Showing papers on "Heat-affected zone published in 2001"

Weld metal composition change during conduction mode laser welding of aluminum alloy 5182

Abstract: Selective vaporization of volatile elements during laser welding of automotive aluminum alloys affects weld metal composition and properties. An experimental and theoretical study was carried out to seek a quantitative understanding of the influences of various welding variables on vaporization and composition change during conduction mode laser welding of aluminum alloy 5182. A comprehensive model for the calculation of vaporization rate and weld metal composition change was developed based on the principles of transport phenomena, kinetics, and thermodynamics. The calculations showed that the vaporization was concentrated in a small high-temperature region under the laser beam where the local vapor pressure exceeded the ambient pressure. The convective vapor flux driven by the pressure gradient was much higher than the diffusive vapor flux driven by the concentration gradient. The computed weld pool geometry, vaporization rates, and composition changes for different welding conditions agreed well with the corresponding experimental data. The good agreement demonstrates that the comprehensive model can serve as a basis for the quantitative understanding of the influences of various welding variables on the heat transfer, fluid flow, and vaporization occurring during conduction mode laser welding of automotive aluminum alloys.

Mechanical properties and microstructure of gas tungsten arc welded magnesium AZ91D plates

Abstract: Gas tungsten arc (GTA) welds were performed on 4-mm thick plates, machined from as-cast magnesium AZ91D ingots. The microstructure and defect formation was investigated by optical and scanning electron microscopy. Mechanical properties were determined by standard tensile tests on small-scale specimens. A wide heat affected zone (HAZ) (>3 mm) was created adjacent to the fusion line that consisted of two regions: (1) a partially melted zone (PMZ), created near the fusion line; and (2) a wide region, which was heat affected without melting. It was found that, after resolidification, a continuous Al12Mg17 phase existed along the PMZ grain boundaries that markedly reduced the joint strength to below that of fully annealed Mg-AZ91D alloys. In contrast, specimens machined from welded metal exhibited improved strength and ductility that resulted from the microstructural refinement caused by the rapid cooling during resolidification of the fusion zone. When the creation of a large PMZ was prevented, as in electron beam welding, the joint strength was comparable to that of the base metal.

Comparison of keyhole and conventional gas tungsten arc welds in commercially pure titanium

Abstract: Keyhole gas tungsten arc welding (GTAW), a novel process recently developed at CSIRO Manufacturing Science and Technology, Australia, has been used to join commercially pure (CP) titanium. The process enables single pass, out of chamber welding of 12.7 mm thick CP titanium without expensive filler metal addition or joint preparation. The microstructure and mechanical properties of the resultant weld joint have been compared with those of a multipass conventional GTAW joint in CP titanium prepared from plates of the same thickness with a double-V edge preparation and the addition of a matching filler metal. It was shown that the differences in the net heat input and weld thermal cycles associated with the two processes led to similar microstructures, albeit of different degrees of refinement. As a result, the tensile and hardness properties of the two welded joints were similar to each other and comparable to those of the base material. The acicular nature of the fusion zone microstructure in both welds led to a significant increase in their notch toughness over that of the base material. It is concluded that the keyhole GTAW, with its significantly higher productivity combined with the simplicity of proven technology and low capital investment requirements, can be successfully applied in the welding of heavy section CP titanium, without sacrificing the metallurgical quality associated with the GTAW process.

Effect of vacuum on penetration and defects in laser welding

Abstract: The effect of vacuum on weld penetration and porosity formation was investigated in high-power cw CO2 and YAG laser welding. It was consequently confirmed in welding with both lasers that the penetration was slightly deeper in aluminum alloys and austenitic stainless steel with a decrease in the ambient pressure. It was also revealed that no porosity was present in the materials welded at lower pressures. The reason for no porosity formation in vacuum was examined by observing keyhole behavior, bubble and porosity formation situation, and liquid flow in the molten pool during high-power YAG laser welding under various conditions through the microfocused x-ray real-time observation system. It was confirmed in the coaxial Ar or He shielding gas that a lot of bubbles were generated near the bottom part of the molten pool from the tip of a fluctuated keyhole and resulted in large pores. On the other hand, under the vacuum conditions, no bubbles were formed in the melt pool from the keyhole, although the middl...

