Showing papers on "Heat-affected zone published in 2021"

Experimental investigation on laser cutting of PMMA sheets: Effects of process factors on kerf characteristics

Abstract: This paper reports an experimental investigation on continuous CO2 laser cutting of polymethylmethacrylate (PMMA) sheet. The influence of four process factors (laser power, cutting speed, assisting gas pressure and sheet thickness) on five process responses (kerf deviation, top heat affected zone, bottom heat affected zone, maximum surface roughness and rough area) has been investigated. The experimental plan was established based on Taguchi L18 mixed design. The kerf geometry and heat affected zones have been measured using polarising light microscopy technique, while the surface roughness was evaluated using 3D laser scanning confocal microscope. Regression models have been derived to correlate different process responses with different process factors. The cut surface could be classified into three zones: rough zone, moderate zone and soft zone. The rough area is increased by increasing gas pressure and laser power and by decreasing the sheet thickness and cutting speed. Increased kerf deviation has been observed at high cutting speed, laser power and gas pressure. High laser power and low cutting speed produced worst surface roughness and wide heat affected zone. Therefore, it is recommended to use low laser power and high cutting speed to minimize the heat affected zone and the surface roughness. However, increasing the cutting speed may result in high kerf deviation.

Optimization of postweld tempering pulse parameters for maximum load bearing and failure energy absorption in dual phase (DP590) steel resistance spot welds

Abstract: Resistance spot welds of dual phase (DP) steels are prone to low fracture toughness due to the formation of brittle martensitic structure in the fusion zone (FZ). In-process tempering of martensite via applying second pulse current is considered a new pathway to improve mechanical performance of the welds. The success of in-process tempering depends upon precise controlling the amount of heat input and uniform temperature distribution which in turn influenced by postweld tempering pulse parameters. This paper aims to investigate the effect of three postweld tempering pulse parameters such as welding current, welding time and cooling time applied after main pulse current on microstructure and mechanical properties of DP590 steel resistance spot weld. Mechanical properties in terms of peak load and failure energy were determined after performing cross tension (CT) test. Taguchi quality design based on L16 orthogonal array has been used to determine the optimum conditions for maximum peak load and failure energy. Moreover, microstructure-property relationship is also studied. The results show that at optimum conditions maximum improvement of 62% in peak load and 62.3% in failure energy is achieved in double pulse welds compared with conventional single pulse weld. It was found that improvement in peak load and failure energy resulted from (i) enhanced weld nugget size (WNS) and (ii) tempering of martensite in FZ and heat affected zone (HAZ). These factors are influenced by heat input (Q) during postweld heating cycle (PWHC) which in turn increased with increasing second pulse current and time.

The significant impact of introducing nanosize precipitates and decreased effective grain size on retention of high toughness of simulated heat affected zone (HAZ)

Abstract: The study of toughening mechanism associated with the heat-affected zone (HAZ) continues to be of significant interest for improving the low temperature (−40 °C) toughness. In this study, the effect of different heat input on microstructure, hardness and low temperature impact toughness of coarse-grained HAZ (CGHAZ) in a high-strength low-alloy (HSLA) plate steel was explored by using welding thermal simulations. After different welding thermal cycles with heat input of 60, 90, 120, and 180 kJ cm−1, average effective grain size of 5.7, 5.9, 6.2, and 6.7 ± 0.2 μm, were respectively obtained. Although the effective grain size marginally increased with the increase of heat input, the values were remarkably smaller than the prior austenite grain size of 54, 93, 104, and 118 μm. Compared to the as-processed HSLA steel, impact toughness of steel welded with a heat input of 60 kJ cm−1 decreased dramatically from 318 ± 20 to 31 ± 8J·cm−2. But interestingly, impact toughness was increased to a peak value of 325 ± 20 J cm−2 when the heat input was increased to 180 kJ cm−1. Toughness was enhanced by grain refinement (D = 6.7 ± 0.2 μm) and through the nano-size precipitates (d = 10 nm), which were induced by medium heat input of 180 kJ cm−1. The values of precipitation strengthening were 203 MPa and 90 MPa while the critical stress values of crack propagation were 32 GPa and 10 GPa for the steels welded at 180 and 270 kJ cm−1, respectively. A critical analysis of fracture toughness and yield strength in terms of theoretical predictions is presented.

