Formation Of Heat-affected Zone Research Articles

Multi-pass laser cutting of carbon/Kevlar hybrid composite: Prediction of thermal stress, heat-affected zone, and kerf width by thermo-mechanical modeling

Numerical modeling offers considerable promise to reduce costs associated with trial-and-error process in the manufacturing industry. In laser cutting of fiber-reinforced composites, the developed thermal stress in the cut region has considerable influence on the application of the machined composite and the end product quality. Nevertheless, measurement of the thermal stress is quite challenging in practice. Here, an uncoupled thermo-mechanical finite element model is developed to accurately predict formation of heat-affected zone, kerf width, thermal field, and thermal residual stress of an anisotropic carbon/Kevlar fiber reinforced composite during multi-pass laser cutting process. A novel approach of element deletion incorporating temperature-dependent Hashin failure criteria and VUMAT subroutine is proposed. The study is carried out using Abaqus interlinked with Fortran compiler to define laser Gaussian beam (DFLUX subroutine) and material removal (VUMAT subroutine) for determining the temperature gradient and cut characteristics, respectively. The numerical results agree well with the experimental scanning electron micrographs of heat-affected zone and kerf width. In addition, residual temperature after subsequent pass results in greater temperature distribution and heat accumulation. It has also been established that the strength of composite gradually decays with the increase of temperature due to stiffness (elastic moduli) degradation in the area of the cutting zone, accelerating damage initiation in both fibers and matrix.

Laser polishing of additively manufactured Ti-6Al-4V: Microstructure evolution and material properties

Laser polishing of metals consists of irradiating the part's surface with a laser beam, thus generating a molten layer that is redistributed and resolidified to create a surface with reduced roughness. However, the process is also characterized by an instantaneous formation of heat-affected zones with consequent microstructural changes that influence the mechanical properties. In order to understand the microstructural evolution during laser polishing of Ti-6Al-4V laser-based powder bed fusion samples, a thermal model is applied in the current study to predict the dimensions of the melted zones and the heat-affected areas. Furthermore, the results obtained through simulations are discussed and compared to the experimental data, thereby establishing the validity of the process models. Finally, the experimental studies also include the evaluation of material hardness and residual stresses after laser polishing.

Experimental and numerical investigation on multi-pass laser cutting of natural fibre composite

In this paper, an experimental and numerical investigation of low power laser cutting of cotton fibre laminate (CFL) is presented. CFL is very useful for electrical insulation applications at low voltages, and usually used in gears, spacers and coil supports in turbine generator. Analysis of variance (ANOVA) along with Taguchi experimental design is conducted to determine the optimum levels of all input parameters; it is observed that heat-affected zone (HAZ) and kerf width are largely affected by the cutting speed and least affected by the stand-off distance (SOD). Detailed examination using SEM micrographs shows protruding fibres due to the polymeric matrix vaporization under laser processing. Additionally, it was noted that multiple beam passes produce a greater fibre pull-out than a single beam pass. In order to understand the underline physics, detailed numerical simulation of the problem is also performed in order to predict formation of heat-affected zone (HAZ), kerf width and thermal residual stress. Owing to fatigue, higher values of residual stresses result in cracking, delamination and failure. Using Abaqus software, user defined routines for defining laser beam profile and material removal are carried out for determining temperature gradient and cut characteristics. The approach for material removal uses element deletion based on temperature-dependent Hashin failure criteria when the fibres lose their strength at ablation point. The numerical results match reasonably well with the experimental measurements of HAZ and kerf width using the temperature-dependent material removal approach.

Experimental Study of Laser Beam Welding Process Parameters on AISI 4130-309 Joint Strength

