Effect of post-weld aging treatment on mechanical properties of Tungsten Inert Gas welded low thickness 7075 aluminium alloy joints

This paper reports the influence of post-weld aging treatment on the microstructure, tensile strength, hardness and Charpy impact energy of weld joints low thickness 7075 T6 aluminium alloy welded by Tungsten Inert Gas (TIG). Hot cracking occurs in aluminium welds when high levels of thermal stress and solidification shrinkage are present while the weld is undergoing various degrees of solidification. Weld fusion zones typically exhibit microstructure modifications because of the thermal conditions during weld metal solidification. This often results in low weld mechanical properties and low resistance to hot cracking. It has been observed that the mechanical properties are very sensitive to microstructure of weld metal. Simple post-weld aging treatment at 140 °C applied to the joints is found to be beneficial to enhance the mechanical properties of the welded joints. Correlations between microstructures and mechanical properties were discussed.     

The formation and control of Laves phase in superalloy 718 welds

Weld heat input/cooling rate (affected by welding process, parameters, technique, tooling, etc.) was found to influence the microstructural characteristics and segregational features in alloy 718 welds, with low heat inputs proving beneficial. Laves phase formed in the interdendritic regions of the weld metals as a result of segregation. The morphology and composition of Laves phase depended strongly on heat input/cooling rate and influenced its response to subsequent homogenization post-weld heat treatment. The various factors affecting the formation and control of Laves phase in alloy 718 welds are highlighted. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of pre- and post-weld heat treatment on metallurgical and tensile properties of Inconel 718 alloy butt joints welded using 4 kW Nd:YAG laser

The effects of pre- and post-weld heat treatments on the butt joint quality of 3.18-mm thick Inconel 718 alloy were studied using a 4 kW continuous wave Nd:YAG laser system and 0.89-mm filler wire with the composition of the parent metal. Two pre-weld conditions, i.e., solution treated, or solution treated and aged, were investigated. The welds were then characterized in the as-welded condition and after two post-weld heat treatments: (i) aged, or (ii) solution treated and aged. The welding quality was evaluated in terms of joint geometries, defects, microstructure, hardness, and tensile properties. HAZ liquation cracking is frequently observed in the laser welded Inconel 718 alloy. Inconel 718 alloy can be welded in pre-weld solution treated, or solution treated and aged conditions using high power Nd:YAG laser. Post-weld aging treatment is enough to strengthen the welds and thus post-weld solution treatment is not necessary for strength recovery.     

Studies on creep/stress rupture behaviour of superalloy 718 weldments used in gas turbine applications

Superalloy 718 in the solution-treated condition was welded autogenously by electron beam welding and gas tungsten are welding processes. The weldrnents after suitable heat treatment were subjected. to creep/stress rupture testing at 650°C and 690 MPa. The results showed that the Laves phase resulting in the weld metalis deterimental to the creep rupture life of weld metals when present With the continuous morphology found in gas tungsten are welds. The lower amounts and discontinuous morphology of the Laves phase in electron beam weldments in combination With favourable gramonentauon resulted in relatively better rupture properties for these weldments.     

Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments

Abstract

The response to post-weld heat treatment of an 18%Ni (250 grade) gas tungsten arc weld metal has been investigated. The post-weld heat treatments are (a) direct aging at 480°C/3 h/air cooling, (b) solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling and (c) homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling. Metallographic characterisation of fusion zone revealed pronounced segregation of titanium and molybdenum along the interdendritic and intercellular boundaries. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Solutionised treatment at 815°C does not remove the segregation. Homogenisation treatment (1150°C/1 h/air cooling) succeeded in making the composition become homogenised. Mechanical properties including tensile, hardness and impact toughness were evaluated. Tensile test results showed that directly aged weldments exhibited lower strength but higher ductility than the other cases; this was attributed to the presence of reverted austenite. Homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling resulted in optimum tensile properties. A substantial increase in fusion zone toughness was observed after homogenisation+solutionising+aged condition due to a decrease in the content of austenite content compared to the directly aged condition. The reduction in microsegregation by diffusion of alloying elements from cell boundaries to the cell during homogenisation treatment is responsible for the decrease in austenite content.     

Influence of phosphorus on weld heat affected zone toughness in niobium microalloyed steels

Abstract

Weld heat affected zone microstructures have been simulated in a niobium microalloyed C–Mn steel. These structures have been Charpy impact tested and examined metallographically. Troughs were observed in the plots of Charpy energy against post-weld heat treatment time at 600°C. The explanation for this behaviour is shown to be associated with non-equilibrium segregation of phosphorus to lath boundaries in the steel. Mathematical modelling of non-equilibrium segregation, together with high spatial resolution analytical electron microscopy results, support this contention. Using the predictions of the model, several suggestions are made concerning the alleviation of phosphorus segregation to boundaries and hence the reduction of cracking susceptibility during post-weld heat treatment of the steel.

