Low Heat Input Conditions Research Articles

Corrosion Behavior of Laser-Brazed Surface Made by Joining of AA6082 and Galvanized Steel

In the present study, a galvanized steel and a AA6082 aluminum alloy sheet of 2 mm thick were weld-brazed by applying laser and using 2-mm-diameter solid Al-12% Si filler wire at 2.5 m/min wire feed rate and 2 m/min laser scan speed at varying laser power (4.0, 3.5, 3.0 kW) in flange configuration. The microstructural investigation of the laser weld-brazed joints revealed the presence of two-layered intermetallic compound at steel/braze seam interface. The corresponding x-ray diffraction analysis of the steel/braze seam interface showed the presence of Al-Fe-Si-based ternary intermetallic phases. SEM and EDS analysis of the braze seam revealed the presence of α-Al grains surrounded by Mg2Si phase in the grain boundary region. Fine intermetallic layer was observed at lower heat-input condition. The corrosion performance of the brazed joints was studied with the help of immersion, salt spray, and electrochemical polarization tests in different electrolytes containing NaCl salt. Brazed joints experienced galvanic corrosion during immersed in corrosive media. From the salt spray test, it was revealed that galvanized steel was highly susceptible to corrosion compared to Al-rich phase. The polarization results showed a substantial change in corrosion resistance from steel interface to the brazed region due to the variation in microstructure. The samples brazed with low heat-input showed better corrosion resistance as compared to the joints made with high heat-input. Post-corrosion microstructure revealed intergranular corrosion at the brazed joint along with pit formation.

Friction stir welding of Monel alloy at different heat input conditions: Microstructural mechanisms and tensile behavior

The mechanisms governing the microstructural evolution during friction stir welding of Monel at different heat inputs and the corresponding mechanical properties were investigated. In the low heat input condition, both continuous and discontinuous dynamic recrystallizations caused the formation of fine grains with an average grain size of 1.7 μm and closely random texture. A high heat input condition resulted in larger average grain size of 23.5 μm with shear texture by only the continuous dynamic recrystallization. Low heat input led to the simultaneous increase of strength and elongation. The origins of the tensile behavior of the joints have been discussed.

Tensile behavior, microstructure, and substructure of the friction stir welded 70/30 brass joints: RSM, EBSD, and TEM study

The effect of bead on plate friction stir welding parameters on the tensile properties of the 70/30 brass joints was investigated using response surface method. The microstructures of the joints were characterized using optical microscopy, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). The tensile test was conducted to measure the ultimate tensile strength and elongation of the joints. In addition, the fracture surfaces of the tensile specimens were analyzed by scanning electron microscopy (SEM). The results showed that the most effective parameters on the strength and elongation of the joints were tool rotational speed and axial force, respectively. Optimizing the parameters revealed that the maximum strength and elongation of 318.5MPa and 54.9% can be achieved at a rotational speed of 1000rpm, a traverse speed of 58.4mm/min, and an axial force of 3kN. The strengthening mechanisms of grain boundary and dislocation density effects were responsible for the higher ultimate tensile strength of the joints welded at the lower heat input conditions. Furthermore, the effect of friction stir parameters on the ultimate tensile strength and elongation of the joints has been discussed, thoroughly.

Investigation of temperature dependent microstructure evolution of pure iron during friction stir welding using liquid CO2 rapid cooling

The microstructure evolution of pure iron during friction stir welding was reconstructed by an ingenious experimental design, in which the rapid cooling friction stir welding combined with the tool “stop action” technique and the subsequent short-time annealing were adopted to “freeze” the transient microstructure during the stirring and reproduce the normal cooling during conventional friction stir welding, respectively. The microstructure evolution during the stirring and normal cooling was investigated along the material flow path and in the annealed “frozen” weld zone by high-resolution electron-backscatter-diffraction technique. The results show that the continuous and discontinuous dynamic recrystallizations occur simultaneously at the severe deformation stage in front of the tool both under low and high heat input conditions. However, during the material flow, the microstructure evolution involves the plastic deformation, recrystallization, high angle boundaries migration and dynamic recovery under the low heat input condition, while in a dynamic balance of deformation, recrystallization and grain growth under the high heat input conditions. At the cooling stage, normal grain growth occurs both for the low and high heat input welding conditions, while it is very limited for the low heat input condition.