Abstract Laser Beam Welding (LBW) is extensively being utilized in manufacturing processes to join dissimilar metals and alloy steels because its special advantages of controlled heating, Low Heat Affected Zone (HAZ) and smaller weld bead formation. However, it is viewed as a strange strategy with applications normally and constrained to welding of thick plates of metals. With a new generation of high power lasers, there has been a renewed interest in thick section LBW (also known as Keyhole Laser Welding (KLW)). KLW makes the process a possibility for industrial applications dealing with thick metals for welding in power plants, pipelines, offshore structures, shipbuilding. The advantages provided by such LBW, is suitable for joining at high process speed, low heat input, and to achieve high productivity, leading to significantly reduction in process costs. LBW of dissimilar metals such as alloy steel and stainless steel is still a challenging task, particularly due to the formation of martensitic formation in HAZ, brittle phases in the weld and solidification cracking in the fusion zone. Such issues can significantly deteriorate strength of the welded joint. Therefore, the aim of the present work is to examine the basic phenomena that happen inside the different weld zones and their impact on weld quality in the wake of LBW joints. Taguchi L25 is chosen and experiments are conducted with LBW considering 2 mm thick dissimilar metals, i.e. AISI 4130 and AISI 309 stainless steel by varying Laser Power, Welding Speed, Beam Angle, Focal Point Position and Focal Length. The experimental output results that are measured for the mechanical properties like Ultimate Tensile Strength and Impact Strength. The Analysis of Variance (ANOVA) is carried out to obtain the influence of the process parameters and statistical evolution of the results. Firefly optimization algorithm is used to determine the optimal combination of the process parameters of the LBW

Investigation of forming heat affected zone of the deposited weld metal in adaptive-pulse welding in low climatic temperatures

This article discusses the possibilities of forming the structure of the heat-affected zone (HAZ) in adaptive pulsed welding at low temperatures. It is determined that the use of adaptive pulsed welding allows the formation of a dispersed and uniform structure of the heat-affected zone. In order to identify the effect of current pulses on the formation of HAZ, the same technological parameters of the welding process were maintained. The results showed the possibility of determining the optimal algorithms for adaptive pulse control of welding processes, both positive and negative temperatures. Metallographic studies showed that in all deposited specimens, the weld structures have a dendritic structure: in the zone adjacent to the transition zone in the form of columnar formations, and closer to the outer surface, the dendrites have a disoriented structure. It was revealed that when using adaptive pulsed welding, the grain size in the heat-affected zone is reduced by 1.3 times both at positive and negative climatic temperatures, which undoubtedly positively affects the increase in the strength characteristics of the welded joint.

Effect of Direct Aging on Heat-Affected Zone and Tensile Properties of Electrospark-Deposited Alloy 718

The degradation of high-temperature components in the aerospace industry becomes a greater concern with the use of higher operating temperatures and increased operating cycles. Although the repair of defects can extend component lifespans, welding often results in a heat-affected zone (HAZ) or fusion zone with reduced mechanical properties. Due to the low energy input of electrospark deposition (ESD), repaired components should be less susceptible to mechanical property deterioration. ESD of alloy 718 on solution-annealed and aged alloy 718 base metal is evaluated in the as-deposited and direct-aged condition. HAZ formation is measured at 80 µm on an annealed substrate and 40 µm on an aged substrate. Direct aging of depositions eliminates the heat-affected zone and introduces strengthening phases in the deposition that results in a hardness equivalent to that of the aged base metal. The yield strength of as-deposited and direct-aged alloy 718 depositions is equivalent to the annealed and aged base metal, respectively, whereas the ultimate strength is, respectively, 16 and 8 pct lower. Decreased ultimate strength is attributed to lower fracture toughness of brittle secondary phases and splat boundaries from the ESD process that remain after the direct aging heat treatment.

Parametric study of laser welding of copper to austenitic stainless steel

Welding of copper to stainless steel is challenging because of sharp difference in thermophysical properties of materials and the presence of miscibility gap in Fe-Cu system. The parametric study of continuous Yb:YAG laser welding between copper and austenitic stainless steel 316L has been performed. The influence of laser power, welding speed and beam offset from joint line on weld composition, microstructure and tensile properties was studied. The corrosion behaviour of the welds was evaluated in 0.1M NaCl with the potentiostatic pulse testing method, salt fog and immersion tests.In function of copper dilution in the melted zone, different types of microstructure were observed: homogenous solid solution (≥2 at.% Cu), copper-rich net between austenite cells (2-5 at.% Cu) and formation of Cu-rich droplets (> 20 at.% Cu). Selective corrosion of Cu-rich microstructures took place in the melted zone. Tensile properties of the welds were determined by the formation of heat-affected zone in solid copper, where ductile fracture took place.

