As-welded Samples Research Articles

Linear friction welding of AD730™ Ni-base superalloy: Process-microstructure-property interactions

Linear friction welding (LFW) is a near net shape solid state joining technology for aerospace applications. In this study LFW of a recently introduced Ni-based superalloy, AD730™, with superior properties for use in the hot section of gas turbines was studied. In order to minimize the number of experiments for achieving sound welds, an analytical method was developed that allowed to determine the optimum process parameters with a limited number of experiments. The predictions of the method were validated by LFW experiments, and sound samples without defects were produced. Microstructure evolution of the as-welded samples from the weld center to the base metal was investigated using laser confocal and field emission scanning electron microscopy (SEM) including electron back scatter diffraction (EBSD), energy dispersive spectrometer (EDS) and backscattered imaging (BSE). Post-weld heat treatment (PWHT), consisting of a γ′ sub-solvus solutionizing followed by aging, was conducted on LFWed blocks. Then, microstructure evolution and mechanical testing were conducted on PWHTed samples and compared with the as welded blocks. The obtained results were correlated with microhardness as well as tensile testing at room temperature and 650 °C, and interpreted in terms of fundamental metallurgical processes. Macroscopic examination of the PWHTed joints revealed that the samples failed out of the weld zone, further demonstrating the appropriate selection of LFW processing parameters using the proposed analytical method. PWHTed samples exhibited better room and high temperature tensile properties compared to those of the as-welded samples. Microscopic examination of the fracture zones of the samples showed that the higher levels of reprecipitation of γ′ particles in the thermomechanically-affected zone (TMAZ) of the PWHT samples were associated to their higher tensile properties compared to the as welded ones.

The corrosion behaviour of as-welded and post-weld heat treated Ti6Al4V alloy in simulated body fluid

In this study, the corrosion behavior of Ti6Al4V alloy base metal, laser welded and post-weld heat treated (PWHT) samples in simulated body fluid was investigated. The potentiodynamic polarization curves of the as-welded and PWHT samples were compared with each other and with the base metal. Surface morphologies of samples were evaluated. The polarization curves indicate that, the lower corrosion resistance obtained for as-welded samples as compared to base metal. The corrosion resistance of welded joints improved with increasing welding speed. PWHT process has positively affected to the corrosion properties of the laser welded Ti6Al4V joints.

Enhancement of weld strength of laser-welded joints of AA6061-T6 and TZM alloys via novel dual-laser warm laser shock peening

In this paper, an experimental study is presented on an investigation to improve the weld strength of laser-welded joints via post-processing by warm laser shock peening (wLSP). A dual-laser setup was utilized to simultaneously heat the sample to a prescribed temperature and to perform the wLSP process on the laser-welded joints of AA6061-T6 and TZM alloys. Joints in overlap and bead-on-plate configurations were created by laser welding by a high-power fiber laser and post-processed with wLSP. The tensile tests carried out on wLSP-processed AA6061-T6 samples demonstrate an enhancement in the strength by about 20% over as-welded samples and the ductility of samples processed by wLSP improved by 30% over as-welded samples. The bead-on-plate (BOP) welds of TZM alloy processed with wLSP demonstrated an enhancement in strength by about 30% and the lap welds processed with wLSP demonstrated an increase in the joint strength by 22%. Finite element analysis revealed that the depth and magnitude of compressive stresses imparted by wLSP were greater than room temperature laser shock peening (rtLSP), which contributed to the enhancement of the joint strength for processed samples.

Effect of post weld heat treatment and welding parameters on mechanical and corrosion characteristics of friction stir welded aluminium alloy AA2014-T6

This paper addresses the effect of post weld heat treatment methods on the mechanical and corrosion characteristics of friction stir welded aluminum alloy AA2014-T6. Aluminum alloy AA2014 is mainly used in applications that demand high strength to weight ratios, such as aerospace, marine, and industrial applications. In this work, AA2014-T6 plates of 6 mm thick were butt welded using a tool with a square profile. Tensile strength, hardness, and corrosion characteristics were compared between the samples as welded and post weld heat treated. Welded samples that were heat treated for a shorter ageing period (8 h) showed improved tensile strength irrespective of welding process parameters, compared to as-welded samples. The samples heat treated for a longer ageing period (9 h) showed a decline in tensile strength for low tool rotation speed. Hardness increased in welded samples heat treated for 8 h. Welded samples heat treated for 9 h show high passivity in corrosion media.

