Heat Affected Zone Boundary Research Articles

Influence of HAZ microstructure on RCF under twin-disc test of a flash-butt welded rail

A new twin-disc test was performed using discs containing all microstructures of the heat-affected zone (HAZ) of flash-butt welded rails. It was observed that the lower hardness regions (spheroidized cementite in a ferritic matrix) at the HAZ boundary underwent a local deformation, generating valleys and reproducing a common defect type observed in rail welds: double squat-like defect. Longer crack lengths or crack clusters were observed in the HAZ boundary and central weld regions. The cross-section metallography showed that these cracks are directly related to the microstructure of spheroidized cementite at the HAZ edges or pro-eutectoid ferrite at the central region. The decrease in surface roughness before the cracks, the presence of oxide on the bottom crack surface, and superior spheroidized cementite microstructure in the upper crack region suggest that these larger/cluster cracks are predominated by extrusion of the softer material above the harder material (pearlitic/base metal).

Estimation of Heat Source Model’s Parameters for GMAW with Non-linear Global Optimization—Part I: Application of Multi-island Genetic Algorithm

Estimating the thermo-elasto-plastic deformation by arc welding through finite element analysis has been used in various industrial fields. The Goldak heat source model is one of the most important and widely used models in finite element analysis, and its parameters are estimated based on the results of previous studies and tests. Part I of this study focused on the adequate heat source model, and the study for the welding deformation with the moving heat source will be done on the latter research. This study used the parameters of Goldak’s heat source model, weld efficiency, and the location of the heat source as design variables, and defined the Heat Affected Zone (HAZ) boundary line of Bead on Plate (BOP) welding as the target. BOP welding was performed using SS400 plates, the HAZ boundary line was determined based on examining the shape of the cross-section, and the optimization condition was that temperature inside the boundary line exceeded 727 °C while the temperature outside the line did not exceed 727 °C during the welding process. During this process, a multi-island genetic algorithm (non-linear global optimization method) was used to obtain the optimal results out of 1000 candidate groups, in which the HAZ boundary was similar to the experimental results. Applying a global optimization algorithm to determine the parameters of the most important heat source model to analyze welding deformation is significant, and this may be applied in various industrial fields that use welding including shipbuilding, aviation, and machinery industries.

Characterization of drilled hole in low melting point material during low power microwave drilling process

Drilling of micro-holes for micro-electrical-mechanical systems and its applications are critical. In present work, micro-holes are drilled on a low melting point material, perspex. Microwave energy is used to drill holes through thermal ablation at 2.45 GHz. The drilled holes are analysed under field emission scanning electron microscope (FESEM) based on backscattering. The experimental results show a fine drilled hole with minimum heat affected zone (HAZ) and no delamination. A smooth curve with an average diameter of 846 micron was successfully drilled on a 5 mm thick perspex sample. Surface integrity around the hole was studied by atomic force microscope (AFM) through Ra, Rp, Rz, Rv, Rt, Rku and Rq values. AFM is used to generate values at three variants as near to hole, at average HAZ and at the end of HAZ boundary. Energy dispersive spectroscopy (EDS) is used to study the material degradation in terms of changes in composition at five different zones. The Vickers hardness tester at Hv0.05 was used to understand the hardness variation at three regions of HAZ. The XRD analysis was used to confirm the formation of the crystalline surface around the amorphous hole. The customised process is developed in-house to study the performance of microwave power, drill time, ablation, residue and (HAZ). The results confirm enhanced drilling of the hole at low power (90 W) by a steel alloy concentrator, which otherwise worsens with an increase of power beyond 180 W without significant variation in HAZ beyond 360 W.

Prediction of Creep Rupture Time Using Constitutive Laws and Damage Rules in 9Cr–1Mo–V–Nb Steel Welds