Assessment of delta ferrite in multipass TIG welds of 40 mm thick SS 316L: A comparative study of ferrite number (FN) prediction and measurements

Austenitic stainless steels are widely used in the fabrication of fusion reactor major systems like vacuum vessel, divertor, cryostat and other structural components development. Multipass welding is used for the development of thick plates for the structural components fabrication. Due to the repeated weld thermal cycles, the microstructure adversely alters owing to the presence of complex phases like austenite, ferrite and delta ferrite and subsequently influences the mechanical properties like tensile and impact toughness of joints. The present paper reports the detail analysis of delta ferrite phase in welded region of 40 mm thick SS316L plates welded by special design multipass narrow groove TIG welding process under three different heat input conditions. The correlation of delta ferrite microstructure of different type structures acicular and vermicular is observed. The chemical composition of weld samples was used to predict the Ferrite Number (FN), which is representative form of delta ferrite in welds, with Schaeffler’s, WRC-1992 diagram and DeLong techniques by calculating the Creq and Nieq ratios and compared with experimental data of FN from Feritescope measurements. The low heat input conditions (1.67 kJ/mm) have produced higher FN (7.28), medium heat input (1.72 kJ/mm) shown FN (7.04) where as high heat input (1.87 kJ/mm) conditions has shown FN (6.68) decreasing trend and FN data is compared with the prediction methods.

Underwater dissimilar friction stir welding of aluminum alloys: Elucidating the grain size and hardness of the joints

Underwater dissimilar friction stir welding of the AA6061 and AA7075 aluminum alloys was performed in this study. The effect of friction stir welding parameters on the grain size and hardness of the joints was studied using empirical models. The microstructure of the joints was characterized by means of light and transmission electron microscopes. The Vickers hardness test was conducted to measure the hardness of the joints. In addition, the process parameters including traverse and rotational speeds, grain size and hardness of the joints were correlated. The results revealed that the developed models predicted the hardness and grain size of the joints, precisely. Higher traverse speed and lower rotational speeds resulted in finer grain size and larger hardness. The grain boundaries and dislocations were identified as responsible for the higher hardness of the joints welded at lower heat input conditions. Moreover, the Hall–Petch relationship showed a deviation from its linear classical equation, which was due to the formation of substructures such as dislocations inside the grains.

Mg–6%Al–1%Zn–2%Ca合金のMIG溶接

Effect of metal inert gas (MIG) welding parameters on tensile strength of weld joints in a Mg–6%Al–1%Zn–2%Ca (AZX612) flame retardant magnesium alloy was examined. Proper welding conditions were found by examining welding current from 70 to 140 A and welding speed from 45 to 70 cm/min. The ultimate tensile strength of weld beads was more than 70% of the strength of the base metal in all the conditions examined. The fracture origin was not Al2Ca second phase or Al–Mn particles, which was the fracture origin of the base metal, but blowhole or oxide inclusion. High weld strength was obtained in low heat input conditions, in which blowhole formation was suppressed. In case that an oxidized MIG welding wire was used for welding, the weld strength decreased and the large oxide fracture origin with the size of 2 mm was found. To obtain high weld strength, it is important to choose proper welding conditions in which blowhole formation was suppressed and to suppress the oxidation of MIG welding wire.

Formation mechanism of typical onion ring structures and void defects in friction stir lap welded dissimilar aluminum alloys

The formation mechanism for typical onion ring structure and void defect with heat input during FSLW was continuously visualized by an exit-hole continuous observation technique. Based on this result, the compatibility between microstructure, microtexture, element maps and strain maps using electron backscattered diffraction (EBSD) with the chemical indexing assisted by EDS analysis was simultaneously investigated. The results revealed that the threaded probe was significantly correlated to typical onion ring structure and the onion structure formed as soon as it touched the probe. This result is different from the results so far. On the other hand, the remnant of original interface between top and bottom plates after FSLW and asymmetrical flow around rotating tool were significantly correlated to the formation of void defect in low heat input condition.