Effect of post-weld heat treatment on the microstructure and tensile properties of electron-beam-welded 21st century nickel-based super alloy 686

This work examines the influence of post-weld heat treatment (PWHT) on the fusion zone microstructure, and mechanical properties of electron beam (EB)-welded alloy 686. Comparative studies have been made on weld microstructure and tensile properties of the weldments both in as-welded and post-welded heat-treated conditions. PWHT consists of direct aging (DA/480°C for 3 h) and solution treatment (ST, 980°C for 1 h) followed by ageing and finally, homogenizing treatment (HT, 1200°C for 1 h) followed by DA and ST. The secondary topologically closed packed (TCP) phases formation, their distribution and microsegregation characteristics are studied with the aid of scanning electron microscope (SEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) is also performed to estimate the microsegregation of alloying elements in the dendritic core and interdendritic regions of the weldments. The result shows that there is no significant change in the microstructure of DA and solution-treated sample as compared to the as-welded sample. The microstructure of HT sample was entirely different from those of as-welded and other HT samples. The SEM/EDS analysis illustrates the presence of secondary TCP phases (σ, P and μ) in the as-weld, DA and ST condition, whereas HT weldments did not show the presence of TCP phases. Tensile test results show higher tensile strength in ST condition whereas homogenization samples show higher ductility compared with others.

Improvement of mechanical properties in treated spot welded joint

Abstract Post Weld Impact Treatment (PWIT) is one of the treatment methods that is usually applied on weld region to reduce the level of residual stress originated from the welding process. During the spot weld process, the formation of stress is produced around the weld area and degrades the mechanical properties of the joint. This paper focused on the improvement of mechanical properties of the spot welding joint via Pneumatic Impact Treatment (PIT) process. This study uses the low-carbon steel with a welded single lap shear joint that has a constant thickness of 1.2 mm. All the welded samples were subjected to tensile-shear test, failure mode and hardness test. The tensile-shear load and fusion zone hardness of RSW joint after PIT was improved by 8% and 27%, respectively. The results indicated improvement in tensile-shear strength and hardness of treated samples as compared to the as-welded samples. From the observation done, the failure mode type was generally accepted as failure for spot-weld joint. The improvement of mechanical properties is envisaged as the result of the mechanical pulses produced by unique fluidic muscle conveyed to the surface of the RSW joint during PIT process.

Role of evolving microstructure on the mechanical behaviour of P92 steel welded joint in as-welded and post weld heat treated state

An autogenous gas tungsten arc welded P92 weld joint was subjected to two different post weld heat treatment (PWHT). One PWHT involved tempering of the as-welded sample at 760 °C for 2 h (referred to as PWDT) followed by natural air cooling. The other PWHT involved normalizing (re-austenitizing) the as-welded sample at 1040 °C for 40 min followed by PWDT (referred as PWNT). The cross-section of the as-welded sample exhibited a higher degree of microstructural heterogeneity. Both the PWDT and PWNT heat treatment procedures reduced the heterogeneity gradient along the weld cross section. The heat affected zone (HAZ) of the as-welded and PWDT samples showed Charpy toughness values of 3 ± 4 J and 64 ± 6 J, respectively, which were lower than that of the base metal (72 ± 5 J). The PWNT treated sample exhibited a HAZ Charpy toughness value of 83 ± 4 J which was higher than that of the base metal. The brittle mode of the fracture with river patterns was observed for the as-welded and PWDT treated sample while a ductile mode of fracture with fine and shallow dimples was observed for the PWNT condition. The PWNT treatment resulted in dissolution of the ferrite patches and formation of a uniform microstructure along the weld cross section. The PWNT treated samples exhibited the lowest yield strength to tensile strength ratio supporting the enhanced ductility as a result of this re-austenizing heat treatment. The as-welded and PWDT treated weld joints showed the presence of detrimental δ ferrite phase in the weld fusion zone and the coarse grained heat affected zone. The PWNT completely removed the δ ferrite patches from the microstructure whereas the PWDT treatment merely reduced the range of the hardness of the δ ferrite from 179 to 301 HV (as welded) to 204–228 HV.