A study on the micromachining of molybdenum using nanosecond and femtosecond lasers

Laser micromachining is an advanced machining process in which machining is achieved by focusing a laser beam to melt and vaporize the material. The primary aim of this work is to fabricate a control grid for an electron gun using laser micromachining. Initially, line scribing and 2D profiling experiments are performed on a 130-μm molybdenum plate to compare the surface quality and material removal rate of nanosecond and femtosecond lasers. The effects of laser processing parameters such as average power, repetition rate, and the feed rate on the width, depth, material removal rate, and cut quality of both the nanosecond and femtosecond lasers are studied. During micromachining using the nanosecond laser, melting and recasting of the metal around the machined sites are observed, resulting in the formation of heat-affected zone. During machining using the femtosecond laser, ultrafast laser pulses are used, which result in the absence of heat-affected zone. The surface roughness obtained using the femtosecond laser for creating a 2D profile is 0.187 μm, while using the nanosecond laser, the roughness value obtained is 1.89 μm. The femtosecond laser is used to successfully machine the 3D profile of the control grid, adopting the optimized parameters obtained from the line scribing and 2D profiling experiments. The average width of the grid line was measured as 149.89 μm which is very close to the required dimension of 150 μm.

Characterization and Strain-Hardening Behavior of Friction Stir-Welded Ferritic Stainless Steel

In this study, friction stir-welded joint of 3-mm-thick plates of 409 ferritic stainless steel (FSS) was characterized in light of microstructure, x-ray diffraction analysis, hardness, tensile strength, ductility, corrosion and work hardening properties. The FSW joint made of ferritic stainless steel comprises of three distinct regions including the base metal. In stir zone highly refined ferrite grains with martensite and some carbide precipitates at the grain boundaries were observed. X-ray diffraction analysis also revealed precipitation of Cr23C6 and martensite formation in heat-affected zone and stir zone. In tensile testing of the transverse weld samples, the failure eventuated within the gauge length of the specimen from the base metal region having tensile properties overmatched to the as-received base metal. The tensile strength and elongation of the longitudinal (all weld) sample were found to be 1014 MPa and 9.47%, respectively. However, in potentiodynamic polarization test, the corrosion current density of the stir zone was highest among all the three zones. The strain-hardening exponent for base metal, transverse and longitudinal (all weld) weld samples was calculated using various equations. Both the transverse and longitudinal weld samples exhibited higher strain-hardening exponents as compared to the as-received base metal. In Kocks–Mecking plots for the base metal and weld samples at least two stages of strain hardening were observed.

Laser Cutting of Square Blanks in Stainless Steel-304 Sheets: HAZ and Thermal Stress Analysis

Laser cutting is a non-traditional cutting process and cutting of square blank in stainless steel-304 sheets cause heat affected zone (HAZ) and thermal stress. Formation of HAZ is undesirable and excessive stress cause surface defects. Thus, it is necessary to analyze them intensively. The process of laser cutting is a complex thermo-mechanical process. Hence, in this study a thermo-mechanical finite element model has been introduced by ANSYS to predict the temporal variation together with thermal stress and width of heat affected zone (HAZ). CO2 laser is used to cut 10 × 10 mm square blank in a 3 mm thick stainless steel-304 sheet. Optical microscope and SEM are used to analyse the parametric effect on surface quality at the cutting edge. The results showed that maximum temperature at the cutting edge is about to melting temperature and independent to laser power and cutting speed. Importantly, cutting speed has significant effect on rate of temperature variation. Moreover, the width of HAZ increases with the increase of laser power and decrease of cutting speed. However results of ANOVA suggested that laser power is the most significant parameter having 64.21% of contribution to width of HAZ. Furthermore, maximum stress is observed at the corner; which is supported by SEM analysis.

Heat-Affected Zone Formation in Electrospark-Deposition Additive Manufacturing on Ultrahigh-Strength Steel

Electrospark deposition (ESD), an additive microweld-surfacing process, is typically used as a new or repair technology to enhance wear and corrosion resistance for build-up and special surface modifications. ESD employs short-duration, high-current electrical pulses that result in a fused metallurgical bond to the substrate. The pulse microsecond duration splat transfer produces micro- and nanostructures and a very narrow heat-affected zone. The limited corrosion and hydrogen-induced cracking resistance of conventional ultrahigh strength 4340 type steel, greater than 1500-MPa yield strength, led to the development of precipitation-hardened, corrosion-resistant, ultrahigh-strength steel alloys such as Custom 465 and Ferrium S53. Additive surface repair technologies of these alloys in the heat-treated condition are very limited. Preheating requirements, distortion, heat-affected zones, and post-additive heat-treatment requirements prevent surface treatments in the heat-treated condition. This paper characterizes ESD applied to ultrahigh-strength steel with respect to the heat-affected zone and the deposit–substrate mixed zone through micro- and nanohardness measurements and through microstructural and microchemical characterization. The additive alloy was Stellite 21, a cobalt-based, high-chromium alloy capable of developing high hardness and high corrosion resistance. Results show that, properly applied, the heat-affected zone width ranged from 8 to 15 μm. Microhardness analysis revealed a drop in hardness within this zone. The weld dilution zone width in the deposit is on the order of 8 to 10 μm within the first two deposit layers.