MST/994     

Influence of Aging treatment on the microstructure and mechanical properties of T92/Super 304H dissimilar metal welds

Influence of aging treatment on the microstructure and mechanical properties of T92/Super 304H dissimilar steel joints was investigated. The microstructure of T92/Super 304H dissimilar steel joints was characterized using optical microscopy, scanning electron microscopy and energy dispersive spectrometer. The results show that the tensile strength of dissimilar metal welds (DMWs) after 10,000 h aging treatment met the ASME T92 and Super 304H standards. Rupture positions were located in the T92 base metal because of the precipitates formed along the sub grain and prior grain boundaries. The tensile strength of DMWs initially increased with time up to 4000 h, then decreased between 4000 to 6000 h, and finally came to almost a constant value from 8000 to 10,000 h exposure. The decrease in the tensile strength resulted from the nucleation and growth of Laves phases at the sub-grains and prior austenitic grain boundaries. The low absorption of impact energy in the weld metal was related to the coarse grains and its grain orientation.     

High temperature mechanical properties of cryogenically cooled alloy 718 weldments

The micro-segregation of niobium and Laves phase formation during Gas Tungsten Arc (GTA) welding of alloy 718 were studied by employing different cooling techniques which resulted in weld cooling rates ranging from 43.2 °C/s to 508.7 °C/s. The weld cooling rates were achieved by the combinations of modified pulse wave form, argon and helium shielding gases, copper heat sink and liquid nitrogen cooling. A combination of compound current pulse (CCP) waveform with helium shielding and liquid nitrogen cooling resulted in lesser niobium segregation and discontinuous, finer Laves particles in the interdendritic regions compared to that of the conventional constant current GTA weld metal. This process yielded better ageing response and improved high temperature mechanical properties of the weldments.     

Effect of post-weld heat treatment on heat affected zone microstructures of microalloyed C–Mn submerged arc welds

Abstract

The effect of various post-weld heat treatment (PWHT) cycles on the as welded heat affected zone (HAZ) microstructures of C–Mn steels microalloyed with niobium, or niobium plus vanadium, has been studied. Single pass welds were produced at an arc energy of 3·5 kJ mm^?1; examination was carried out using optical and electron microscopy, along with hardness and crack tip opening displacement testing. As welded, the C–Mn–Nb HAZ contained a significant proportion of auto tempered martensite. After PWHT at 550°C, isolated hard regions remained, but at 600°C all hard regions had been removed, with a concomitant increase in cleavage resistance. In contrast, ferrite with aligned second phase with lower hardness was found mainly in the as welded HAZ of the C–Mn–Nb–V steel. When the PWHT temperature was raised, HAZ hardness increased to a maximum at 600°C; overaging would be required to obtain improved toughness, although this would soften the parent plate. The results indicate that the current practice of specifying a common heat treatment procedure for steels to a given specification is not satisfactory; allowance should be made for the particular composition and as welded HAZ microstructure.

MST/1190     

Microstructure evolution of friction welded dissimilar joint during subsequent hot work

Abstract

The present paper reports the influence of hot working conditions on the microstructure of Ti₂AlNb/TC11 dissimilar joint. Linear friction welding technique was used to fabricate the joints. The microhardness and tensile properties of the joints have been tested. It was found that the fine structure of linear friction welds underwent abnormal grain growth and abnormal grain boundary phase growth in the post-weld solution heat treated condition. This phenomenon significantly deteriorated the ductility of the joint. After appropriate hot work, abnormal big grains/phases disappeared, and the joint exhibited good tensile properties due to its fine structures.     

Tensile properties and fracture behavior of friction stir welded joints of Fe-32Mn-7Cr-1Mo-0.3N steel at cryogenic temperature

This study investigates the cryogenic tensile properties and fracture behavior of fiction stir welded and post-weld heat-treated joints of 32 Mn-7 Cr-1 Mo-0.3 N steel. Cryogenic brittle fracture, which occurred in the as-welded joint, is related to the residual particles that contain tungsten in the joint band structure. Post-weld water toughening resulted in the cryogenic intergranular brittleness of the joint, which is related to the non-equilibrium segregation of solute atoms during the post-weld water toughening.Annealing at 550℃ for 30 min can effectively inhibit the cryogenic intergranular brittleness of the postweld water-toughened joint. The yield strength, ultimate tensile strength, and uniform elongation of the annealed joint are approximately 95%, 87%, and 94% of the corresponding data of the base metal.     