Study on Microstructure and Mechanical Properties of Laser Welded Dissimilar Joint of P91 Steel and INCOLOY 800HT Nickel Alloy.

Abstract: This investigation attempts to explore the weld characteristics of a laser welded dissimilar joint of ferritic/martensitic 9Cr-1Mo-V-Nb (P91) steel and Incoloy 800HT austenitic nickel alloy. This dissimilar joint is essential in power generating nuclear and thermal plants operating at 600–650 °C. In such critical operating conditions, it is essential for a dissimilar joint to preserve its characteristics and be free from any kind of defect. The difference between the physical properties of P91 and Incoloy 800HT makes their weldability challenging. Thus, the need for detailed characterization of this dissimilar weld arises. The present work intends to explore the usage of an unconventional welding process (i.e., laser beam welding) and its effect on the joint’s characteristics. The single-pass laser welding technique was employed to obtain maximum penetration through the keyhole mode. The welded joint morphology and mechanical properties were studied in as-welded (AW) and post-weld heat treatment (PWHT) conditions. The macro-optical examination shows the complete penetrations with no inclusion and porosities in the weld. The microstructural study was done in order to observe the precipitation and segregation of elements in dendritic and interface regions. Solidification cracks were observed in the weld fusion zone, confirming the susceptibility of Incoloy 800HT to such cracks due to a mismatch between the melting point and thermal conductivity of the base metals. Failure from base metal was observed in tensile test results of standard AW specimen with a yield stress of 265 MPa, and after PWHT, the value increased to 297 MPa. The peak hardness of 391 HV was observed in the P91 coarse grain heat-affected zone (CGHAZ), and PWHT confirmed the reduction in hardness. The impact toughness results that were obtained were inadequate, as the maximum value of impact toughness was obtained for AW P91 heat-affected zone (HAZ) 108 J and the minimum for PWHT Incoloy 800HT HAZ 45 J. Thus, difficulty in obtaining a dissimilar joint with Incoloy 800HT using the laser beam welding technique was observed due to its susceptibility to solidification cracking.

Friction stir welding joints of 2195-T8 Al–Li alloys: Correlation of temperature evolution, microstructure and mechanical properties

Abstract: 7.5 mm thick 2195-T8 Al–Li alloy plates were manufactured by friction stir welding (FSW). Thermal history of different regions in welding joints was recorded during welding. Microstructure characterization was conducted out by electron back scattered diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mechanical properties were measured using micro-hardness and tensile test. The correlation of temperature evolution, microstructure and mechanical properties of individual regions in welding joints was discussed. It was shown that peak temperature was 358 °C–376 °C in the heat affected zone (HAZ) where joint failure occurred due to the dissolution and coarsening of T1 and development of coarse grain boundary phase and precipitate free zones (PFZs). Peak temperature of thermo-mechanically affected zone (TMAZ) ranged from 376 °C to 401 °C, leading to complicated precipitation behavior. The dissolution and coarsening of precipitates resulted in joint softening, while a layer of fine equiaxed grains at the interface of the AS-TMAZ/NZ increased the hardness of this region. The base material (BM) mainly consisted of rolling textures, while the NZ and TMAZ had strong shear textures. Moreover, the intensity of shear textures was evidently higher in the NZ, but relatively lower in the RS-TMAZ. The hardness and tensile strength profiles of welding joints appeared in “W” shape. From the base material zone to the nugget zone within a welding joint, fracture mechanism changed from brittleness fracture to toughness fracture.