An investigation of laser cutting quality of 22MnB5 ultra high strength steel using response surface methodology

In hot press forming, changes of mechanical properties in boron steel blanks have been a setback in trimming the final shape components. This paper presents investigation of kerf width and heat affected zone (HAZ) of ultra high strength 22MnB5 steel cutting. Sample cutting was conducted using a 4kW Carbon Dioxide (CO2) laser machine with 10.6µm wavelength with the laser spot size of 0.2mm. A response surface methodology (RSM) using three level Box-Behnken design of experiment was developed with three factors of peak power, cutting speed and duty cycle. The parameters were optimised for minimum kerf width and HAZ formation. Optical evaluation using MITUTOYO TM 505 were conducted to measure the kerf width and HAZ region. From the findings, laser duty cycle was crucial to determine cutting quality of ultra-high strength steel; followed by cutting speed and laser power. Meanwhile, low power intensity with continuous wave contributes the narrowest kerf width formation and least HAZ region.

Modeling of the material removal and heat affected zone formation in CFRP short pulsed laser processing

Short pulsed laser milling is a novel method to process the carbon fiber reinforced plastics (CFRP) which has bad machinability. This paper presents a numerical model studying the material removal mechanism of CFRP laser milling. It is confirmed by both the experiment and the simulation that laser ablation and mechanical erosion caused by the polymer pyrolysis are all involved in the material removal. Because the heating and cooling rate in short pulsed laser milling is high, ablation of two adjacent laser pulses almost has little influence on each other. By conducting the parametric analysis, it was found that the spacing distance under which the matrix between two adjacent laser pulses was completely degraded should be adopted to utilize the mechanical erosion effectively. Laser milling experiments of CFRP laminates were performed using a nanosecond pulsed laser system. The established model could predict the average ablation depth per scanning pass at an optimal spacing distance.

Effect of clamping area and welding speed on the friction stir welding-induced residual stresses

Effects of clamping area and welding speed on the development of residual stress fields in aluminum plates joined by the friction stir welding process were investigated in this paper. Friction stir welding on two aluminum alloy 6061-T6 plates was simulated in a three-dimensional, two-step thermo-mechanical analysis using the finite element program ANSYS. In the simulation, effects of welding tool plunging force, formation of heat-affected zone, and clamping fixture release were considered. The finite element results revealed that the clamping area plays a significant role on the formation and magnitude of friction welding-induced residual stresses. The models with greater clamping areas showed a 40 % lower maximum tensile residual stress in comparison with models with small clamping areas. Furthermore, for models with small clamping area, the effect of speed on the magnitude of residual stress was negligible. For models with large clamping area, the magnitude of maximum tensile residual stress was increased up to about 50 % with an increase of welding speed.

Laser-Assisted Waterjet Micro-Grooving of Silicon Wafers for Minimizing Heat Affected Zone

Laser-assisted waterjet micro-machining can significantly reduce the thermal damages to the workpiece as compared to the traditional laser machining process, and hence can overcome the problems associated with laser machining, such as the formation of heat-affected zone, which is a serious issue for thermal sensitive and functional materials. An experimental study on micro-grooving of monocrystalline silicon wafers is reported in this study to explore the effects of process parameters on the groove depth and width as well as the heat-affected zone (HAZ) width. Predictive models based on dimensionl analysis are then developed for estiamting the groove characteristics.

EXPERIMENTAL EVALUATION OF CO2 LASER CUTTING QUALITY OF UHSS USING OXYGEN AS AN ASSISTED GAS

Development of new material known as Ultra High Strength Steel (UHSS) able to improve the vehicle mass thus reflecting better fuel consumption. Transformation into high strength steel has been a significant drawback in trimming the UHSS into its final shape thus laser cutting process appeared to be the solution. This study emphasizes the relationship between Carbon Dioxide (CO2) laser cutting input parameters on 22MnB5 boron steel focusing on the kerf width formation and Heat Affected Zone (HAZ). Experimental research with variation of laser power, cutting speed and assisted gas pressure were executed to evaluate the responses. Metrological and metallographic evaluation of the responses were made on the outputs that are the kerf width formation and HAZ. Positive correlation for power and negative interaction for cutting speed were found as the major factors on formation of the kerf. For the HAZ formation, thicker HAZ were formed as bigger laser power were applied to the material. Cutting speed and gas pressure does not greatly influence the HAZ formation for 22MnB5 boron steel.

