Effect of post-weld heat treatment on the microstructure and plastic deformation behavior of friction stir welded 2024

To improve the plasticity of friction stir welded joints for plastic processing applications, post-weld heat treatment (PWHT) of 2024-O aluminum alloy friction stir welding joints was carried out at annealing temperatures from 250 °C to 450 °C with an interval of 50 °C for 2 h, followed by cooling to 200 °C in the furnace. The effect of PWHT on the microstructure and plastic deformation behavior of the joints was investigated. It was found that the fine-equiaxed grains are stable and retained in the nugget of the joints even after annealing at 450 °C for 2 h. However, the grains in the thermo-mechanically affected zone (TMAZ) of the joints become coarse and equiaxed as annealing temperature increases. The plastic deformation of as-welded joint is very heterogeneous. In contrast, the plastic deformation of PWHT joint is relatively homogeneous by both the nugget and the base material showing large deformation. The decrease in elongation of as-welded joints is completely recovered by PWHT. The high ductility of the joint is mainly attributed to the retention of the fine-equiaxed grains in the nugget during PWHT.     

On the weldability of grey cast iron using nickel based filler metal

Shielded metal arc welding process using nickel based filler metal was used to join grey cast iron. The effect of post weld heat treatment (PWHT) on the microstructure and hardness was studied. PWHT included heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling. By using nickel based filler metal, formation of hard brittle phase (e.g. carbides and martensite) in the fusion zone is prevented. Before PWHT, heat affected zone exhibited martensitic structure and partially melted zone exhibited white cast iron structure plus martensite. Applied PWHT resulted in the dissolution of martensite in heat affected zone and graphitization and in turn the reduction of partially melted zone hardness. Results showed that welding of grey cast iron with nickel based filler metal and applying PWHT can serve as a solution for cast iron welding problems.     

Warm and room temperature deformation of friction stir welded thin aluminium sheets

Friction stir welding is a welding solid state process of large potential advantages for aerospace and automotive industries dealing with light alloys. The metal to be welded is not melted and this avoids welding defects such as cracks and porosity. Moreover, there is no significant deterioration in mechanical properties due to phase transformations in the joint and low-cost and high-quality joints can be produced even from heat-treatable aluminium alloys, notably difficult to weld. In this study, very thin rolled sheets (0.8 mm in thickness) of 2024T3 and 6082T6 were friction stir welded, parallel to the rolling directions, obtaining similar joints (2024T3–2024T3 and 6082T6–6082T6) and dissimilar joints (6082T6–2024T3). Tensile tests at temperatures and strain rates of 170–230 °C and 10⁻³–10⁻⁵ s⁻¹ respectively were performed on the thin joints. The flow stress decreased with increasing temperature and decreasing strain rate. The ductility was quite independent from temperature and strain rate. The tensile stress–strain curves of the thin dissimilar joints placed at an intermediate level between the high strength 2024T3–2024T3 and low strength 6082T6–6082T6 flow curves. The fracture occurred in the middle of the stir zone for all the investigated joints and was of ductile type. Microhardness profiles were slightly modified by straining.     

Mechanical properties evolution during post-welding-heat treatments of double-lap Friction Stir Welded joints

The present study focuses on double-lap Friction Stir Welded (FSW) joints in 2024T3 and 7075T6 aluminium alloys subjected to several post-welding-heat treatments at warm (typical aging) and high temperature (solution range) followed by room temperature deformation (tensile tests). The effect of post-welding-heat treatments on the microstructure and mechanical properties of double lap FSW joints were investigated. Polarized Optical Microscopy (POM) and Scanning Electron Microscopy (SEM) analysis reveal a progressive change in grain size and morphology in high temperature post-welding-heat treated joints, leading to Abnormal Grain Growth in the stir zone. Stress–strain curves are rather flat for 200° and 300 °C post-welded heat treated joints while, for the other set of samples, stress increases with strain to reach maximum stress of 140–160 MPa. Micro-hardness profiles measured on transversal sections of post-welded heat treated joints reveal conditions (temperature and time) of hardness homogeneity at top, bottom and central nugget zone and/or along the whole measured profile. When homogeneity is reached, fracture occurs in the nugget. A relationship between hardness and tensile properties has been applied in the nugget.     