Influence of heat input conditions on microstructure evolution and mechanical properties of friction stir welded pure copper joints

The effect of heat input on microstructure and mechanical properties of friction stir welded pure copper was investigated. For this purpose, 5 mm thick copper plates were friction stir welded at different traverse and rotational speeds. Also, the peak temperatures of the joints during friction stir welding were recorded using accurate thermocouples. Moreover, the microstructure, hardness and tensile properties of the joints were examined. The results showed that the outline of the stirred zone were observed at lower heat input conditions but disappeared when heat input increased. The thermomechanically affected zone was clearly characterized by elongated and recrystallized grains, respectively at lower and higher heat input conditions. Moreover, the lower heat input conditions caused to finer grain sizes of the nugget zone, and hence higher hardness and tensile strength. The joints welded at higher heat input conditions had more elongation and showed more ductile fracture mode.

Heat affected zone microstructures and their influence on toughness in two microalloyed HSLA steels

Microstructures and Charpy impact properties have been examined in two microalloyed steels following heat treatments to simulate weld heat affected zone (HAZ) structures over a range of heat input conditions, characterised by the cooling time from 800 to 500°C (Δt8/5). The base materials were low carbon structural steel plates microalloyed with vanadium and nitrogen (V–N) and niobium (Nb), respectively. The toughnesses of the HAZs displayed remarkably different behaviours as shown by their impact transition temperatures. For the V–N steel, the toughness improved with increasingly rapid cooling (low heat input conditions) whereas the Nb steel showed an opposite trend. Some of this behaviour could be explained by the presence of coarse ferrite grains in the slowly cooled V–N steel. However, other conditions where all the structures were bainitic and rather similar in optical micrographs gave widely different toughness values. The recently developed method of five dimensional boundary analysis based on electron backscattering diffraction has been applied to these cases for the first time. This showed that the lath boundaries in the bainite were predominantly on {110} planes of the ferrite and that the average spacing of these boundaries varied depending on steel composition and cooling rate. Since {110} is also the slip plane in ferrite, it is considered that close spacing between the lath boundaries inhibits general plasticity at stress concentrations and favours initiation of fracture. The differences between the two steels are believed to be due to their transformation behaviours on cooling where precipitation of vanadium nitride in austenite accelerates ferrite formation and raises the temperature of the phase transformation in V–N steels.

Elucidating of tool rotational speed in friction stir welding of 7020-T6 aluminum alloy

The 7020-T6 aluminum alloy plates of 4 mm thickness were friction stir welded at rotational speeds of 400, 600, 800, and 1000 rpm and constant traverse speed of 100 mm/min. The peak temperatures of the joints were recorded by precise thermocouples. Microstructure, hardness, tensile properties, and fracture surfaces of the joints were analyzed. The results showed that decreasing the tool rotational speed from 1000 to 400 rpm decreased the peak temperature from 311 to 209 °C, and hence caused a lower heat input. In addition, lower rotational speeds result in higher hardness and tensile strengths. The higher hardness and ultimate tensile strength were related to the grain boundary, precipitation, and substructure strengthening mechanisms. In addition, the fracture surfaces of the joints welded at higher heat input conditions showed more ductile mode in comparison with that of welded at lower heat input condition, which confirmed the lower tensile elongation of the joints welded at lower rotational speeds.

Effect of tool traverse speed on microstructure and mechanical performance of friction stir welded 7020 aluminum alloy

The effect of tool traverse speed on microstructure and mechanical properties of friction stir welded 7020-T6 aluminum alloy was investigated. For this purpose, 5 mm thick 7020-T6 aluminum alloy plates were friction stir welded at traverse speeds of 50, 100, 150, and 200 mm/min and constant rotational speed of 900 r/min. Also, the peak temperatures of the joints during friction stir welding were recorded by accurate thermocouples. Moreover, the microstructure, hardness, tensile properties, and fracture surfaces of the joints were examined. The results showed that increasing the tool traverse speeds from 50 to 200 mm/min decreased the peak temperature from 331 ℃ to 211 ℃, and hence caused to lower heat input during friction stir welding. Furthermore, the higher hardness and ultimate tensile strength of the joints welded at higher traverse speeds was related to the grain boundary, precipitation, and substructure strengthening mechanisms. In addition, the fracture surfaces of the joints welded at higher heat input conditions showed more ductile mode in comparison with those welded at lower heat input condition, which confirmed the lower tensile elongation of the joints welded at higher traverse speeds.