A computational study of heat transfer and material removal in picosecond laser micro-grooving of copper

Abstract: A 2D two-temperature model (TTM) is developed for picosecond laser micro-grooving of copper (Cu) and discretized using the finite difference scheme, to explore the temperature evolution and groove formation process. For lattice and electron subsystems, as well as the coupling strength between them, comprehensive temperature-dependent properties are employed and updated in real time to make the calculation more accurate. The model verification is then conducted and a reasonable good agreement has been found between predicted and measured groove depths. Using the verified model, computational studies of temperature field evolution are carried out in different fluence levels. For low fluence of around 0.283 J/cm2, phase change can be neglected, and the temperature difference between two subsystems decreases with an increase in pulse width and can be neglected when the pulse width is over 200 ps. In contrast, for high fluence of about 10.61 J/cm2, the maximum lattice temperature increases rapidly to boiling point within 3 ps, and the peak electron temperature is around 10 times higher, leading to material removal via evaporation. Under this high fluence level, groove depth around 0.25 μm is obtained within a 12 ps single pulse irradiation. In addition, thermal diffusion occurs from the residual hot zone adjacent to groove edge towards surrounding area during the off-pulse period, as the heat affected zone depth is up to 1 μm after 500 ps and over 4 μm after 6 ns, and the peak residual lattice temperature drops to about 396 K from boiling point within 53 ns. Moreover, the groove depth and groove profile show a periodically evolving manner with respect to scanning times, and for each scanning, a slow-fast-slow pattern has been observed in groove depth increase and profile development.

Pulsed laser butt welding of AISI 1005 steel thin plates

Abstract: Pulsed laser welding is a joining process, which takes advantage of the laser energy quality with precise control equipment. Its characteristics allow joining thinner materials with reduced thermal distortion without filler material, fulfilling the industrial interest in this process. In the present work, AISI 1005 steel sheets of 1.7 mm thickness were subjected to different pulse Nd:YAG laser treatments to obtain one side bead-on-plate weld samples and double sided butt welds. Since the quality of the joints is strongly influenced by the process parameters, an experimental design methodology was adopted to define a suitable combination of the laser beam power, pulse duration and spot diameter. The Box-Behnken technique and response surface methodology associated with analyses of defects allowed the determination of the appropriate welding parameters. The microstructure characterisation revealed the melted fusion zones originated from the thermal effect of the overlapped pulses. Additionally, microhardness measurements at fusion zone exhibited higher values than the base material and heat affected zone, due to the hard microconstituents developed. The tensile test performed at samples extracted from double sided laser welds showed good resistance results, fracturing at the base material of joints. The present work demonstrates that pulsed laser welding methodology is suitable to obtain high quality joints of steel thin plates.

A review of advances in resistance spot welding of automotive sheet steels: emerging methods to improve joint mechanical performance

Abstract: The use of resistance spot welding (RSW) in the automotive industry is by far the most preferred and widely used joining technique for sheet metal parts and is likely to continue for the foreseeable future. Advanced high strength steels (AHSSs) are most commonly used in automotive structural components due to their attractive strength-ductility combinations. However, several challenges are faced during RSW including (1) complex phase transformations such as hardening of nugget due to brittle martensitic structure and softening in the heat affected zone due to martensite tempering present in base metal, cast like solidification structure of nugget (2) elemental segregation leading to grain boundary embrittlement, (3) solidification defects such as porosity and void formation in nugget, and (4) liquid metal embrittlement cracking. All of above factors contributes in degradation of joint mechanical properties. In recent years, interlayer assisted RSW, magnetically assisted RSW, and pulsed-RSW have emerged as potential methods to improve the joint mechanical performance. This review paper seeks to summarize the recent technological advances of three modified RSW processes. A comprehensive analysis of the effect of welding process parameters on metallurgical features of the weld joint, mechanical performance, and failure behaviour under different loading conditions, i.e. cross tension, tensile shear, and cyclic (i.e. fatigue), is presented. In addition, the process feasibility to various AHSS grades is also discussed. Finally, current challenges and new opportunities arising from three modified RSW processes are discussed to provide a basis for future research.