Cyclic overaging pre-weld heat treatment of Rene 80: effect of solution treatment and end aging temperatures

Abstract

The present work investigated the effect of a cyclic overaging pre-weld heat treatment proposed by Lim (US Patent no. 5 509 980, 1996) on the microstructure, tensile ductility, and weldability of Rene 80 nickel based superalloy (approximate composition 60Ni–14Cr–9.5Co–4Mo–5Ti–3Al– 0.17C–Zr–B, wt-%), with a focus on the role of solution treatment and end aging temperatures. The results showed that maximum ductility corresponds to a microstructure consisting of very coarse and widely spaced γ', with fine γ' not resolvable even at a magnification of × 10 000. This was achieved by cyclic cooling the material from the solution temperature to a temperature in the range 500–750 ° C followed by fast furnace cooling to suppress the precipitation of fine γ'. The solution treatment temperature was found to control the number density of coarse γ', which, in turn, slightly influenced the optimum end aging temperature. After the optimum cyclic overaging heat treatment with a solution temperature of 1080°C and an end aging temperature of 550°C, the material exhibited a tensile type fracture path with a high registered tensile ductility of ~ 30% in elongation. Fractographic studies revealed that the necked γ ligaments were thicker and taller, with an increased frequency of cleaved coarse γ' occurring at different levels in the material. Precision welding tests showed that the weldability of the optimally overaged alloy was significantly improved, owing to both the resultant soft and ductile base metal and the reduced heat affected zone size.     

Correlation of microstructure with mechanical properties of TIG weldments of Ti–6Al–4V made with and without current pulsing

This paper deals with the influence of direct current pulsing on the microstructure, room temperature hardness and tensile properties at four different temperatures of tungsten inert gas (TIG) weldments of Ti–6Al–4V. Autogenous full-penetration bead-on-plate TIG welds were made with and without direct current pulsing. A few coupons were subjected to a post-weld heat treatment (PWHT) at 900 °C. Room temperature hardness and tensile properties at four different temperatures (25, 150, 300 and 450 °C) of the weldments in both as-welded and PWHT conditions were studied and correlated with the microstructure. Current pulsing resulted in slight refinement of prior β grains leading to higher hardness, tensile strength and ductility of weldments in the as-welded condition. The post-weld heat treatment at 900 °C resulted in improvement in ductility and reduction in strength of weldments (both unpulsed and pulsed) owing to more coarsening of α, reduction in defect density and decomposition of martensite to equilibrium α and β. Both pulsed and unpulsed weldments after PWHT exhibited almost the same values of strength and ductility. This may be attributed to the width of the α plates being almost the same in both welds.     

热处理工艺对Inconel718合金多层夹芯板结构的微观组织与力学性能的影响

采用LBW+SPF组合技术制造Inconel718合金多层夹芯板结构.为了增强多层夹芯板结构使用时的安全性,研究其热处理技术.结果表明:Inconel718合金在焊接过程产生了Nb含量较高的Laves沉淀相;超塑成形后焊缝中的Nb元素的偏析问题得了缓解;经980℃固溶30min处理后,焊缝中的δ相完全回溶母体γ相,焊缝...     

A comparative evaluation of friction and electron beam welds of near-α titanium alloy

Electron beam and friction welding studies have been carried out on a TIMET 834 a near-α titanium alloy. The study was also focused on the post-weld heat treatment aspects and their influence on mechanical properties. The influence of parent metal microstructure on the toughness has been evaluated to explain the trends observed on the toughness of welds. Electron beam welds contained micro-porosity while friction welds were free from such solidification related defects. Friction welds exhibited very fine equiaxed prior β grains as compared to electron beam welds. Electron beam welds exhibited better creep and stress rupture properties and marginally higher strength. The impact toughness of both the welds was comparable. Post-weld ageing of welds resulted in reduction in impact toughness due to precipitation of silicides along the martensite lath boundaries.     

Influence of precipitation and segregation on hydrogen embrittlement in aged 9Cr–1Mo steel

Abstract

A study is reported of temper embrittlement and hydrogen embrittlement in a series of model 9Cr–1Mo steel alloys in which the levels of silicon and phosphorus have been varied to separate the formation of the brittle intermetallic (Laves) phase from the segregation of phosphorus during aging. Phosphorus segregation was mildly detrimental to ductility properties, Laves phase formation was more detrimental, and their effects combined produced the most severe loss in ductility. Hydrogen effects were additive to those of aging. In unaged material without silicon enrichment, only M₂₃C₆ precipitates were detected, with little phosphorus segregation. With silicon enrichment, phosphorus segregation to lath and grain boundaries was enhanced. This enhancement increased the susceptibility of the materials to hydrogen embrittlement, promoting transgranular cleavage and chisel fracture. In aged material, the high phosphorus alloys showed some grain boundary segregation, but only limited interaction with hydrogen. In the high silicon alloys, the formation of Laves phase was most evident. This enhanced hydrogen embrittlement resulted in extensive chisel, transgranular cleavage, and some intergranular fracture. In the high silicon high phosphorus alloy, both Laves phase formation and phosphorus segregation were evident. This resulted in enhanced susceptibility to hydrogen embrittlement, producing intergranular fracture. Thus, silicon controls the susceptibility to hydrogen embrittlement in unaged alloy by promoting phosphorus segregation and in aged alloy by promoting Laves phase formation. In the aged alloy, segregation of phosphorus can enhance the effect of silicon.