Effect of a two-step solution heat treatment on the microstructure and mechanical properties of 332 aluminium silicon cast alloy

This paper investigated the effect of a two-step solution heat treatment on the mechanical properties and silicon-rich phase of 332 aluminium alloy. Traditional single-step T6 solution treatment (495 °C/6 h) increased the hardness value of the alloy by 5.96%, increased the tensile strength by 20.42% and reduced the elongation by 3.97%. Two-step solution treatment of the alloy (495 °C/2 h followed by 515 °C/4 h) increased the hardness value by 6.64%, increased the tensile strength by 16.01%, and reduced the elongation by 4.67% compared to the as-cast samples. Both solution treatments were followed by hot water quenching (75–90 °C) and artificial aging at 250 °C for 4 h. The difference in mechanical properties after heat treatment can be linked to the refinement and the spheroidisation of the silicon-rich phase in the alloy.     

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.     

The effect of post weld heat treatment on the properties of 6061 friction stir welded joints

Post weld heat treatment (PWHT) of Friction Stir Welds (FSW) was carried out at solutioninsing temperatures of 520, 540, and 560°C followed by ageing at 175°C or 200°C. It was found that the weld (stir) region exhibited very coarse grains after the PWHT. The hardness was found to be uniform across the weldment after the PWHT. The samples failed after PWHT during root bend test. Heat treatments to reduce the grain size did not reduce the brittleness of the welds. The brittleness was attributed to the presence of precipitate free zones adjacent to the grain boundaries and the equiaxed structure of the weldment microstructure and the failure was due to a ductile intergranular fracture mechanism.     

Improvement of weld temperature distribution and mechanical properties of 7050 aluminum alloy butt joints by submerged friction stir welding

Submerged friction stir welding (FSW) in cold and hot water, as well as in air, was carried out for 7050 aluminum alloys. The weld thermal cycles and transverse distributions of the microhardness of the weld joints were measured, and their tensile properties were tested. The fracture surfaces of the tensile specimens were observed, and the microstructures at the fracture region were investigated. The results show that the peak temperature during welding in air was up to 380 °C, while the peak temperatures during welding in cold and hot water were about 220 and 300 °C, respectively. The temperature at the retreated side of the joint was higher than that at the advanced side for all weld joints. The distributions of microhardness exhibited a typical “W” shape. The width of the low hardness zone varied with the weld ambient conditions. The minimum hardness zone was located at the heat affected zone (HAZ) of the weld joints. Better tensile properties were achieved for joint welded in hot water, and the strength ratio of the weld joint to the base metal was up to 92%. The tensile fracture position was located at the low hardness zone of the weld joints. The fracture surfaces exhibited a mixture of dimples and quasi-cleavage planes for the joints welded in cold and hot water, and only dimples for the joint welded in air.     

Mechanical properties of additive manufactured titanium (Ti–6Al–4V) blocks deposited by a solid-state laser and wire

In this paper, the mechanical properties and chemical composition of additive manufactured Ti–6Al–4V blocks are investigated and compared to plate material and aerospace specifications. Blocks (seven beads wide, seven layers high, 165 mm long) were deposited using a 3.5 kW Nd:YAG laser and Ti–6Al–4V wire. Two different sets of process parameters are used and three different conditions (as-built, 600 °C/4 h, 1200 °C/2 h) of the deposit are investigated. The particular impurity levels of the blocks are considerably below those tolerated according to aerospace material specifications (AMS 4911L). Static tensile samples are extracted from the blocks in the deposition direction and punch samples are extracted in the building direction. The experiments show that as-deposited Ti–6Al–4V can achieve strength and ductility properties that fulfill aerospace specifications of the wrought Ti–6Al–4V material (AMS 4928). The 600 °C/4 h heat treatment leads to a significantly higher strength in the deposition direction, but can also decrease ductility. The 1200 °C/2 h treatment tends to decrease the alloy’s strength.     

Effect of rotation speed on microstructure and mechanical properties of Ti–6Al–4V friction stir welded joints

The effect of tool rotation speed on microstructure and mechanical properties of friction stir welded joints was investigated for Ti–6Al–4V titanium alloy. Joints were produced by employing rotation speeds ranging from 400 to 600 rpm at a constant welding speed of 75 mm/min. It was found that rotation speed had a significant impact on microstructure and mechanical properties of the joints. A bimodal microstructure or a full lamellar microstructure could be developed in the weld zone depending on the rotation speeds used, while the microstructure in the heat affected zone was almost not influenced by rotation speed. The hardness in the weld zone was lower than that in the base material, and decreased with increasing rotation speed. Results of transverse tensile test indicated that all the joints exhibited lower tensile strength than the base material and the tensile strength of the joints decreased with increasing rotation speed.     

Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy

Abstract

Post-weld heat treatment (PWHT) of 2219-O aluminium alloy friction stir welding joints was carried out at solution temperatures of 480, 500 and 540°C for 32 min followed by aging at 130°C for 9 h. The effects of PWHT on the microstructure and mechanical properties of the joints were investigated. Experimental results show that PWHT causes coarsening of the grains in the weld, and the coarsening degree increases with increasing solution temperature. The tensile strength of the heat treated joints increases with increasing solution temperature. The maximum tensile strength can reach 260% that of the base material at the solution temperature of 540°C. PWHT has a significant effect on the fracture locations of the joints. When the solution temperature is lower than 500°C, the joints fracture in the base material; when the temperature is higher than 500°C, the joints fracture in the weld. The change of the fracture locations of joints is attributed to the presence of precipitate free zones beside the grain boundaries and coarsening equiaxed grain structures in the weld.     

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.     

Characterization of the mechanical properties changes in an Al–Zn–Mg alloy after a two-step ageing treatment at 70° and 135 °C

Fine-scale precipitation of the η′ phase and its precursors are essential for the mechanical properties of Al-4.6 wt%Zn-1.2 wt%Mg alloy. This paper deals with an investigation of precipitation in an industrial Al–Zn–Mg alloy at various stages of a conventional two-step ageing treatment at 70 °C and 135 °C. The effect of microstructure on the mechanical properties was performed using microhardness and tensile tests, together with optical, scanning and transmission electron microscopy. After ageing at 135 °C, corresponding to the maximum value of hardness, small η′ precipitates were observed in the alloy matrix. After two-step ageing at 70 °C plus at 135 °C, the volume fraction of this precipitate becomes higher. Consequently, the yield strength of the material increases and it maintains its ductility. This high precipitate density slows the dislocation movement and thus a higher stress was required for its bowing.     

Investigation of mechanical properties of boro-tempered ductile iron

In this study, the effects of boro-tempering heat treatment on mechanical properties of ductile iron were investigated. Standard tensile test samples and unnotched Charpy specimens were boronized at 900 °C for 1, 3 and 5 h and then tempered at four different temperatures (250, 300, 350 and 450 °C) for 1 h. Micro-hardness measurements were performed on cross-section of the metallographically prepared samples, where cut from fractured impact test samples. The hardness of boride layers was measured in the range of 1654–1867 HV_0.05. It was observed that tempering temperature was more effective on the mechanical properties of the material than boronizing time. Optimum mechanical properties were obtained for the samples boronized for 1–3 h and then tempered between 250 and 350 °C for 1 h.     

Enhancement of mechanical properties and failure mechanism of electron beam welded 300M ultrahigh strength steel joints

In this study, four post-weld heat treatment (PWHT) schedules were selected to enhance the mechanical properties of electron beam welded 300M ultrahigh strength steel joints. The microstructure, mechanical properties and fractography of specimens under the four post-weld heat treatment (PWHT) conditions were investigated and also compared with the base metal (BM) specimens treated by conventional quenching and tempering (QT). Results of macro and microstructures indicate that all of the four PWHT procedures did not eliminate the coarse columnar dendritic grains in weld metal (WM). Whereas, the morphology of the weld centerline and the boundaries of the columnar dendritic grains in WM of weld joint specimens subjected to the PWHT procedure of normalizing at 970 °C for 1 h followed by conventional quenching and tempering (W-N2QT) are indistinct. The width of martensite lath in WM of W-N2QT is narrower than that of specimens subjected to other PWHT procedures. Experimental results indicate that the ductility and toughness of conventional quenched and tempered joints are very low compared with the BM specimens treated by conventional QT. However, the strength and impact toughness of the W-N2QT specimens are superior to those of the BM specimen treated by conventional QT, and the ductility is only slightly inferior to that of the latter.     

Effect of heat input on the microstructure and mechanical properties of gas tungsten arc welded AISI 304 stainless steel joints

Influence of heat input on the microstructure and mechanical properties of gas tungsten arc welded 304 stainless steel (SS) joints was studied. Three heat input combinations designated as low heat (2.563 kJ/mm), medium heat (2.784 kJ/mm) and high heat (3.017 kJ/mm) were selected from the operating window of the gas tungsten arc welding process (GTAW) and weld joints made using these combinations were subjected to microstructural evaluations and tensile testing so as to analyze the effect of thermal arc energy on the microstructure and mechanical properties of these joints. The results of this investigation indicate that the joints made using low heat input exhibited higher ultimate tensile strength (UTS) than those welded with medium and high heat input. Significant grain coarsening was observed in the heat affected zone (HAZ) of all the joints and it was found that the extent of grain coarsening in the heat affected zone increased with increase in the heat input. For the joints investigated in this study it was also found that average dendrite length and inter-dendritic spacing in the weld zone increases with increase in the heat input which is the main reason for the observable changes in the tensile properties of the weld joints welded with different arc energy inputs.     