Characterization on the Microstructure Evolution and Toughness of TIG Weld Metal of 25Cr2Ni2MoV Steel after Post Weld Heat Treatment

The microstructure and toughness of tungsten inert gas (TIG) backing weld parts in low-pressure steam turbine welded rotors contribute significantly to the total toughness of the weld metal. In this study, the microstructure evolution and toughness of TIG weld metal of 25Cr2Ni2MoV steel low-pressure steam turbine welded rotor under different post-weld heat treatment (PWHT) conditions are investigated. The fractography and microstructure of weld metal after PWHT are characterized by optical microscope, SEM, and TEM, respectively. The Charpy impact test is carried out to evaluate the toughness of the weld. The optical microscope and SEM results indicate that the as-welded sample is composed of granular bainite, acicular ferrite and blocky martensite/austenite (M-A) constituent. After PWHT at 580 °C, the blocky M-A decomposes into ferrite and carbides. Both the number and size of precipitated carbides increase with holding time. The impact test results show that the toughness decreases dramatically after PWHT and further decreases with holding time at 580 °C. The precipitated carbides are identified as M23C6 carbides by TEM, which leads to the dramatic decrease in the toughness of TIG weld metal of 25Cr2Ni2MoV steel.

Temper-treatment development to decompose detrimental martensite–austenite and its effect on linepipe welds

ABSTRACTA tempering cycle was developed via heat treatment to study the decomposition of detrimental martensite–austenite (MA). The heat treatment cycle was found to preferentially decompose hard MA with a critical size of ≥1 µm, which decreases the hardness of these microconstituents after tempering at 300°C for 10 min. The dislocation density inside the MA and in the surrounding matrix was also decreased. Tempering of the X80 weldments containing MA reveals a similar decomposition behaviour. During transverse weld tensile testing of welds, a fracture occurred in the heat-affected zone due to void formation at the MA/ferrite interfaces. However, a fracture occurred in base metal with improved strength and ductility after tempering, in comparison to the fracture in the heat-affected zone of as-welded samples.

The Evolution of the Weld Metal Microstructures in Dissimilar Titanium Welds Based on Al and Mo Equivalents

CO2 laser welding of Ti-15V-3Cr-3Al-3Sn to Ti-4.5Al-3V-2Fe-2Mo was conducted in the study. The notched fracture and microstructures of the weld metal (WM) with various post-weld aging treatments were studied. In the as-welded sample, the WM comprised only β phase, which was relatively soft and ductile. Moreover, the phase constituent of the as-welded WM was related to the Al and Mo equivalents and further compared with other welds. The microstructural morphologies and microhardness of the WMs of dissimilar welds exhibited significant changes after post-weld aging at distinct temperatures. Increasing the Al equivalent ([Al]EQ) of the WM caused an increased response to age-hardening during post-weld aging treatments. When the aging temperature was increased from 426 to 593 °C, the α precipitates in the WM grew in size, causing a decrease in hardness, but an improvement in toughness.

High temperature creep properties of a linear friction welded newly developed wrought Ni-based superalloy

AD730™ Ni-based superalloy specimens in solution-treated conditions were linear friction welded. Then, post-weld heat treatment (PWHT), consisting of γ′ sub-solvus solution treatments followed by aging, was conducted on the linear friction welded samples. High temperature creep tests were performed on the as-welded and PWHTed joints at two different temperatures: 700°C under 600 and 750MPa stress levels, and 850°C under 100 and 200MPa stresses. The creep resistance of the PWHTed joints was higher than that of the as-welded samples. The PWHTed joints exhibited better ductility than that of the base material at 850°C, while they showed slightly lower creep life at 700°C in comparison to the base metal. Microstructure examination showed that cracks initiated at the interface of oxidized particles at 700°C. The decrease in creep resistance of the AD730™ Ni-based superalloy at 850°C was related to a combination of the formation of precipitate-free zones (PFZ) in the vicinity of the grain boundaries (GBs) and microcracking assisted by oxidation. The Larson-Miller Parameter (LMP) was used to correlate the creep strength, temperature and time to failure for the as-welded and PWHTed samples. LMP values varied between 21.5 × 103 and 24.5 × 103. It was found that in the investigated temperature range, the PWHTed AD730™ has similar creep characteristics as Udimet™720 Li and Inconel 738LC at low values of LMP and better creep properties than those of the Inconel 617 alloy at higher LMP values.

Dynamic Strain Aging Behaviour in AISI 316L Austenitic Stainless Steel under As-Received and As-Welded Conditions

In the current study, dynamic strain ageing (DSA) phenomena in 316L austenitic stainless steel was investigated under as-received and as-welded conditions. A tensile test was carried out on as-received and as-welded samples for the temperatures of 25–800 °C at a strain rate of 1 × 10−3 s−1. Microstructure and fracture surfaces were investigated by optic and scanning electron microscopes (SEM). 316L austenitic stainless steel showed different DSA behavior under as-received and as-welded conditions, which are discussed in terms of microstructure and mechanical properties.

