Heat Affected Zone Regions Research Articles

Post-weld heat treatment effects on the tensile properties of cold metal arc welded AA 6061-T6 aluminum joints

Abstract Heat treatable aluminium alloys of the 6xxx series are widely used in the hood, trunk and roof sections of a car body as they offer better corrosion resistance, high strength to weight ratio and excellent weldability characteristics. Gas metal arc welding (GMAW), a high productivity process is extensively employed in the automobile sector due to its ease of automation. HAZ softening is one of the most common problems that occurs during the welding of aluminium alloys by GMAW because of its high heat input. Cold metal transfer (CMT) is the most suitable welding process for welding aluminium alloys especially for controlling HAZ softening issues that occur due to the low heat input involved in this process. However, softening continues to exist in the welded joints. In this investigation, Post -weld heat treatment (PWHT) was employed to increase joint efficiency and to minimize the HAZ softening of AA6061-T6 aluminum alloy joints welded by GMAW and CMT. The results show that the tensile strength of a post-weld heat treated CMT joint is 10 % higher i. e., 242 MPa (76 % of joint efficiency), than that of the as welded CMT joint. This is due to the nucleation of strengthening precipitates in the weld metal and heat affected zone regions.

Effect of Different Arc Welding Processes on the Metallurgical and Mechanical Properties of Ramor 500 Armor Steel

Abstract In the present study, Ramor 500 armor steel plates were automatically welded using cold metal transfer arc welding (CMT), gas metal arc welding (GMAW), and hybrid plasma arc welding (HPAW) methods. To investigate the effects of three different fusion welding methods on metallurgical and mechanical properties, the welded joints were examined using optical microscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) and also subjected to radiographic, hardness, tensile, and notched impact tests. The weld metal (WM) region of the GMAW and HPAW joints consisted of massive austenite. In the CMT welded joint, the WM consisted mainly of dendritic austenite and a minor amount of δ-ferrite. Regardless of the welding process, the hardness of both the WM and heat-affected zone (HAZ) regions was found to be higher than the base metal (BM). The tensile strengths obtained by CMT, GMAW, and HPAW were 45%, 50%, and 65% of the BM, respectively. Cleavage-type brittle fractures occurred in the GMAW and HPAW welded joints, while localized ductile fractures occurred in the CMT joints. Tensile test specimens of the CMT joints fractured in the WM, while the GMAW and HPAW joints fractured in the HAZ. In terms of notch toughness, the CMT joints exhibited better impact resistance compared with the BM. GMAW and HPAW joints displayed less impact resistance than the BM, with values comparable with previous studies in the literature.

Evaluation of microstructural and mechanical properties of dissimilar Inconel 625 nickel alloy–UNS S32205 duplex stainless steel weldment using MIG welding

Inconel 625 nickel alloy and UNS S32205 duplex stainless steel (DSS) were welded with ER2209 filler metal using MIG (Metal Inert Gas) welding method. The weld metal obtained by DSS filler metal was subjected to mechanical and microstructural evaluation. Toughness and hardness properties had been examined by mechanical and microstructural characterization of weld metal, fusion line, and HAZ (Heat Affected Zone) region, and precipitations were investigated by light microscopy (LM), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). Corrosion behavior of base metals and face and root of weld metal were examined by using potentiodynamic polarization test. Additionally, detailed elemental analysis and mapping of weld metal with both optical emission spectrometer (OES) and X-Ray spectrometer were attained. Results demonstrate a significant decrease in toughness of the welding due to the presence of Nb and Mo rich intermetallic precipitations in the Inconel HAZ and root region, although there is no significant increase in hardness. Potentiodynamic polarization test shows that the dilution-induced microstructural transformation in the root of weld has the worst corrosion resistance in the weld metal. Therefore, this dissimilar welding does not have optimum properties for neither toughness nor corrosion.

Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420

Dissimilar laser welding of AISI 304 and AISI 420 stainless steel sheets was performed with pulsed Nd: YAG laser so as to evaluate the temperature distribution, microstructure and mechanical properties of the welded zone. The effects of pulsed laser process parameters including (pulse duration, frequency, current, focal length and welding speed) on the fusion zone temperature variation were investigated. Furthermore, the weld joint microstructure and mechanical properties were evaluated according to the temperature distribution of the fusion zone. Different thermal conductivity of the austenitic and ferritic stainless steel resulted in higher temperature of the welded zone for austenitic sample because of more heat concentration. The focal length and peak power had the significant influence on the AISI 304 temperature rise. The fusion zone microstructure composed of coarse ferrite grains austenitic grains. The heat affected zone (HAZ) was including coarse ferrite grains near the ferritic side and austenitic grains with stringers of ferrite δ adjacent to the austenitic side of the weld. The fracture of the welded joint initiated at the HAZ region of AISI 420 stainless steel. Increasing the welding speed decreased the peak temperature of the fusion zone which clearly reduced the strength of the joint. Increasing the welding speed remarkably reduced the weld tensile strength of the about 30% and the temperature of the melt pool adjacent area about 140 °C. Also, the porosity formed at the melt pool region when the temperature of the ferritic sample decreased from 250 °C to 130 °C during welding process. The fusion zone microstructure analysis shows that the predominant microstructure was composed of coarse ferritic grains that include more than 70% of the melt pool total volume.

Fatigue Behaviors of Resistance Spot Welds for 980 MPa Grade TRIP Steel

The fatigue life of the resistance spot weld of 980 MPa grade transformation induced plasticity (TRIP) steel was investigated and failure modes and fracture surfaces according to the fatigue load were analyzed. The fatigue life according to the nugget size was observed by using two electrodes with face diameters of 8 mm and 10 mm. When an electrode face diameter with 10 mm was used, the nugget size was large, and the fatigue life was further increased. After the fatigue test, three types of failure modes were observed, namely pull-out, plug, and heat affected zone (HAZ) failure, depending on the fatigue load. The fracture surfaces in each failure mode were analyzed. In all failure modes, a crack was initiated in the HAZ region, which is the interface between the two materials in all failure modes. In the case of pull-out failure, the crack propagates as if it surrounds the nugget at the outer edge of the nugget. In the case of HAZ failure, the crack propagates in the thickness direction of the material and outward in the nugget shell. Plug failure occurs with pull-out failure and HAZ failure mixed. The propagation patterns of cracks were different for each failure mode. The reason why the failure mode and the fracture surface are different according to the fatigue load is that the propagation speed of the fatigue crack is fast when the fatigue load is relatively large and is slow when the fatigue load is low.

Indentation for investigation of strain rate effect on mechanical properties in structural steel weld zone

In this study, instrumented indentation testing was conducted at room temperature for the investigation of the effect of strain rate on the hardness and yield strength in the weld zone of a commonly used structural steel, SM520. A number of indentation tests were undertaken at a number of strain rate values from 0.02 s-1 to 0.2 s-1 in the weld metal (WM), heat-affected zone (HAZ), and base metal (BM) regions of the weld zone. The mechanical properties including yield strength (σy) and hardness (H) in WM, HAZ, and BM were then computed from the applied load-penetration depth curves using a proposed method. As the result, the effects of strain rate indentation on yield strength and hardness in all regions of the weld zone were evaluated. The results displayed that hardness and yield strength in the weld zone’s components are influenced on the strain rate, where both hardness and yield strength decrease with the decreasing strain rate.
 Keywords:
 indentation; mechanical properties; strain rate effect; structural steel; weld zone.

Improvement in Mechanical and Metallurgical Properties of Friction Stir Welded 6061-T6 Aluminum Alloys through Cryogenic Treatment

An investigation has been made to improve the properties of the friction stir welded (FSW) 6061-T6 aluminum alloys. A cryogenic thermal treatment is developed for the joints during welding and its effects on mechanical and metallurgical properties, and precipitates are evaluated at various welding parameters. The friction stir welded joints with cryogenic treatment attained the better properties than the without cryogenic treatment. The improvement of properties was attributed to the refinement of grain size and to the introduction of a reduction in the softening region of the welded joints. Under cryogenic cooling rates, joints were experienced to the low temperature environments and faster cooling rates, which are contributed to enhance the hardness of the stir zone and heat affected zone regions and the formation of fine grain structure in the stir zone. The results indicated that the formation of finer grains of less than 5 µm in the stir zone, which is smaller than the joints of without cryogenic treatment. Subsequently, mechanical properties drastically improved and the joints achieved a maximum joint efficiency of 74% of the base material

Multiphysics modeling of phase transformation and microhardness evolution in laser direct deposited Ti6Al4V