Some Studies on Heat-Affected Zone (HAZ) Toughness Behavior of API 5L X52 Steel

Heat Affected Zone (HAZ) simulation by Gleeble is a useful method for evaluating welding process parameters to improve the toughness of weld HAZ by optimizing process parameters. The HAZ Toughness in low alloy steels is strongly influenced by the welding parameters (as HAZ microstructure is influenced by the heat input and cooling rate). In the present work different regions of weld HAZ of API 5L X52 steel have been simulated by using Gleeble @ 3800 Thermal- Mechanical Simulator. Two cooling rates (t 8/5 ) 10 s to simulate low heat input (LHI) welding conditions without preheating, (t 8/5 ) 40s to simulate high heat input (HHI) welding were used. Charpy V-notch impact tests were conducted on the simulated as well as on the base material at room temperature (25° C) and at -40° C. Results showed that HHI simulated coarse grain HAZ (CGHAZ) region attributed highest Impact energy of 300 J in comparison to 199 J of base material at 25° C. In HAZ regions, lowest room temperature toughness was observed in sub critical HAZ (SCHAZ) for both HHI and LHI conditions. Lowest impact energy at -40° C was observed in fine grained HAZ (FGHAZ) for both HHI and LHI conditions. Hardness test showed that LHI samples exhibited slightly higher hardness than HHI condition and the hardness profile for HAZ regions was similar for both HHI and LHI conditions.

Some Studies on Heat-Affected Zone (HAZ) Toughness Behavior of API 5L X52 Steel

Heat Affected Zone (HAZ) simulation by Gleeble is a useful method for evaluating welding process parameters to improve the toughness of weld HAZ by optimizing process parameters. The HAZ Toughness in low alloy steels is strongly influenced by the welding parameters (as HAZ microstructure is influenced by the heat input and cooling rate). In the present work different regions of weld HAZ of API 5L X52 steel have been simulated by using Gleeble @ 3800 Thermal- Mechanical Simulator. Two cooling rates (t 8/5 ) 10 s to simulate low heat input (LHI) welding conditions without preheating, (t 8/5 ) 40s to simulate high heat input (HHI) welding were used. Charpy V-notch impact tests were conducted on the simulated as well as on the base material at room temperature (25° C) and at -40° C. Results showed that HHI simulated coarse grain HAZ (CGHAZ) region attributed highest Impact energy of 300 J in comparison to 199 J of base material at 25° C. In HAZ regions, lowest room temperature toughness was observed in sub critical HAZ (SCHAZ) for both HHI and LHI conditions. Lowest impact energy at -40° C was observed in fine grained HAZ (FGHAZ) for both HHI and LHI conditions. Hardness test showed that LHI samples exhibited slightly higher hardness than HHI condition and the hardness profile for HAZ regions was similar for both HHI and LHI conditions.

Effects of Welding and rotational speeds on the Microstructure and Hardness of Friction Stir Welded Single-Phase Brass

This study focuses on the effects of rotational and welding speeds on the microstructure and hardness of joints in friction stir welded single-phase brass. Welds were achieved under low heat input conditions at rotational and welding speeds of 400–800 r/min and 100–300 mm/min, respectively. In order to characterize the obtained welds, optical microscopy and Vickers hardness measurements were taken on the weld cross sections. According to the obtained results, increasing the welding speed and/or decreasing the rotational speed caused the grain size of the stir zone to decrease and, hence, improved the average hardness of this region. These results are discussed with respect to the interplay between the welding parameters and the peak temperature in the weld thermal cycle.

Influence of rotational speed on the formation of friction stir processed zone in pure copper at low-heat input conditions

The aim of this work is to study the influence of rotational speed on the formation of friction stir processing (FSP) zone in commercial pure copper at low-heat input conditions. The experiments were conducted using K-type thermocouples to record the peak temperature history at different locations on the workpiece. The results suggest that the temperature achieved during processing plays an important role in determining the microstructure and properties of the processed metal. FSP produced very fine and homogenous grain structure and it is observed that smaller grain size structure is obtained at lower rotational speed whereas a tunnel defect was formed at lower speed of 250rpm. It is also observed that the hardness of the processed copper depends strongly on the heat input during FSP. Tensile tests were carried out and the tensile strength of the FSPed samples was compared to that of the base metal. For a successful FSP at low-heat input condition, the minimum rotational speed was found to be 350rpm.