Improvement of Impact Toughness of the Welding Heat-Affected Zone in High-Strength Low-Alloy Steels through Ca Deoxidation

Abstract: Thick high-strength low-alloy (HSLA) steel plates with excellent large heat input welding properties are increasingly demanded to build super-large container ships for the globally flourishing marine trade. This article describes the effect of Ca content on the TiN particles, microstructure and impact toughness of the simulated coarse-grained heat-affected zone (CGHAZ) in HSLA steels after large heat input welding at 400 kJ/cm. The quantitative analyses on TiN particles demonstrate that the number densities of the submicron- and nano-scale particles increase with increasing Ca content, but the particle sizes appear to be independent of the Ca content. The prior austenite grains in the CGHAZs of the HSLA steels are comparably coarse, including a small number of extremely large grains, except for 25Ca steel, whose grains are relatively fine and uniform. Moreover, only the impact toughness of the CGHAZ in 25Ca steel is satisfactorily improved with a small test variability, and this could be attributed to the improved precipitation of the submicron-scale TiN particles that effectively refine the grains. Based on the experimental and thermodynamic results, it is strongly recommended to increase the Ca content to > 0.0019 wt pct for the related HSLA steel system in the future steel optimization.

Titanium alloy components fabrication by laser depositing TA15 powders on TC17 forged plate: Microstructure and mechanical properties

Abstract: The manufacturing of titanium alloy components is of great significance for many industrial applications. High-performance titanium alloy parts are generally produced by a serial operation of hot forging process and extensive machining. Recently, additive manufacturing (AM) technologies such as direct laser deposition (DLD) has been developed to produce net-shape components. In this study, the parts were fabricated by depositing TA15 titanium alloy powders via DLD process on forged TC17 titanium alloy plates. Microstructure, microhardness and tensile properties of as-fabricated sample were examined. By using digital image correlation (DIC), the strain fields as well as the global strain behavior of the tensile specimen have been analyzed. The sample consists of three typical zones: the laser deposited zone (LDZ), the forged substrate zone (SZ), and the heat affected zone (HAZ) with good metallurgical bonding. Superfine basketweave microstructure forms within β grains in LDZ and HAZ because of fast cooling rate. The as-fabricated specimens have good tensile properties with an average ultimate tensile strength of 1027 MPa and average elongation of 4.57%. Non-uniform local strain concentration occurs in LDZ, while no strain concentration is found in HAZ and SZ. The remarkable non-uniformity of plastic strain is ascribed to different microstructures.

Effects of laser shock peening on microstructure and mechanical properties of TIG welded alloy 600 joints

Abstract: Laser shock peening (LSP) is a competitive surface strengthening technology for post-weld treatment. In this work, tungsten inert gas (TIG) welded joints of Alloy 600 were treated by LSP to enhance the mechanical properties. The results showed that the tensile residual stress in weld surface was transformed into the compressive stress state after LSP. LSP treatment caused a significant improvement in microhardness and dislocation density in the weld surface. The tensile strength and yield strength of joints after peening on the full gauge part of tensile samples were increased by 10% and 75%, respectively. However, the fatigue life could be improved much more by only peening on the fusion zone and heat affected zone than that of samples peened on the full gauge part. The peening region played a crucial role on the difference of tensile and fatigue properties due to the residual stress distribution, dislocation strengthening and work hardening induced by LSP.

Effect of porosity morphology and elements characteristics on mechanical property in T-joints during dual laser-beam bilateral synchronous welding of 2060/2099 Al-Li alloys

Abstract: The 2 mm thick 2060-T8/2099-T83 Al-Li alloy T-joints were welded by dual laser-beam bilateral synchronous welding (DLBSW) with ER4047 filler wire. The metallurgical porosity defects under different welding parameters were investigated to study the influence of laser power and weld speed on the porosity distribution, formation mechanism and mechanical properties during DLBSW process. In order to explore the special effect of welding heat input on the difference of alloying elements content around porosity, a comparative analysis of elements content between the porosity boundary and matrix around porosity was studied. It was found that the weld porosity defects were increased significantly as the laser power increased. Besides, the heat affected zone (HAZ) and branches of dendritic crystals were the key positions of porosity nucleation and formation. The results of tensile test and SEM of failure surface demonstrated that porosity was one of the important factors leading to weld bead failure. In particular, the porosity in the bottom fusion zone and EQZ significantly weakened the performance of the T-joint.