Long-term creep tests for the parent metal, a simulated heat-affected zone (HAZ) and a weld joint were conducted based on 9Cr–1Mo–V–Nb steel. The creep rupture time, tr, for the weld joint was predicted by computationally simulating creep damage based on Norton’s law combined with the time exhaustion rule (TER), where the equivalent stress (σeq), maximum principal stress (σ1), or Huddleston stress (σhud) was used to evaluate the rupture time. Creep damage analysis was also conducted based on the Hayhurst-type damage mechanics rule (HDR), in which creep rupture time was evaluated in relation to the rupture stress, σr. The computed creep rupture times and damage distributions were compared with the experimentally obtained rupture time and void distribution of the actual weld joint, respectively. Furthermore, the effect of the bevel angle of the HAZ was examined. The key findings from this study were as follows: (1) cautious predictions could be obtained for a loading stress of 80 MPa and higher; (2) the computed fracture initiation positions were on the HAZ boundaries, consistent with the type IV fracture observed at the stress of 80 MPa; (3) the magnitudes of the predicted rupture times were tr (σ1) < tr (σhud) ≈ tr (σr) < tr (σeq); (4) the bevel angle dependence previously reported was reasonably reproduced with the TER models that used σeq and σhud and with the HDR model.

Modeling of the Heat-Affected and Thermomechanically Affected Zones in a Ti-6Al-4V Inertia Friction Weld

Inertia friction welding has been used across the aerospace, automotive, and power-generation industries for the fabrication of complex axisymmetric components for over forty years. The process involves one axisymmetric piece being held stationary and another piece being brought into contact set to rotate about its axis of symmetry by a flywheel with the system under an applied load across the joint. Plasticization at the joint interface through the frictional heating sees the two pieces bond together. The titanium alloy Ti-6Al-4V has been widely studied for inertia welding applications. A successful selection of processing parameters (flywheel energy and mass, applied load) allows an inertia welding process which produces a very high-integrity weld, with a minimal heat-affected zone (HAZ) and thermomechanically affected zone (TMAZ), formed as a narrow band at the interface and extending further into the material. The width of this narrow band of heated material is dependent upon the process parameters used. A series of experimental inertia friction welds were performed using Ti-6Al-4V, and a finite element (FE) modeling framework was developed using the FE code Deform in order to predict the widths of the HAZ and TMAZ at the weld interface. The experimentally observed HAZ boundaries were correlated with the thermal fields from the FE model, while TMAZ boundaries were correlated with the Von Mises plastic strain fields.

Modeling Particle Spray and Capture Efficiency for Direct Laser Deposition Using a Four Nozzle Powder Injection System

Powder capture efficiency is indicative of the amount of material that is added to the substrate during laser additive manufacturing (AM) processes, and thus, being able to predict capture efficiency provides capability of predictive modeling during such processes. The focus of the work presented in this paper is to create a numerical model to understand particle trajectories and velocities, which in turn allows for the prediction of capture efficiency. To validate the numerical model, particle tracking velocimetry (PTV) experiments at two powder flow rates were conducted on free stream particle spray to track individual particles such that particle concentration and velocity fields could be obtained. Results from the free stream comparison showed good agreement to the trends observed in experimental data and were subsequently used in a direct laser deposition (DLD) simulation to assess capture efficiency and temperature profile at steady-state. The simulation was validated against a single track deposition experiment and showed proper correlation of the free surface geometry, molten pool boundary, heat affected zone boundary, and capture efficiency.

Finite element modeling of the electron beam welding of Inconel-713LC gas turbine blades

Electron beam welding (EBW) of shrouds in Inconel-713LC low-pressure gas turbine blades has been associated with cracking in fusion zone (FZ) and heat affected zone (HAZ) leading to a high-scrap rate in manufacturing of gas turbine blades. A comprehensive numerical analysis have been carried out using ABAQUS software to predict the level of thermal and residual stresses as well as distortions in FZ and HAZ of the electron beam welded shrouds. A particular effort has been made to develop an accurate model for the heat input since it has a significant influence on temperature distribution and has a direct correlation with the melt pool shape. As a result of the analysis, it was observed that the highest residual stress arises in FZ and HAZ boundary. Furthermore, it was concluded that the cosmetic pass narrows down the high-stress region. To reduce the level of tensile stresses, which are known as main cracking causes in FZ and HAZ, it is imperative to understand the influence of weld parameters on the residual stresses. To this end, a sensitivity analysis was carried out and the impact of EBW process parameters on temperature distribution, residual stress, and distortion were determined. From the numerical calculations, it was concluded that higher amount of power results in larger size of HAZ, wider regions exposed to tensile residual stresses and higher distortions. On the other hand, by increasing the welding speed, the amount of heat input is decreased which results in lower keyhole temperature, smaller size of HAZ, narrower regions exposed to tensile residual stresses, and lower amount of distortions.