On the Influence of Welding Stainless Steel on Microstructural Development and Mechanical Performance

In the present work, the influence of welding stainless steel on the microstructural development and associated mechanical performance were investigated. Gas tungsten arc welding (GTAW) process was used to weld 6 mm thick AISI 304L stainless steel using three levels of welding arc energy inputs. Different weld thermal cycles that resulted due to different combinations of heat input used caused significant microstructural changes in the weldments, which consequently affected the tensile and impact properties of these joints. Mechanical and material characterization of these joints have revealed that the weld metal under low welding heat input condition possessed relatively fine columnar dendritic microstructure which resulted in high microhardness and impact toughness of this zone. For every joint, the weld metal exhibited superior mechanical strength (transverse tensile strength, elongation, and Charpy impact energy) than the corresponding heat-affected zone. This work establishes that GTAW process when used for manufacturing operations involving AISI 304L stainless steel fabrications is capable of giving superior mechanical properties provided parametric combinations leading to low welding heat input are used.

Laser brazing of TiAl intermetallic compound using precious brazing filler metals

The applicability of laser brazing technique to bonding of TiAl intermetallic compound was investigated. Five kinds of filler metals such as gold, sliver, palladium and titanium alloys were employed for brazing. Diode laser brazing of TiAl intermetallic compound was carried out at laser power 300–450 W, travelling velocity 3.0–5.0 mm/s and wire feeding speed 20.0 mm/s with shield gas (Ar) at flow rate 15 L/min. According to the preliminary investigation of the filler metal selection, the gold-silver-copper filler metal (BAu-12) was selected as the suitable brazing filler metal for TiAl intermetallic compound. The filler metal did not completely penetrate and infiltrate the joint gap at lower heat input conditions, and the centreline cracking as well as serious erosion occurred in the braze metal at higher heat input conditions, while sound joints could be obtained by optimising processing parameters. The centreline cracking in the braze metal would be caused by the formation of brittle compounds attributed to the contamination (erosion) of the base metal into the filler metal. The theoretical approaches to the erosion and wetting/flowing phenomena during laser brazing process were made by the computer simulation. We customised the flow modelling software (FLOW-3D) to enable us to analyse the metal flow problem during laser brazing by coupled with the erosion behaviour. The simulations of the filler metal BAu-12 showed that it wetted/spread the base metals and infiltrated the joint gap with 0.5 mm when the laser power was increased. However, it did not completely infiltrate the joint gap when the brazing clearance was 0.3 mm. The amount of base metal erosion concurrently increased with an increase in the laser power at any brazing clearances. The computed wetting/flowing and erosion profiles in laser braze joints were fairly consistent with the experimental ones. The joint strength of TiAl intermetallic compound with the filler metal BAu-12 at laser power of 380 W attained to approx. 350 MPa being higher than 80 % of the base metal strength at any brazing clearances between 0.3 and 0.5 mm.

Effects of heat input on microstructure of tungsten inert gas welding used hastelloy X

The present work provides a microstructure examination of the weld zone (WZ) and heat affected zone (HAZ) associated with the low and high heat input tungsten inert gas (TIG) welding condition of used hastelloy X (HX). For both welding conditions, the segregation of the elements during solidification led to the formation of second phases (σ and P phases). In low heat input condition, the heat from the WZ caused the dissolution of carbide in the HAZ, whereas in high heat input condition, there was no enough time for carbide to dissolve but simply melt in the grain boundaries. Liquid phase in the HAZ during welding decreased the tensile properties of this alloy. Transmission electron microscopy (TEM) and scanning electron microscope (SEM) were used to evaluate the formation of phases and liquated product respectively. Additionally, the geometry analysis by using ANSYS software was introduced to ensure the different in stress concentration.

Study of friction stir processed zone under different tool pin profiles in pure copper

Friction Stir Processing (FSP) is a solid-state surface modification technique to alter the properties of metals and alloys. This work studies the effect of FSP on pure copper with six different tool pin profiles ( plain cylindrical, threaded cylindrical, triflute, triangle, square and hexagonal ) at low-heat input condition. The tool rotational speed and tool traverse speed were kept constant to maintain the low heat input. Temperature distribution, microstructure, microhardness and tensile strength were analyzed to evaluate the modifications occurred in the mechanical properties. From the investigation, it is observed that threaded cylindrical pin profile tool is more effective in bringing about a favourable mechanical modification in pure copper than other pin profiles under low-heat input condition.