Recent developments in underwater wet welding

Abstract: Developments in underwater wet welding processes over the past 25 years are reviewed. Shielded metal arc welding with rutile base coated electrodes is still by far the most common wet welding process in use. Research and development of wet welding electrodes has led to improvements in the control of hydrogen content, porosity, chemical composition, and microstructure of the weld metal. Additional work is required to develop welding consumables with improved control over diffusible hydrogen and porosity. Development of techniques such as temper bead welding has allowed successful wet welding repairs on steels having carbon equivalents greater than the traditional limit of 0.40. Alternative wet welding processes such as flux cored arc welding and friction welding are under development, but have yet to become widely accepted.

Marangoni effect in laser deep penetration welding of steel

Abstract: In welding, the resulting weld-seam geometry may vary significantly although using constant process parameters and steels with the same material number. One likely reason for this are small variations in the concentration of sulfur, phosphorus, oxygen, and other chemical elements that are well within the tolerance of the standard of a specific alloy. These substances act as surfactants and even marginal changes strongly effect the temperature-dependent coefficient of surface tension. In simulations of conventional electric arc welding and laser heat conduction welding, the effect of the temperature-dependent coefficient of surface tension (Marangoni effect) has been identified as one of the primary driving forces of the liquid melt. In laser deep penetration welding simulations this effect has been widely neglected, so far. In this contribution, simulations of flow fields in the weld pool resulting from different temperature dependencies of the coefficient of surface tension are presented. The simulations...

Friction stir welding of polymeric materials

Abstract: A method and apparatus for welding thermoplastic materials with a stir welding system using a rotating element pin or element with a geometry to induce frictional energy from the rotating element. A floating constraining surface that has a motion independent of the rotation the pin (preferably stationary) is used to constrain ejection of material from the zone where the weld is being formed. Molten material is retained in the weld region, avoiding weakness and voids at the weld joint. Optionally a system for introducing thermal energy into the weld zone in addition to the frictional energy is used. The system for introducing the thermal energy may be in association with the constraining shoe and/or the rotating tool, or a system independent of these.

Degradation of Creep Strength in Welded Joint of 9%Cr Steel

Abstract: The degradation of creep strength in the heat affected zone (HAZ) of welded joint has been investigated for a tungsten-strengthened 9Cr steel, 9Cr-0.5Mo-1.8W-VNb. The creep test was carried out for the simulated HAZ specimens and the welded joint at 923 K. The creep rupture strength of the welded joint is almost the same as that of the base metal at high stresses but it decreases rapidly and then it becomes almost the same as that of the Ac3 simulated HAZ specimen at low stresses. The creep fracture of the welded joint occurs at the fine-grained zone of HAZ, corresponding to the Ac3 heating, at low stresses. The fine-grained zone of HAZ contains higher density of dislocations than the base metal. The recovery of higher density of dislocations and the sparse distribution of large M23C6 carbides promote the formation of coarse subgrains near prior austenite grain boundaries. This results in the concentration of creep deformation in the coarse subgrains, which accelerates eventual creep fracture.

Characterisation of microstructure and texture in friction stir welded joints of 5754 and 5182 aluminium alloy sheets

Abstract: The microstructure and texture of friction stir welded joints of hot mill stock 5754 and 5182 aluminium alloy sheets were examined after a commercial joining process. The as received sheet was cut into two pieces, one piece was cold rolled with 20 reduction in thickness, whereas the other was subsequently aged at 230C for 100 h after the 20 prestrain. These two pieces with different thermal pretreatments were joined by friction stir welding FSW. On FSW, a uniform fine grained microstructure in the through thickness of the sheet having a width corresponding to the diameter of tool shoulder was observed in both the 5754 and 5182 joints. Moreover, the microhardness profile did not reveal a softened heat affected zone between the weld and the base metal. An X-ray pole figure study showed that the 5754 and 5182 weld zones were not highly textured and both comprised of similar texture components. The calculated average sheet formability of the weld zones predicts a more isotropic behaviour than that for...