Inverse Thermal Analysis of Ti-6Al-4V Laser Welds Using Solidification and Heat-Affected Zone Boundaries

Temperature histories of Ti-6Al-4V laser welds are presented, which are calculated using numerical-analytical basis functions and boundary constraints based on measured solidification and heat-affected zone cross sections. These weld temperature histories can be adopted as input data to various types of computational procedures, which include numerical models for prediction of solid-state phase transformations and mechanical response. In addition, these temperature histories can be used parametrically for inverse thermal analysis of welds corresponding to other welding processes whose process conditions are within similar regimes. The present study applies an inverse thermal analysis procedure that uses three-dimensional constraint conditions whose two-dimensional projections are mapped within transverse cross sections of experimentally measured solidification and heat-affected zone boundaries.

Analysis of microstructure and mechanical properties change in laser welding of Ti6Al4V with a multiphysics prediction model

This paper reports on a study involving examination of the geometry, microstructure and microhardness of welds produced on a 1.625mm thick Ti6Al4V titanium alloy using the IPG YLR-1000 fiber laser. A numerical model has been developed to predict the fusion zone (FZ), heat affected zone (HAZ) and microhardness of the weld. The thermal history from the multi-physics model is incorporated into a two-dimensional phase prediction model to predict the α′ martensite formation in the heat-affected zone and fusion zone. The multi-physics phase prediction model is applied to simulate the laser keyhole welding process with varying laser power and welding speed. It was observed that the weld geometry and microhardness predictions from the multi-physics model were in good agreement with the experimental data. The results show that the peak hardness consistently appear at the FZ and HAZ boundaries due to the high cooling rate. In addition, it has been found that laser welding speed has a profound influence on acicular α′ martensite formation leading to hardness change. Consequently, to minimize the weld hardening, use of lower power and lower welding speed parameters is recommended in the power range of 800–1000W and welding speed range of 1.73–4m/min.

Prediction and Optimization of TIG Weldment Macrostructure Zones Using Response Surface Methodology

The present work discusses the modeling and optimization of tungsten inert gas arc welding (TIG) and graphical representation of weldment macrostructure zones' shape profiles based on full factorial design of experiments and response surface modeling. To accomplish the task, an in-house developed low cost arc image magnifying system was used. The welds were produced based on a full factorial design matrix of TIG process parameters such as welding current, arc length and traverse speed. Interaction effect plots of control factors indicated weld width is affected more by the welding arc length and current. Similar effects were also observed for depth of penetration and heat affected zone (HAZ). The full factorial experimental data was further analyzed for response surface regression modeling and the response surface regression equations were tested for a number of test case data for their adequacy. The response surface regression relations were further used to optimize the TIG welding process parameters using multi response optimization technique. A mapping scheme was also developed to map the TIG weldment cross-sections' macrostructure zones including that of HAZ. The responses predicted by the surface response regression equations for the test cases were further utilized for graphical representation of the welment shape profiles including that of HAZ. The modeling methodology, regression relations and weldment mapping scheme used in the present investigation were found to be adequate for the estimation of fusion and heat affected zone boundaries.

Effect of Welding Parameters and Post-Weld Heat Treatment on the Cracking Sensitivity of Weldments in Electron Beam Welded Rene 41 with Incoloy907

The cracking sensitivity of weldments in electron beam welded (EBW) Rene 41 with Incoloy 907 superalloy was studied under various welding and post-weld heat treatment condition. The results revealed that the optimum heat input for the weldments was about 714 J/cm. The weldments exhibited relatively higher cracking sensitivity when the post-weld heat treatments were carried out at 760°C for 16hrs. In this case, the carbides in the grain boundaries commonly distributed as thin film. It is found that the lower heating rate is favor to the formation of thin carbide film, which results in the increased cracking sensitivity in the heat affected zone (HAZ). When the weldments were heated at 950°C for 12h followed with furnace cooling to 620°Cand holding for 12hrs, then air cooling to room temperature, no cracks were observed in the HAZ of Rene 41. In this case, the carbides precipitated as stagger and refined granules in the grain boundaries. When the heat treatment temperature was up to 1040°C, some overfiring cracks occurred on the grain boundary of HAZ.

