional arc welding processes, leading to a rather small weld zone that usually exhibits a high hardness for C-Mn structural steels due to the formation of martensite. It is rather difficult to determine the tensile properties of a laser weld joint area due to the small size of the fusion zone. Complete information on the tensile and fracture toughness properties of the fusion zone is essential for prequalification and a complete understanding of the joint performance in service, as well as for conducting the defect assessment procedure for such weld joints. Therefore, an experimental investigation on the mechanical properties of laser welded joints using flat microtensile specimens (0.5 mm thick, 2 mm wide was carried out to establish a testing procedure to determine the tensile properties of the weld metal and heat-affected zone (HAZ) of the laser beam welds. In the present work, two similar joints, namely, ferritic-ferritic and austenitic-austenitic and one dissimilar ferritic-austenitic joint were produced with a CO 2 laser using 6-mm-thick steel plates. In addition to the testing of flat microtensile specimens, the mechanical properties were examined by microhardness survey and conventional transverse and round tensile specimens. The results of the microtensile specimens were compared with standard round tensile specimens, and this clearly showed the suitability of the microtensile specimen technique for such joints. The crack tip opening displacement (CTOD) tests were also performed to determine the fracture toughness of the LB welds using three-point bend specimens. The effect of strength heterogeneity (mismatchingj across the weld joint and at the vicinity of the crack tip on the CTOD fracture toughness values was also discussed.

Process behavior and stability in short circuit gas metal arc welding

Abstract: In this paper, the results of an investigation dealing with short circuit gas metal arc welding with the emphasis on process stability are presented. Welding runs were made under different conditions and, during each run, the different process parameters were continuously monitored. It was found that maximum process stability is reached under specific welding conditions. Outside this maximum, either irregular material transfer takes place with a tendency for open arc droplet transfer or stubbing of the welding wire in the weld pool starts to occur, accompanied by highly irregular melt-off behavior. The results show that process stability is directly related to weld pool oscillation. More specifically, it appears that process stability is maximum when the short circuit frequency equals the oscillation frequency of the weld pool. Under these conditions, the weld pool touches the droplet at the end of the electrode at every oscillation, which results in regular droplet transfer and high stability of the overall welding process.

Effect of welding parameters on the solidification microstructure of autogenous TIG welds in an Al-Cu-Mg-Mn alloy

Abstract: The weld metal microstructures of autogenous TIG welds have been investigated for a range of welding conditions using an Al–Cu–Mg–Mn alloy. It was found that a combination of high welding speeds and low power densities provide the thermal conditions required for the nucleation and growth of equiaxed grains in the weld pool, providing heterogeneous nucleation sites are available. The most likely origin of the nucleants is from a combination of dendrite fragments and TiB2 particles that survive in the weld pool. The finest microstructure was observed in the centre of the weld and is attributed to the higher cooling rates which operate along the weld centreline. Composition profiles across the dendrite side arms were measured in the TEM and were found to follow a Scheil type segregation behaviour where there is negligible back diffusion in the solid. The measured core concentration of the dendrite side arms was found to rise with increasing welding speed and was attributed to the formation of significant undercoolings ahead of the primary dendrite tip, which enriched the liquid surrounding the dendrite side arms.

Nature and evolution of the fusion boundary in ferritic-austenitic dissimilar weld metals : Part 1 - Nucleation and growth

Abstract: A fundamental investigation of fusion boundary microstructure evolution in dissimilar-metal welds (DMWs) between ferritic base metals and a face-centered-cubic (FCC) filler metal was conducted. The objective of the work presented here was to characterize the nature and character of the elevated-temperature fusion boundary to determine the nucleation and growth characteristics of DMWs. Type 409 ferritic stainless steel and 1080 pearlitic steel were utilized as base metal substrates, and Monel (70Ni-30Cu) was used as the filler metal. The Type 409 base metal provided a fully ferritic or body-centered-cubic (BCC) substrate at elevated temperatures and exhibited no on-cooling phase transformations to mask or disguise the original character of the fusion boundary. The 1080 pearlitic steel was selected because it is austenitic at the solidus temperature, providing an austenite substrate at the fusion boundary. The weld microstructure generated with each of the base metals in combination with Monel was fully austenitic. In the Type 409/Monel system, there was no evidence of epitaxial nucleation and growth as normally observed in homogeneous weld metal combinations. The fusion boundary in this system exhibited random grain boundary misorientations between the heat-affected zone (HAZ) and weld metal grains. In the 1080/Monel system, evidence of normal epitaxial growth wasmore » observed at the fusion boundary, where solidification and HAZ grain boundaries converged. The fusion boundary morphologies are a result of the crystal structure present along the fusion boundary during the initial stages of solidification. Based on the results of this investigation, a model for heterogeneous nucleation along the fusion boundary is proposed when the base and weld metals exhibit ferritic (BCC) and FCC crystal structures, respectively.« less

Localized Corrosion and Stress Corrosion Cracking Resistance of Friction Stir Welded Aluminum Alloy 5454