Microstructure and Mechanical Properties of Weld Fusion Zones in Modified 9Cr-1Mo Steel

Abstract: Modified 9Cr-1Mo steel finds increasing application in power plant construction because of its excellent high-temperature properties While it has been shown to be weldable and resistant to all types of cracking in the weld metal and heat-affected zone (HAZ), the achievement of optimum weld metal properties has often caused concern The design of appropriate welding consumables is important in this regard In the present work, plates of modified 9Cr-1Mo steel were welded with three different filler materials: standard 9Cr-1Mo steel, modified 9Cr-1Mo, and nickel-base alloy Inconel 182 Post-weld heat treatment (PWHT) was carried out at 730 and 760 °C for periods of 2 and 6 h The joints were characterized in detail by metallography Hardness, tensile properties, and Charpy toughness were evaluated Among the three filler materials used, although Inconel 182 resulted in high weld metal toughness, the strength properties were too low Between modified and standard 9Cr-1Mo, the former led to superior hardness and strength in all conditions However, with modified 9Cr-1Mo, fusion zone toughness was low and an acceptable value could be obtained only after PWHT for 6 h at 760 °C The relatively poor toughness was correlated to the occurrence of local regions of untransformed ferrite in the microstructure

Welding superalloy articles

Abstract: A process is provided for welding a nickel or cobalt based superalloy article to minimize cracking by preheating the entire weld area to a maximum ductility temperature range, maintaining such temperature during welding and solidification of the weld, raising the temperature for stress relief of the superalloy, then cooling at a rate effective to minimize gamma prime precipitation.

An assessment of creep deformation and fracture behavior of 2.25Cr-1Mo similar and dissimilar weld joints

Abstract: The evaluation of the creep deformation and fracture behavior of a 2.25Cr-1Mo steel base metal, a 2.25Cr-1Mo/2.25Cr-1Mo similar weld joint, and a 2.25Cr-1Mo/Alloy 800 dissimilar weld joint at 823 K over a stress range of 90 to 250 MPa has been carried out. The specimens for creep testing were taken from single-V weld pads fabricated by a shielded metal arc-welding process using 2.25Cr-1Mo steel (for similar-joint) and INCONEL 182 (for dissimilar-joint) electrodes. The weld pads were subsequently given a postweld heat treatment (PWHT) of 973 K for 1 hour. The microstructure and microhardness of the weld joints were evaluated in the as-welded, postweld heat-treated, and creep-tested conditions. The heat-affected zone (HAZ) of similar weld joint consisted of bainite in the coarse-prior-austenitic-grain (CPAG) region near the fusion line, followed by bainite in the fine-prior-austenitic-grain (FPAG) and intercritical regions merging with the unaffected base metal. In addition to the HAZ structures in the 2.25Cr-1Mo steel, the dissimilar weld joint displayed a definite INCONEL/2.25Cr-1Mo weld interface structure present either as a sharp line or as a diffuse region. A hardness trough was observed in the intercritical region of the HAZ in both weld joints, while a maxima in hardness was seen at the weld interface of the dissimilar weld joint. Both weld joints exhibited significantly lower rupture lives compared to the 2.25Cr-1Mo base metal. The dissimilar weld joint exhibited poor rupture life compared to the similar weld joint, at applied stresses lower than 130 MPa. In both weld joints, the strain distribution across the specimen gage length during creep testing varied significantly. During creep testing, localization of deformation occurred in the intercritical HAZ. In the similar weld joint, at all stress levels investigated, and in the dissimilar weld joint, at stresses ≥150 MPa, the creep failure occurred in the intercritical HAZ. The fracture occurred by transgranular mode with a large number of dimples. At stresses below 150 MPa, the failure in the dissimilar weld joint occurred in the CPAG HAZ near to the weld interface. The failure occurred by extensive intergranular creep cavity formation.