Measured residual stress distributions for low and high heat input single weld beads deposited on to SA508 steel

In steel welds, the development of residual stress is often complicated by the solid state phase transformations that occur upon cooling. Here the authors present bulk residual stress distributions, as measured by neutron diffraction, for the configuration of a single pass gas tungsten arc weld bead deposited on to a 20 mm thick SA508 steel substrate. Two specimens were manufactured, one with a low heat input (1·2 kJ mm−1) and another with a high heat input (2·4 kJ mm−1). The resulting microstructures and residual stress distributions are discussed in terms of the differing thermal excursions across each weld. In regions that are austenitised during welding, the transformation strain on cooling is shown to compensate in part for thermal contraction strains. Accordingly, phase transformations in SA508 steel led to significant reductions in stress within the weld bead and much of the heat affected zone, while the highest tensile stresses were located immediately outside the heat affected zone boundary in the untransformed region.

Stress corrosion cracking behavior of friction-stir-welded Al 6061-T651

In the present study, the stress corrosion cracking (SCC) behavior of friction-stir-welded AI 6061-T651 alloy was examined of −650 mV vs Ag/AgCl using a slow strain rate testing technique. The resistance to SCC was correlated to the percent change in tensile elongation with exposure to 3.5 pct NaCl aqueous solution with respect to the reference environment. It was demonstrated the the SCC resistance of friction-stir-welded Al 6061-T651 was considerably higher than that of parent material at an anodically applied potential. In friction-stir-welded Al 6061-T651 alloy, the stress corrosion cracks occur only locally in the boundary region between the dynamically recrystallized zone (DXZ) and the heat affected zone (HAZ) regions. However, the HAZ has much lower strength properties compared with the rest of the material, and thus, fracture occurs there despite the increase in stress intensity due to corrosion at the DXZ and HAZ boundary. Eventually, the tensile fracture in friction-stir-welded A1 6061-T651 was relatively unaffected by the SCCs formed in 3.5 pct NaCl aqueous solution.

Metallurgical investigation of laser welds in wrought Waspaloy

Wrought Waspaloy was autogenously welded by means of Nd-YAG pulsed and CO2 continuous wave lasers and the microstructures of laser welds investigated using scanning and transmission electron microscopy. The fusion zones of as welded specimens contained fine γ′ and isolated MC carbides. The hardness profile of laser welds in wrought Waspaloy peaked at the fusion zone and heat affected zone (HAZ) boundaries, owing to fine γ′ precipitation and MC carbides in the fusion zone, and γ′ and M23C6 precipitates at the HAZ boundaries. After a full post-weld heat treatment, the hardness of the bead can be recovered to around 400 IIV but the hardness peaks at the HAZ boundaries became higher relative to the fusion zone and base material after aging at 450°C for 20 h. The microstructures in the fusion zone of post-weld heat treated welds consist of M23C6, MC, and γ′ which are quite uniform in size. The low cycle fatigue (LCF) strength of laser welds decreased to 400 MPa compared with 450 MPa for the base material. Fatigue cracks are initiated at the boundaries of the fusion zone and grow into the HAZ or along the boundary of the fusion zone. Fractography of LCF specimens showed ductile transgranular fracture.

単純モデル化によるビード溶接部の引張変形の解析

From a simple model of the bead weld constructed by three zones (base metal (BM) without heat affected zone, heat affected zone (HAZ) and weld metal (WM)), the approximate analysis for the tensile deformation of bead welds (mild steel (arc): three.zones, and copper (TIG and electron beam) and aluminum (TIG): two zones of BM and WM) have been carried out. And the confirmation for the propriety of this analysis is made by the tensile test of their bead welds.As the results, it is found that absolute values of both ratios (σ/σx) and (ε/εx) are larger than 0.85 (σ and ε: equivalent stress and strain, and σx and εx: normal stress and longitudinal strain), and that the tensile deformation of bead weld is approximately expressed by the tensile deformation of the heterogeneity distribution. Moreover, it is found that initial yield for mild steel, copper (TIG) and aluminum occurs in BM in the vicinity of WM, and yield for copper (electron beam) occurs near the ditch (made by the weld), and that the yielding progresses along the boundary of HAZ (for arc and TIG). Also, it is recognized that the inclination (not perpendicular in the tensile direction) of boundary of HAZ and the contignity between HAZ and WM are necessary for the occurrence of the initial yield and for the yield progress and that the variations of the stress and the strain near boundaries between their zones occurs remarkably.