This paper investigates the phase transformations occurring in laser direct deposited Ti6Al4V and assesses the microhardness of the as-built components using predictive modeling tools. A three-dimensional laser direct deposition (LDD) model was utilized to predict the geometry and thermal history of the multi-track LDD process. The predicted free surface geometry, fusion zone (FZ) and heat affected zone (HAZ) boundaries were in good agreement with experimental results. Temperature profiles and cooling rates were extracted from the validated LDD model and incorporated into a two-dimensional phase prediction model to determine phase transformations in FZ and HAZ regions. In the FZ, the high peak temperature and high cooling rate produced a complete α’ martensitic microstructure, which was predicted with the phase transformation model and observed experimentally. Within the HAZ, a low peak temperature and correspondingly low heating and cooling rates led to a mixture of α’ martensite, transformed and untransformed α phase in the final microstructure. When depositing the subsequent tracks, the LDD process produces overlapped HAZ regions, which contain a higher volume fraction of α’ martensite due to the repeated heating cycles. Based on the volume fraction of α’ martensite and α phase in the final microstructure, the microhardness within different zones of a three-track Ti6Al4V deposit was accurately predicted.

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.

Evaluation of double-layer weld deposition technique on ASTM A182 F22 steel without post-weld heat treatment

The present work evaluates the mechanical and metallurgical behavior of ASTM A182 F22 steel heat-affected zone (HAZ) through the double-layer weld deposition technique, with different welding energies between the first and second layers. This methodology leads to the reduction of hardness levels in the HAZ and can exempt the need for post-weld heat treatment on low-alloy steel after buttering welding. The welding was performed by the automated GMAW process in flat position. As the filler metal, the AWS ER309L, ER 80S-G and ER NiCrMo-3 (Inconel 625) alloys were applied on ASTM A182 F22 steel. The analysis of the refining and/or tempering in coarse-grained HAZ (CGHAZ) of the first layer was performed by metallographic tests and microhardness measurements. The tempering of the first-layer CGHAZ is dependent on the correct overlap of the weld layers, weld bead geometry and the highest imposed heat of the second layer. The results revealed that the Higuchi technique is effective for obtaining welding parameters for the second-layer deposition that promotes thermal treating of the hard region of the first-layer HAZ. However, this technique was not able to reduce the hardness values at the 250 HV level required by NACE 0175 for the test conditions in this work.

Nanoindentation creep behavior of RPV’s weld joint at room temperature

SA508 Gr.3 steel has been widely used in nuclear reactor pressure vessels (RPV). Nuclear components are generally combined through arc welding processes, which always produces heterogeneous mechanical properties in heat affected zone (HAZ) of weld joint. In order to study mechanical heterogeneity of weld joint, HAZ was been divided into five small regions (HAZ1 to HAZ5) based on the distance from the weld center line. The elastic modulus, hardness, and creep deformations of five regions in HAZ were measured through nanoindentation, as well as base and weld metals. According to the experimental results, the HAZ2 region (belonging to the fine-grained HAZ) exhibited a significantly lower hardness and creep behavior. Strain rate sensitivities (SRS) in different regions were then estimated from the steady-state creep, and the HAZ2 region showed a relatively higher value. The influence of grain boundary fraction on the creep behavior of weld joints was discussed later. Furthermore, the results of SRS also indicated that the creep mechanism of tested regions could be dominated by dislocation activities.

MICROSTRUCTURE EVALUATION OF DISSIMILAR LASER BEAM WELDS –MDN 250 AND 15CDV6 STEELS

A novel dissimilar Laser Beam Weld of MDN 250 and 15CDV6 steels with adequate joint strength was successfully achieved using CO2 laser source. Electron probe microanalysis of the weld, weld microstructure, micro-hardness measurement across the weldment and transverse tensile properties of the weldment were evaluated. The fusion zone of the dissimilar welds exhibits an epitaxial grain growth. In dissimilar metal welds, HAZ (Heat Affected Zone) width is more on 15CDV6 steel side compared to MDN 250 steel side. An asymmetrical hardness profile across the dissimilar weld joint was observed, and HAZ region on 15CDV6 steel exhibited a high hardness. The tensile strength values of the welded specimens were close to that obtained for sheet specimens.
 Keywords: Laser beam welding, MDN 250 steel, 15CDV6 steel, Mechanical properties, Microstructure.