Abstract: The susceptibility of welded and unwelded samples of Al 5454 (UNS A95454) in the -O and -H34 tempers to pitting corrosion and stress corrosion cracking (SCC) in chloride solutions was studied. Welded samples were fabricated using the relatively new friction stir welding (FSW) process as well as a standard gas-tungsten arc welding process for comparison. Pitting corrosion was assessed through potentiodynamic polarization experiments. U-bend and slow strain rate tests were used to determine SCC resistance. The FSW samples exhibited superior resistance to pitting corrosion compared to the base metal and arc-welded samples. U-bend tests indicated adequate SCC resistance for the FSW samples. However, the FSW samples exhibited discontinuities that probably were associated with remnant boundaries of the original plates. These defects resulted in intermittent increased susceptibility to pitting and, particularly for Al 5454-H34 samples, poor mechanical properties in general.

Laser welding of thin sheet steel with surface oxidation

Abstract: The joining of thin sheet steel generally involves conduction mode welding, in which the reflection of the laser beam by the sheet surface is high. The absorption of laser energy by the workpiece increases significantly during keyhole laser welding, in which a vapor-plasma-filled cavity is formed. The reflectivity of cold-rolled thin sheet steel was found to be in the range of 65--80% in CO{sub 2} laser welding. The reflectivity decreased to about 30% when the sheet surface was oxidized before laser welding. In the laser welds with surface oxidation, the oxygen inclusions and porosity were not found, but the grain size was large. However, the tensile strength of all welds--with or without surface oxidation--was higher than the base metal. The toughness of the welds with surface oxidation degraded, because of the small amount of oxygen content, but it was still comparable to the toughness of the welds without surface oxidation. The oxygen content in the welds with surface oxidation was found to be slightly higher than in the welds without surface oxidation. The mechanical properties of the welds with surface oxidation were found to improve when steel powders consisting of manganese and silicon were used during welding.

Heat treatment for weld beads

Abstract: Two parts are welded together at a weld spot in a groove. A spot adjacent the weld spot is heated simultaneously and independently of the welding so that the heating and welding form a heat-treated weld bead along the groove. Preheat treatment, postheat treatment, or both, may be used simultaneously with welding to maximize performance of the resulting weld joint. Preheat and postheat treatment may be performed using lasers while the welding may be performed by TIG welding or laser welding. The welding torch and preheat and postheat lasers are coplanarly aligned.

Effect of coarse initial grain size on microstructure and mechanical properties of weld metal and HAZ of a low carbon steel

Abstract: In this study, the effects of coarse initial grain size with varying heat inputs on microstructure and mechanical properties of weld metal and heat-affected zone (HAZ) were investigated. In the welding experiments, SAE 1020 steel specimens in hot-rolled and in grain-coarsened conditions were used. The specimens taken from the hot-rolled steel (original) plate were heat treated at 1100°C for 45 min and then cooled in a furnace in order to obtain a coarse initial grain size. The original and grain-coarsened specimens were welded using a submerged arc welding machine with heat inputs of 0.5, 1 and 2 kJ/mm. Following the welding, microstructure, hardness and toughness of weld metals and HAZs were investigated. From the results, we tried to establish a relationship between initial grain size, microstructure, hardness and toughness of weld metals and HAZs. From the results of the toughness tests, it was seen that the weld metals of coarse initial grain sized specimens and original specimens exhibited nearly the same toughness values with the same heat input, whereas different HAZ toughness values were obtained with the same heat input. Maximum toughness of HAZ of the coarse initial grain sized specimen was achieved with a high input, while maximum toughness of original specimen was obtained with a medium heat input. As a result, considering the heat input, it was observed that the coarse initial grain size had a great influence on the microstructure, hardness and toughness of HAZ of a low carbon steel. Thus, taking into consideration the plate thickness, a higher heat input should be used with respect to the maximum toughness of the HAZ in the welding of grain-coarsened low carbon steels.

An analysis of heat transfer and fume production in gas metal arc welding. III

Abstract: A two–dimensional dynamic theory for predictions of arc and electrode properties in arc welding has been used to investigate heat transfer phenomena in the welding wire in gas metal arc welding (GMAW). The theory is a unified treatment of the welding wire, the plasma and the workpiece and includes a free surface treatment for the welding drops, accounting for the effects of inertia, gravity, surface tension, arc pressure, magnetic forces, and viscous drag by the gas flow around the drop. Also, the theory accounts for the variation of the surface tension coefficient with temperature and includes thermal and dynamic phenomena within the solid and liquid phases of the wire, together with a detailed treatment for the electrode sheath regions. Calculations are made for arcs in argon with wires of mild steel at currents between 150 and 325 A. Results of calculations for heat fluxes within the wire suggest that evaporation from the surface of the droplet during droplet growth has an important influence on the he...