A prediction of laser spot-to-cutting tool distance in laser assisted micro milling Inconel 718

Laser assisted micro milling (LAMM) is one of the techniques used to enhance the ability to manufacture microscale products. Laser is used to preheat the workpiece materials prior the machining process. Compared with conventional machining, LAMM is a green manufacturing method due to reduction in cutting force during machining process. However, it needs to be controlled to avoid the melting and changing of materials properties. Determining the processing parameters and their effects on the processing characteristics, temperature measurement and the laser spot-to-cutting tool distance are crucially important. In this study, finite element analysis using ANSYS software was used to predict the heat distribution to characterise the melting and heat affected zone formation. The data at the centre of laser spot were recorded to obtain the appropriate distance between laser spot-to-cutting tool. From the results, the estimations of laser spot-to-cutting tool distance of Inconel 718 together with the allowable depth of cut were determined and applied in the actual LAMM experiment. Meanwhile, the analyses of cutting force and tool wear as an evidence in conjunction with the laser heating application to reduce the materials strength were performed.

Impact of scribe width reduction on the back-contact insulation process for the series connection of thin-film silicon solar cells

The loss of active area due to the integrated series connection of thin-film solar cells is one of the major reasons for reduced cell-to-module efficiencies. Therefore, it is crucial to reduce the dead area losses to increase the module efficiency. In this work, the back-contact insulation process (P3) for thin-film silicon modules was evaluated for substrate side laser scribing with 532 nm. A special emphasis was placed on the influence of the P3 scribe width reduction on the electrical properties of the solar module. A strong dependence of the ablation behavior on the processing beam radius and absorber thickness was observed. Calculations of the laser beam intensity distribution predict a decrease in heat-affected zone formation when the processing beam radius decreases. However, due to an increased ablation threshold fluence, measurements showed severe deterioration of the module properties for reduced beam radii. Raman microscopy revealed good agreement between extent/degree of absorber modification at the direct scribe edge, electrical evaluation, and scribe morphology for a-Si:H solar cells, while for a-Si:H/μc-Si:H tandem cells the P3 process evaluation remains challenging.

Prediction and characterization of heat-affected zone formation in tin-bismuth alloys due to nickel-aluminum multilayer foil reaction

Reactive multilayer foils have the potential to be used as local high intensity heat sources for a variety of applications. Most of the past research effort concerning these materials have focused on understanding the structure-property relationships of the foils that govern the energy released during a reaction. To improve the ability of researchers to more rapidly develop technologies based on reactive multilayer foils, a deeper and more predictive understanding of the relationship between the heat released from the foil and microstructural evolution in the neighboring materials is needed. This work describes the development of a numerical model for the purpose of predicting heat affected zone size in substrate materials. The model is experimentally validated using a commercially available Ni-Al multilayer foils and alloys from the Sn-Bi binary system. To accomplish this, phenomenological models for predicting the variation of physical properties (i.e., thermal conductivity, density, and heat capacity) with temperature and composition in the Sn-Bi system were utilized using literature data.

Finite element simulation and experimental validation of pulsed laser cutting of nitinol

Nitinol (NiTi) alloys are widely used in laser cutting of cardiovascular stents due to excellent biomechanical properties. However, laser cutting induces thermal damage, such as heat affected zone (HAZ), micro-cracks, and tensile residual stress, which detrimentally affect product performance. The key process features such as temperature distribution, stress development, and HAZ formation are difficult to measure experimentally due to the highly transient nature. In this study, a design-of-experiment (DOE) based 3-dimensional (3D) finite element simulation was developed to shed light on process mechanisms of laser cutting NiTi. The effects of cutting speed, peak pulse power, and pulse width on kerf width, temperature, stress, and HAZ were investigated. A DFLUX user subroutine was developed to model a moving volumetric (3D) heat flux of a pulsed laser. Also, a material user subroutine was used that incorporated superelasticity and shape memory of NiTi.