Fracture Assessment of Weld Joints of High-Strength Steel in Pre-Strained Condition

Unstable fractures tend to occur after ductile crack initiation or propagation. In most collapsed steel structures, a maximum 15% pre-strain was recorded, at the steel structural connections, during the great earthquake of 1995, in Japan. Almost-unstable fractures were observed in the beam-to-column connections, where geometrical discontinuities existed. Structural collapse and unstable failure occurred after large-scale plastic deformations. Ship structures can also suffer from unstable fractures in the welded joints. The fracture resistance of butt-welded joints subjected to tension in the pre-strained condition was estimated by considering the toughness deterioration, due to pre-strain and toughness correction for constraint loss in a tension specimen. The target specimen for this fracture assessment was a double-edged, through-thickness crack panel, with a crack in the weld joint (heat-affected zone (HAZ)). The critical fracture toughness value (crack tip opening displacement (CTOD)) of a large structure with pre-strain, which was applied to the HAZ region, was estimated from a small-scale, pre-stained, three-point bend specimen. Fracture toughness values, evaluated by a CTOD test, were recently mandated for shipbuilding steel plates. The critical fracture toughness value is a very useful parameter to evaluate the safety of huge ship structures.

Microstructure and strain rate sensitivity behavior of SM490 structural steel weld zone investigated using indentation

In this study, a series of optical microscope examinations, constant loading rate indentation (CLR), and creep indentation (CI) tests were performed on room temperature to investigate the strain rate sensitivity behavior of the SM490 structural steel weld zone. Microstructures of weld metal (WM), fine-grain heat-affected zone (HAZ), and base metal (BM) in the weld zone were observed using optical microscopy. Numerous CLR tests were performed at a wide range of strain rate indentation from 0.02 s−1 to 0.2 s−1 in the middle regions of WM, fine-grain HAZ, and BM across the weld zone, and as a result strain rate indentation effects on both indentation hardness and yield strength were investigated. The results showed that both yield strength and indentation hardness of WM, fine-grain HAZ, and BM exhibit the strain rate-dependent behavior, in which yield strength and indentation hardness tend to increase with increasing strain rate indentation. Strain rate sensitivity (SRS) values of WM, fine-grain HAZ, and BM were determined by using three different models: indentation hardness, yield strength, and creep indentation models. The SRS behavior of SM490 structural steel weld zone was investigated. The results indicated that, while both yield strength and indentation hardness of BM are lower than those when compared with HAZ and WM, BM has the highest SRS value than does fine-grain HAZ and WM. Furthermore, WM has the lowest SRS value compared with the SRS values of other components in the weld zone. The results of this study were used to assess and understand the strain rate sensitivity behavior of SM490 structural steel weld zone.

Analysis of creep behavior of welded joints of P91 steel at 600 °C

The martensitic P91 steel (9Cr-1Mo-V-Nb) has been developed for ultra-supercritical pressure application in steam power plants. The creep rupture strength of the welded joint of this steel is limited by the fine-grained region of its heat affected zone (HAZ). The factors which result in the reduction of the creep rupture strength of the fine-grained region of HAZ were discussed. It was found that the most effective factor reducing the creep rupture strength of the fine-grained region of the HAZ of the welded joint of P91 steel, in comparison with other regions of the welded joint, is the finer prior austenite grain size. These fine prior austenite grains of the fine-grained region of the HAZ accelerate the rate of growth of martensite lath subgrains which results in a softer martensite matrix.The minimum creep rate dependence on applied stress for base metal, welded joints and the samples simulating the fine-grained region of the welded joints was described by means of the conventional power-law equation. The experimental results, both in the form of strain versus time creep curves and of minimum creep rate versus time creep curves were used to study the creep behavior of the welded joints of P91 steel at 600 °C.

HAZ characterization of welded 42SiCr steel treated by quenching and partitioning

An experimental steel 42SiCr was heat treated with a quenching and partitioning treatment in order to achieve high strength and at the same time acceptable ductility. This crucial combination requires the formation of a martensitic microstructure with a small volume fraction of retained austenite embedded on a microscopic scale. The objective of this work was to investigate the evolution of microstructure and hardness in the heat affected zone (HAZ) of welds in this 42SiCr and its major influence factors. Thermo-physical simulations with a dilatometer yielded samples of each zone in the HAZ and allowed a detailed investigation of HAZ microstructures and properties. A decrease in hardness from 580 HV in the quenched and partitioned base material to the HAZ, with minimum hardness values of 370 HV in a region exposed to temperatures close to the Ac3 temperature, was revealed. Simulation showed that the experienced peak temperature is predominantly determining the HAZ properties. The microstructure after quenching and partitioning treatment consisted of fine martensite with presumed retained austenite. The HAZ regions that were heated above Ac3 temperature can be characterized to have a fine bainitic/martensitic microstructure, while the regions heated to more than 1100 °C show coarser grain, with bainitic/martensitic features.To support the HAZ simulation results sheets of this steel were rolled and resistance spot welds were produced. The welds were characterised with respect to the microstructure in the heat affected zone, as well as the mechanical properties of the joint. Results proved that the data obtained by thermo-physical simulation is in very good agreement with the spot welding results. On a macroscopic scale the welding results indicate very good joint formation but very brittle fracture behaviour using a conventional resistance spot welding process for the quenching and partitioning steel. However, despite the relatively high carbon equivalent of the material, also ductile aspects of fracture were observed.