Effect of residual stress relaxation by means of local rapid induction heating on stress corrosion cracking behavior and electrochemical characterization of welded Ti-6Al-4V alloy under slow strain rate test

In this study, a welded Ti-6Al-4V alloy was treated by means of local rapid induction heating in order to relax the residual stress existed in the weldment. The welded samples were heat treated at the different temperatures. The stress corrosion cracking behavior and electrochemical characterization of the as-welded samples before and after the post weld heat treatment as a function of residual stress were investigated. Electrochemical impedance spectroscopy measurements of the samples under slow strain rate test were performed in a LiCl-methanol solution. The results demonstrated that the residual stress in the as-welded sample was dramatically reduced after the post weld heat treatment, and the residual stress decreased with the increase in the heat treatment temperature. The stress corrosion cracking susceptibility and electrochemical activity of the as-welded sample were significantly reduced after the heat treatment due to the relaxation of the residual stress, which gradually decreased with the decreasing value of the residual stress distributed in the heat treated samples.

Effect of Post Weld Toe Treatments on Fatigue Life of Welded Structures using FEA

Welding joints is the weak region for the components of all earth-moving machines as they are exposed to the repeated fluctuating stresses which cause fatigue cracks at the toe and throat of welded joints due to the metallurgical imperfections. This is why welded joints must be fully understood to improve the fatigue and service life of the earth-moving machines. Here failure at throat can be minimized by achieving optimum strength but problem of weld toe cracking only be eliminated by reducing stress concentration at weld toe. To do this, R & D work has been initiated by preparing set of five T-joint weld samples having 6mm (sample A to E) throat thickness with one as-welded sample i.e. E (without post weld toe treatment) using GMAW welding process. As per IIW guidelines, samples A to D has post weld toe treated with cylindrical & hemispherical burr tool, cutter (A24PBF) and flap disc grinder respectivelyto reduce the stress concentration at weld toe by making smooth transition at weld toe with undercut of 0.5mm. Fatigue analysis shows that, sample “B” with 8mm weld size post weld toe treated with hemispherical burr tool has given maximum optimization in fatigue life of 76.22% as compared to as-welded samples i.e. E.Validation carried out by hardness testing at Heat affected zone (HAZ) of etched samples.

Effect of Post-Weld Heat Treatment on Mechanical and Electrochemical Properties of Gas Metal Arc-Welded 316L (X2CrNiMo 17-13-2) Stainless Steel

In the present research work, corrosion behavior of post-weld heat-treated (PWHT) AISI 316L (X2CrNiMo 17-13-2) specimens joined by gas metal arc welding is compared with as-welded samples by using potentiodynamic polarization technique. Welded samples were PWHT at 1323 K for 480 s and quenched. Mechanical properties, corrosion behavior and microstructures of as-welded and PWHT specimens were investigated. Microstructural studies have shown grain size refinement after PWHT. Ultimate tensile strength and yield strength were found maximum for PWHT samples. Bend test have shown that PWHT imparted ductility in welded sample. Fractographic analysis has evidenced ductile behavior for samples. Potentiodynamic polarization test was carried out in a solution composed of 1 M H2SO4 and 1 N NaCl. Corrosion rate of weld region was 127.6 mpy, but after PWHT, it was decreased to 13.12 mpy.

Effect of high frequency mechanical impact treatment on fatigue strength of welded 1300 MPa yield strength steel

High frequency mechanical impact (HFMI) is a recent post weld treatment method which can be employed to increase the fatigue strength of welded components. In this paper the fatigue strength of as-welded and HFMI treated fillet welds in a 1300MPa yield strength steel was compared. Fatigue testing was done under fully reversed, constant amplitude bending load. Finite element analysis was used to calculate the stress distribution in the weld toe region to permit evaluation of the fatigue data with the effective notch stress approach. As-welded samples showed a mean fatigue strength of 353MPa and a characteristic fatigue strength of 306MPa. HFMI treatment increased the mean fatigue strength by 26% and the characteristic fatigue strengths by 3%. The weld toe radii in as-welded condition were large. HFMI only increased the weld toe radii slightly but resulted in a more uniform weld toe geometry along the weld. A depth of indentation in the base metal in the range of 0.15–0.19mm and a width of indentation in the range of 2.5–3mm, were achieved. Maximum compressive residual stresses of about 800MPa in the longitudinal and 250MPa in the transverse direction were introduced by HFMI treatment, adjacent to the weld toe. The surface hardness was increased in the entire HFMI treated region. It is concluded that the increase in fatigue strength is due to the combined effects of the weld toe geometry modification, increase in surface hardness and creation of compressive residual stresses in the treated region.