Fine equiaxed zone induced softening and failure behavior of 7050 aluminum alloy hybrid laser welds

Abstract: A fine (~3–10 μm) equiaxed zone (FQZ) between the heat affected zone and the weld metal is a unique microstructural feature associated with fusion welded Zr-containing aluminum alloys, posing a significant threat to the structural integrity of welded components. This work examines the softening and failure behavior of a 7050-T7451 hybrid laser welded joint. Nanoindentation test results show the FQZ to have the lowest hardness across the welded joint, which is attributed primarily to a significant reduction in strengthening precipitates within the grains. The high-angle grain boundaries in the FQZ are decorated with an interconnected network of coarse brittle ω (Al7Cu2Fe) and S (Al2CuMg) phases. The damage sequence recorded by time-lapse X-ray computed tomography (CT) and in situ scanning electron microscopy (SEM) shows that these phases give rise to intergranular failure through the FQZ by void nucleation and link-up at low overall levels of plasticity, aided by the plastic incompatibility with these phases and the strain localisation.

Pulsed Laser Welding Applied to Metallic Materials-A Material Approach

Abstract: Joining metallic alloys can be an intricate task, being necessary to take into account the material characteristics and the application in order to select the appropriate welding process. Among the variety of welding methods, pulsed laser technology is being successfully used in the industrial sector due to its beneficial aspects, for which most of them are related to the energy involved. Since the laser beam is focused in a concentrated area, a narrow and precise weld bead is created, with a reduced heat affected zone. This characteristic stands out for thinner material applications. As a non-contact process, the technique delivers flexibility and precision with high joining quality. In this sense, the present review addresses the most representative investigations developed in this welding process. A summary of these technological achievements in metallic metals, including steel, titanium, aluminium, and superalloys, is reported. Special attention is paid to the microstructural formation in the weld zone. Particular emphasis is given to the mechanical behaviour of the joints reported in terms of microhardness and strength performance. The main purpose of this work was to provide an overview of the results obtained with pulsed laser welding technology in diverse materials, including similar and dissimilar joints. In addition, outlook and remarks are addressed regarding the process characteristics and the state of knowledge.

Fatigue Properties of Resistance Spot Welded Dissimilar Interstitial-Free and High Strength Micro-Alloyed Steel Sheets

Abstract: This study aims to investigate the influence of nugget diameter on tensile-shear and fatigue properties of the dissimilar resistance spot-welded joints of interstitial free and high strength niobium microalloyed steel sheets. The spot-weldings are done at currents of 7 kA and 9 kA at welding time of 300 ms with electrode force of 2.6 kN. Fatigue tests are done at different load amplitudes, considering the maximum tensile-shear load bearing capacity of the joints. The tests are interrupted before the final failure to understand the crack initiation and propagation paths. The study is supplemented by the microstructural and detailed fractographic analyses. The results indicated that the fatigue strength of the spot-welds increased with a decrease in nugget diameter. This could be attributed to the higher microhardness of heat affected zone (HAZ), presence of compressive residual stress and lower variation in thickness reduction on the IF steel side, welded at 7 kA. However, the tensile-shear load bearing capacity of the spot-welds increased with increase in the nugget diameter. The failure of the spot-welded joint under static loading initiates from the HAZ/base metal interface of the IF steel side. Nevertheless, under cyclic loading crack initiates from the notch root at the interface of two sheets lying in the HAZ of IF steel side. Fractographic investigations indicate intergranular fracture features in combination with striations, secondary cracks on the IF steel side under cyclic loading. Fracture surfaces of the specimens failed under static loading shows however, shear dimples on the IF steel side.