MST/1785     

Effect of heat treatment on tensile properties of friction stir welded joints of 2219-T6 aluminium alloy

Abstract

The effect of post-weld heat treatment (PWHT) on the tensile properties of friction stir welded (FSW) joints of 2219-T6 aluminium alloy was investigated. The PWHT was carried out at aging temperature of 165°C for 18 h. The mechanical properties of the joints were evaluated using tensile tests. The experimental results indicate that the PWHT significantly influences the tensile properties of the FSW joints. After the heat treatment, the tensile strength of the joints increases and the elongation at fracture of the joints decreases. The maximum tensile strength of the joints is equivalent to 89% of that of the base material. The fracture location characteristics of the heat treated joints are similar to those of the as welded joints. The defect free joints fracture in the heat affected zone on the retreating side and the joints with a void defect fracture in the weld zone on the advancing side. All of the experimental results can be explained by the hardness profiles and welding defects in the joints.     

Inertia-friction welding of SiC-reinforced 8009 aluminium

Inertia-drive friction welding (IFRW) of an 8009 Al alloy (Al-8.5 Fe-1.7 Si-1.3 V, wt%) reinforced with 11 volume per cent SiC particles (8009/SiC/11_p) has been investigated. Inertia-drive friction welds were made with constant energy at two levels of axial force. The microstructures of the base material and the welds were characterized using optical and scanning electron microscopy, while the mechanical properties were evaluated using microhardness and tensile testing. Examination of weld sections revealed that the hot deformation experienced during welding produced a homogenized microstructure with a uniform distribution of SiC particles along the bond line. No evidence of a chemical reaction between the SiC and the matrix was found in any of the welds, but cracking of some of the larger SiC particles was observed in the base material as well as in the IFR welds. The average microhardness of the various heat-and-deformation affected zones (HDZs) of the welds did not vary greatly from that of the base material, and no weld induced weak regions were discerned. The room-temperature (RT) tensile strength of the IFR welds exceeded 90 per cent of the base material. The weld tensile specimens failed at the outer edge of the HDZ for all of the welds tested. The fracture surface of the 8009 matrix of tensile samples for both the base material and the welds exhibited a dimpled appearance indicating a ductile failure, while fracture through the SiC appeared to occur in a brittle fashion. IFRW has proven effective in joining 8009/SiC/11_p with little loss in RT hardness and tensile properties.     

Study of welding peak temperatures on microstructures and hardness of heat affected zone in 2024-T3 aluminium alloy

Abstract

The influence of welding thermal cycle peak temperatures and post-weld heat treatments on the microstructures and mechanical properties of the heat affected zone (HAZ) for 2024-T3 aluminium alloy have been investigated by Gleeble HAZ simulation. Differential scanning calorimetry (DSC) in conjunction with transmission electron microscopy (TEM) is used to characterise the HAZ microstructures. The welded HAZ in the region of peak temperature 414°C has the lowest hardness after natural aged temper, which is primarily due to the precipitation and coarsening of stable S phases. When the peak temperature of welded HAZ is larger than 414°C, the hardness of HAZ increasing with an increasing peak temperature can be seen, which is due to higher peak temperature thermal cycles treatment inducing the dissolution of precipitations in the matrix, and, after the natural aging treatment, Guinier-Preston (GPB) and GPB2 zones precipitating out in the matrix again can be seen. Post-weld T81 artificial aging (PWAA-T81) heat treatment has no effect on improving the HAZ hardness; the HAZ hardness of the 2024-T3 alloy obtained by PWAA-T81 is less than that obtained by natural aging, and its lowest hardness is shifted to the region of peak temperature, which is 452°C, because overaging induces coarse and sparse amounts of stable S phase.     

Thermal cycles and their effects during friction stir welding of AA7075 thicker plates with and without in-process cooling

Friction stir welding of AA 7075 plates in three different thicknesses such as 10, 16 and 25 mm at natural convection condition was carried out successfully without defects. Water cooled friction stir welds were also produced on 16 mm thick plates. The thermal cycles at different locations of the plate, during the friction stir welding process, were predicted using a three-dimensional thermal model. Mechanical properties of the welds were evaluated using tensile and hardness tests. Weld microstructures were also examined with optical and transmission electron microscopes. The weld hardness values and tensile properties were found to decrease with increase in plate thickness. The use of water cooling was found to improve the weld properties to some extent, although not to the level of base metal. The reasons for this behavior are discussed, correlating thermal cycles, mechanical properties, fracture locations and precipitate morphology.