Quality and productivity improvement in aluminium alloy thin sheet welding using alternating current pulsed metal inert gas welding system

Abstract: An alternating current (ac) pulsed metal inert gas (MIG) welding power source has been developed for welding thin sheets of aluminium alloys and the process features are investigated. Advantages such as high wire melting coefficient, low heat input, shallow penetration, and increased reinforcement height are obtained at high values of electrode negative ratio (ratio of electrode negative current integration to electrode negative plus electrode positive current integration over one pulse cycle). These features successfully counteract the problem of burnthrough in welding of thin sheet joints and greatly improve the bridging ability for wide gap joints. Thin sheet joints can be welded at high speed and with low distortion. By integrating the present welding power source with a welding robot, welding process and current waveform parameters can be defined by key operations in the teach pendant. It is possible to switch between welding processes such as ac pulsed MIG, direct current (dc) pulsed MIG, lo...

Effects of Cooling Time and Alloying Elements on the Microstructure of the Gleeble-Simulated Heat- Affected Zone of 22% Cr Duplex Stainless Steels

Abstract: The effects of austenite stabilizers, such as nitrogen, nickel, and manganese, and cooling time on the microstructure of the Gleeble simulated heat-affected zone (HAZ) of 22% Cr duplex stainless steels were investigated The submerged are welding was performed for comparison purposes Optical microscopy (OM) and transmission electron microscopy (TEM) were used for microscopic studies The amount of Cr2N precipitates in the simulated HAZ was determined using the potentiostatic electrolysis method The experimental results indicate that an increase in the nitrogen and nickel contents raised the δ to transformation temperature and also markedly increased the amount of austenite in the HAZ The lengthened cooling time promotes the reformation of austenite An increase in the austenite content reduces the supersaturation of nitrogen in ferrite matrix as well as the precipitation tendency of Cr2N The optimum cooling time from 800 to 500 °C (Δt8/5) obtained from the Gleeble simulation is between 30 and 60 s, which ensures the austenite content in HAZ not falling below 25% and superior pitting and stress corrosion cracking resistance for the steels The effect of manganese on the formation of austenite can be negligible

Resistance Welding of Metal/Thermoplastic Composite Joints

Abstract: An investigation of the resistance welding between carbon fiber (CF) rein-forced polyetherimide (PEI) and aluminium substrates (7075-T6 grade alloy) is presented. A three-dimensional transient finite element model (FEM) featuring heat transfer, consolidation and thermal degradation was used for simulating the process. Two mechanisms are distinguished in the consolidation model: (1) removal of the initial surface profile of the laminate modelled by the establishment of intimate contact between the two substrate surfaces and (2) penetration of the thermoplastic (TP) polymer in the micro-pores of the aluminium oxide surface modelled using a capillary flow model. The “optimal” welding time based on the maximum lap shear strength (LSS) was determined for various power levels and correlated to the bonding time predicted by the FEM. Consolidation quality and failure mechanisms were discussed in relation to processing parameters. The effect of the welding operation on overaging (annealing) of the aluminium alloy ...

Bacterial attachment to stainless steel welds: Significance of substratum microstructure

Abstract: AISI Type 304 L stainless steel (SS) is a widely used material in industry due to its strength and resistance to corrosion. However, corrosion on SS is reported largely at welds or adjacent areas. Bacteria were observed to colonize preferentially near welds as a result of surface roughness. In the present study, the influence of another important metal surface condition on bacterial adhesion has been evaluated, i.e. substratum microstructure. Type 304 L SS weld samples were prepared and machined to separate weld metal, the heat affected zone (HAZ) and base metal regions. The coupons were molded in resin so that only the surfaces polished to a 3 p.m finish were exposed to the experimental medium with Pseudomonas sp. isolated from a corrosive environment in Japan. The coupons were exposed for varying durations. The area of bacterial attachment showed significant differences with time of exposure and; the type of coupons. Generally, the weld metal samples showed more attachment whilst the base metal showed t...