The Influence of Heat Input to Mechanical Properties and Microstructures of API 5L-X65 Steel Using Submerged Arc Welding Process

API 5L-X65 steel is the type of high strength low alloy (HSLA) steel, widely used in the manufacture of pipe. Submerged arc welding (SAW) is widely used for the fabrication of the pipe, the extent of use submerged arc welding caused it could be done automatically and high reliability. The results of the welding process will lead to differences and changes in the microstructure in heat affected zone (HAZ) and weld metal that will affect the mechanical properties of the output, so as to obtain good welding results required the selection of welding parameters accordingly. As the use of the heat input during welding is very important influence on the mechanical properties and microstructure of the weld. The purpose of this study to determine the effect of heat input on the microstructure, hardness and toughness of welds in submerged arc welding. Welding currents used were 200, 300, 400 and 500 Ampere with a voltage were used 25, 27 and 30 Volt. The results showed that the higher heat input will result in a growing area of HAZ region width and grain size increased. Highest hardness values are the results of the weld heat input with a low of 244.69 HVN caused by the rapid cooling rate of the weld area. The highest toughness values are the results of the highest heat input that was dominated by acicular ferrite phase.

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

ABSTRACTEffect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.

Influence of Angle of Corner Deposition on Temperature and Residual Stress Distributions in the Vicinity of the Deposited Region by a Ti-6Al-4V Wire-Feeding Type of Direct Energy Deposition Process

It is compelling to realize that the additive manufactured part using wire feeding type directed energy deposition (DED) process is subjected to undesired thermal effects, and induced residual stress during the manufacturing process. In order to improve the quality of the manufactured part, the distributions of temperature and residual stress have to be understood to manage the results of the processing of these materials. The objective of this paper is to investigate the influence of the angle of corner deposition on the distributions of temperature and residual stress of the Ti-6Al-4V deposited bead, and the substrate via thermo-mechanical finite element analyses (FEAs). In the same fashion, the formation of the heat affected zone (HAZ) and the stress influenced region (SIR) are estimated from the measured results of the FEAs. Equally important, it can be stated that from the estimated HAZ and SIR regions, the overlapping of undesired thermal effects and residual stress between two beads fabricated by the wire feeding type DED process can be avoided at the design stage.

Dissimilar Friction Stir Welding of Carbon Steel and Stainless Steel: Some Observation on Microstructural Evolution and Stress Corrosion Cracking Performance

Specimens of carbon steel (CS) and stainless steel (SS) 316 was joined together by double sided butt joint Friction Stir Welding method. Effect of the tool rotational speed and specimen preheat temperature on resultant microstructure and mechanical behavior was studied. Constant traveling speed of 100 mm/min, tool rotational speed of 500 and 1000 rpm in combination with preheat temperature of 50 and 100 °C was employed for this study. No sign of root crack was observed in all specimens. Optical microscopy identified four distinct zones characterized as the stir zone, thermal mechanically affected zone (TMAZ) in both SS and CS specimen, and heat affected zone (HAZ) in CS specimen. No HAZ in SS was observed. Scanning electron microscopy revealed formation of a thin void layer on several locations at the boundary between the TMAZ and the re-oriented region of SS side, while on the CS side no voids were identified. The grain size on the TMAZ of CS was observed to be smaller compared to the HAZ region. No sign of root crack was observed in all specimens. The specimens were subjected to U bend. Specimens joined with 1000 rpm rotational speed developed a brittle behavior and failed in U bend test. The extent of the crack and brittleness was proportional to the applied preheat. U bent specimens from 500 rpm rotation demonstrated a good stress corrosion cracking behavior in 30,000 ppm solution of sodium chloride (NaCl).