Ultrasonic Impact Treatment to Improve Stress Corrosion Cracking Resistance of Welded Joints of Aluminum Alloy

Stress corrosion cracking is one of the major issues for welded joints of 6005A-T6 aluminum alloy in high-speed trains. High residual stress in the welded joints under corrosion results in stress corrosion cracking. Ultrasonic impact treatment was used to control the residual stress of the welded joints of 6005A-T6 aluminum alloy. Experimental tests show that ultrasonic impact treatment can induce compressive longitudinal and transverse residual stress in the welded joint, harden the surface, and increase the tensile strength of welded joints. Salt-fog corrosion tests were conducted for both an as-welded sample and an ultrasonic impact-treated sample. The surface of the treated sample had far fewer corrosion pits than that of the untreated sample. The treated sample has higher strength and lower tensile residual stress than the untreated sample during corrosion. Therefore, ultrasonic impact treatment is an effective technique to improve the stress corrosion cracking resistance of the welded joints of 6005A-T6 aluminum alloy.

The effect of annealing on microstructure and tensile properties of Ti–22Al–25Nb electron beam weld joint

This study investigated the microstructure evolution and tensile properties of Ti–22Al–25Nb EBW joints. The fusion zone of the as-welded joint exhibited a fully B2 microstructure. Widmanstätten O particles precipitated out of B2 matrix after annealing and their size increased within a temperature range from 750 °C to 900 °C. In the heat affected zone, there was a transition of microstructure moving away from the fusion zone towards the base material. Strength and elongation of the as-welded sample were significantly improved after annealing, which was attributed to the strengthening effect of O precipitates and the slip transmission between O and B2 phases. Samples tensile tested at 650 °C all failed within the fusion zone and exhibited intergranular failure instead of transgranular failure at room temperature. The room temperature strength and hardness of the joints decreased with annealing temperature due to the coarsening of O precipitates. At 650 °C, failure occurred by intergranular fracture in the fusion zone and the joint strength of all annealed samples was similar due to similar B2 grain boundary strength.

Global and local characteristics of an autogenous single pass electron beam weld in thick gage UNS S41500 steel

Electron beam welding of UNS S41500, a low carbon martensitic stainless steel utilized in hydroelectric turbine manufacturing, was investigated by applying a single pass autogenous process to penetrate a section thickness of 72mm without preheating. In the as-welded and post-weld heat treated conditions, the evolution in microhardness and microstructure across the weldments, as well as the global and local tensile properties, were evaluated. In the as-welded condition, assessment of the microhardness and the associated microstructure across the welds led to the identification of six regions, including the fusion zone, four heat affected zones and the base metal; each of these regions consisted of different phase constituents, such as tempered martensite, untempered martensite, delta ferrite and retained austenite. Post-weld heat treatment, undertaken to temper the untempered martensite in the as-welded microstructure, was effective in homogenizing the hardness across the weldment. The mechanical response of the welds, determined through tensile testing at room temperature with an automated non-contact three-dimensional deformation measurement system, indicated that the global tensile properties in the as-welded and post-weld heat treated conditions met the acceptance criteria in the ASME Section IX standard. Also, evaluation of the local tensile properties in the fusion and heat affected zones of the as-welded samples allowed a more comprehensive understanding of the strength and ductility associated with the different microstructures in the “composite” nature of the weldment. Fractographic analysis demonstrated dimpled features on the tensile fracture surfaces and failure was associated with debonding between the martensitic matrix and the secondary phases (such as delta ferrite and retained austenite) that resulted in the formation, growth and coalescence of voids into a macroscale crack.

Structures and Properties of Welding Seams in Nickel-Base Superalloys

Due to the existence of strengthening alloying elements in nickel-base superalloys, welding is quite difficult and the mechanical properties of the welding seam usually decrease significantly compared to the corresponding base materials. In this paper, a study on laser welding of nickel-base superalloys was carried out, and the as-welded samples were treated with mechanical (Rolling) and heat treatment, and the structures and mechanical properties after treatment were investigated. It was found that before any treatment, the fusion line was obviously observed between the welding seam and base materials, and few heat affected zone was found. After rolling treatment, the tensile properties of as-welded samples were close to the base materials. Aging and solid solution treatment can improve the mechanical properties of welding seams significantly. It was indicated that aging treatment could result in the formation of plenty of TiCN precipitates in welding seams, which could be helpful for strengthening the welding seams.