Effect of post weld heat treatment on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel weldments

The effect of postweld heat treatment (PWHT) on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel joints with ER316LMn filler material was investigated. PWHT aging was performed for 1 h at four different temperatures of 600 °C, 760 °C, 870 °C and 920 °C, respectively. The microstructure revealed the sigma phase precipitation occurred in the weld metals heat-treated at the temperature of 870 °C and 920 °C. The PWHT temperatures have the less effect on the tensile strength, and the maximum tensile strength of the joints is about 630 MPa, reaching the 95% of the base metal, whereas the elongation is enhanced with the rise of PWHT temperatures. Meanwhile, the sigma phase precipitation in the weld metals reduces the impact toughness.     

Effects of thermal treatment on precipitate shape and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy

Precipitation reactions in a Mg–8Gd–4Y–Nd–Zr alloy have been investigated using TEM, HREM, hardness measurements and tensile testing. Globular β′ precipitates, which were different from the typical plate-shaped β′ precipitates usually observed in Mg–Gd-based alloys, were detected in the 160 °C/192 h-aged sample. Instead of dissolution and then precipitating as plate-shaped β′ precipitates, the formed globular β′ precipitates grew up when further aged at 215 °C, which resulted in the decrease in strength comparing with that of the 215 °C single-stage aged samples. Two-stage ageing treatments on the alloy demonstrated that ageing 192 h at 150 °C plus 16 h at 215 °C made the ultimate strength and the yield strength improved 17 MPa and 13 MPa, respectively.     

Influence of fluidized sand bed heat treatment on the performance of Al–Si cast alloys

The present study was undertaken to arrive at a better understanding of the effects of solution heat-treatment time and melt treatment on the microstructure and tensile properties of T6-tempered A356.2 and B319.2 cast alloys heat treated using a fluidized sand bed furnace (FB) as opposed to a conventional convection furnace (CF). The alloys investigated were subjected to solution heat treatment at 530 °C and 495 °C, respectively, for times ranging from 0.5 to 24 h, warm water quenching, and conventional T6 aging at 155 °C and 180 °C, respectively, for times of 0.5, 1, 5, 8 and 12 h. The results revealed that the tensile strength of these alloys is more responsive to an FB heat treatment than to a CF treatment for solution treatment times of up to 8 h. A significant increase in strength is observed in the FB heat-treated samples after short aging times of 0.5 h and 1 h, the trend continuing up to 5 h. Analysis of the tensile properties in terms of quality index charts showed that both modified and non-modified 319 and 356 alloys display the same quality, or better, after only 2 h treatment in a fluidized bed compared to 10 h using a CF treatment. The 319 alloys show signs of overaging after 8 h of aging using a CF, whereas with an FB, overaging occurs only after 12 h. The Si particle characteristics of the alloys investigated show that the smallest particle size is obtained after solution heat treatment using a fluidized sand bed, the optimum solution heat-treatment time being 0.5 h for modified alloys, and up to 5 h for non-modified alloys.     

Characterisation of dissimilar P91 and P92 steel welds joint

In the present study, dissimilar weld joint was prepared using the P91 and P92 steel plate of 8-mm thickness, using the multi-pass gas tungsten arc (GTA) welding with filler (weld 1) and autogenous tungsten inert gas welding (A-TIG) process (weld 2). Evolution of δ-ferrite patches was studied in weld zone and heat affected zone (HAZ) for both weld 1 and weld 2. Effect of varying post weld heat treatment (PWHT) duration was also studied on δ-ferrite patches and mechanical properties of the dissimilar weld joint. PWHT was carried out at 760°C. For weld 2, weld zone showed poor impact toughness and higher peak hardness as compared to weld 1. After the PWHT, a considerable reduction in hardness was obtained for both weld 1 and weld 2,while impact toughness of weld zone showed a continuous increment with PWHT duration. For weldments characterisation, optical microscope, scanning electron microscope (SEM) and microhardness tester were utilised.