Method for laser twist welding of compressor blisk airfoils

Abstract: A method for repair welding a titanium compressor blisk using laser twist welding. A blisk assembly has damaged portions removed. The surface to be repaired is provided with a predetermined geometry. The portions to be repaired are surrounded by a fixture which contains a laser nozzle. The fixture can provide a protective atmosphere to the area under repair while also shielding adjacent areas from damage resulting form welding. The welding is accomplished using preprogrammed patterns. The welding within the fixture is monitored remotely and is adjusted when stand-off between the weld nozzle and the molten metal are not within a predetermined range.

The extent of laser-induced thermal damage of UD and crossply composite laminates

Abstract: Composite materials exhibit poor quality cut surfaces due to spalled fibres, fuzzing, and delamination when routed by conventional tools. Laser beam cutting offers an ideal means for the cutting of fibre composite materials, being a non-contact and virtually force-free manufacturing method. However, in the shaping operation of composite materials after curing, thermal damage associated with laser energy can be produced. It leads to poor assembly tolerance and long-term performance deterioration. In the current investigations, 3-dimensional anisotropic heat conduction models based on moving point heat source for thermal analysis are presented to predict heat affected zone (HAZ). Also, the conductivity models allow to consider the anisotropic heat conductivity for unidirectional (UD) and [0/90] laminates. Taking the immersing heat source and Mirror Image Method into account would further improve the prediction of HAZ. Extensive experiments were conducted on composite materials to examine HAZ, and compared with experimental results. The analytical results show a good agreement with experiments.

Microwave Facilities for Welding Thermoplastic Composites and Preliminary Results

Abstract: The wide range of applications of microwave technology in manufacturing industries has been well documented (NRC, 1994; Thuery, 1992). In this paper, a new way of joining fibre reinforced thermoplastic composites with or without primers is presented. The microwave facility used is also discussed. The effect of power input and cycle time on the heat affected zone (HAZ) is detailed together with the underlying principles of test piece material interactions with the electromagnetic field. The process of autogenous joining of 33% by weight of random glass fibre reinforced Nylon 66, polystyrene (PS) and low density polyethylene (LDPE) as well as 23.3% by weight of carbon fibre reinforced PS thermoplastic composites is discussed together with developments using filler materials, or primers in the heterogenous joining mode. The weldability dependence on the dielectric loss tangent of these materials at elevated temperatures is also described.

Laser Transmission Welding of Thermoplastics: Analysis of the Heating Phase

Abstract: In laser transmission welding, the parts to be joined are brought into contact prior to welding, and the heating and joining phase take place simultaneously. The laser beam of the Nd:YAG laser pene...

The classification of metal weld deposits in terms of the amount of oxygen

Abstract: In analogy to the classification of metal weld deposits and arc welding processes of low-carbon and low-alloy steel in terms of the amount of hydrogen in metal weld deposits, a similar classification was given in terms of the amount of oxygen. Different methods of arc welding processes were chosen, such as welding with coated electrodes (basic, rutile, acid, oxide electrodes), shielded arc welding process MIG/MAG, submerged arc welding process (various wires, fluxes, shielded gases). The amount of oxygen and the percentage of acicular ferrite of metal weld deposits were mainly analysed and the impact toughness of it. Metallographical structures and fractography tests of metal weld deposit with varied amount of acicular ferrite were presented by putting attention to non-metallic inclusions and their morphology presented in metal weld deposit. Additional inclusions observation and measurements were done using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer. The studies were also made on the classification of metal weld deposits and arc welding processes of low carbon and low alloy steel in terms of the amount of oxygen in metal weld deposits on the following processes: low-oxygen processes, medium-oxygen processes, high-oxygen processes.

Method of solid state welding and welded parts

Abstract: A method of solid state welding for joining metal parts having opposed planar and parallel surfaces which includes quickly heating the opposed surfaces of the metal parts with a high frequency induction heater to the hot working temperature of the metal parts in a non-oxidizing atmosphere, continuously moving at least one of the parts relative to the other part generally parallel to the parallel surfaces of the parts, and quickly bringing the opposed surfaces of the parts together with an axial force approximately equal to the conventional friction welding forging force while continuing the relative motion of the parts until the absorbed kinetic energy is approximately equal to ten percent of the energy input required by conventional friction welding The resultant weld is of a quality equal to or greater than normal friction welding but has a much smaller volume of flash and requires significantly less kinetic energy than conventional friction welding and is performed at rotational velocities well below the normal critical surface velocity of friction welding The welded metal part includes a generally planar flash extending radially from the inner section of the opposed planar welded surfaces having a volume corresponding to a combined loss of length of less than 02 axial inches per inch of wall thickness Thus, the disclosed solid state welding method is substantially more efficient than prior friction or other welding methods and results in an improved product

Modified friction stir welding

Abstract: A modified method of friction stir welding of members applying a non-consumable rotating probe further comprises a preheating of the assembled members prior to the welding operation. A welding apparatus is also disclosed provided with a primary heat source attached to the probe, preferentially a high frequency moving induction coil.