Effects of Gravitational orientation on the microstructural evolution of gas tungsten arc welds in an Al-4 wt% Cu alloy

Gas tungsten arc (GTA) welds on Al-4 wt% Cu alloys were investigated to determine effects of gravitational orientation on the weld solidification behavior. A bead-on-plate welding was performed by varying the relation between the arc translation direction and gravity vector, e.g., parallel-up, parallel-down, and perpendicular orientations. A solidification rate (V _S) was calculated from the measured grain orientation, and a thermal gradient (G _L) was estimated from the observed weld pool shape following a linear relation. A primary dendrite spacing (₁) decreased continuously from the s-l boundary to the weld pool surface regardless of the gravitational orientations. Larger ₁ for the parallel-up weld was observed near the boundary and surface than that of the perpendicular and parallel-down welds, which is believed to be associated with a smaller G _L due to larger weld pool dimension and with different solidification morphology. A solidification morphology and orientation in the perpendicular and parallel-up welds was comparable with a loss of columnar directionality near the weld surface and a continuous grain orientation. However, the parallel-down weld exhibited more columnar structure near the surface, which might be associated with the larger G _L and relatively mild convection flows. Outward convection flows in the parallel-down weld might be inhibited because of its reverse direction with respect to the gravity vector. This resulted in abnormal S shape of the trailing s-l interface and the V _S, which was receded toward the weld pool center. Based on these findings, significant influence of gravitational orientation resulted in the variation on the weld pool shape associated with convection flows, which in turn affected solidification orientation/morphology and the primary dendrite spacing.     

Effect of heat input on the microstructure and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates

In this paper, the effects of heat input on the microstructures and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates were investigated by microstructural observations, microhardness tests and tensile tests. The results show that with an increase of the heat input, the grains both in the fusion zone and the heat-affected zone coarsen and the width of the heat-affected zone increased. Moreover, an increase of the heat input resulted in a decrease of the continuous β-Mg₁₇Al₁₂ phase and an increase of the granular β-Mg₁₇Al₁₂ phase in both the fusion zone and the heat-affected zone. The ultimate tensile strength of the welded joint increased with an increase of the heat input, while, too high a heat input resulted in a decrease of the ultimate tensile strength of the welded joint. In addition, the average microhardness of the heat-affected zone and fusion zone decreased sharply with an increase of the heat input and then decreased slowly at a relatively high heat input.     

Microstructural characterization of CO2 laser welds in the Al-Li based alloy 8090

The microstructural development of the Al-Li-Cu-Mg-Zr alloy 8090 has been studied after autogenous CO₂ laser welding. Sheets ranging in thickness from 1–4 mm were welded at speeds of between 20–120 mm s^–1 and powers from 1.5–3.8 kW. Optical microscopy, scanning and transmission electron microscopy were used to study the as-received base metal, the heat-affected zone and the solidified fusion zone. The base metal was supplied in a superplastically formable condition and thus had an unrecrystallized grain structure containing 1–2 m sized sub-grains with sub-micrometre and precipitates in the matrix. In the fusion zone, the as-solidified grain structure was columnar at the interface with the base metal but became equiaxed in the central region of the weld pool. The weld depth and top bead width both increased with decreasing welding speed and increasing beam power within the limits investigated. The fusion zone microstructure was cellular-dendritic. Intermetallic precipitates, which are rich in copper, magnesium, silicon (and presumably lithium), formed in the cell/dendrite boundaries. Very fine-scale precipitates were present in the as-solidified -Al matrix but there was no evidence for the , S and T₁ phases. The heat-affected zone was only 100 m wide and was characterized by regions of partial melting. Radiographs of welds reveal that porosity occurred predominantly along the weld centre-line. In partial penetration welds, two types of pores were observed: near spherical and irregular. However, in fully penetrating welds, only the spherical type of porosity was present. Overall volume fractions of porosity were measured from metallographic sections and were found to vary with welding speed and weld type, i.e. partial or full penetration.     

Effect of Ti on hot cracking and mechanical performance in the gas tungsten arc welds of copper thick plates

The new welding material – ERCuTi alloys filler metals were developed for gas tungsten arc welding (GTAW) of copper. The cracking susceptibility of the welds with ERCuTi and ERCu separately in GTAW of 10 mm copper thick plates was investigated. The formation causes of hot cracking was researched by using ERCu and the suppression mechanism of hot cracking when using ERCuTi alloy filler was proposed. It has been found that, when element Ti is added into the welding pool, the Ti will combine with O preferentially rather than Cu to generate TiO₂, which process can suppress the formation of Cu₂O. The hot cracking force and the hot ductility of the welds in brittle temperature range (BTR) could be improved effectively by adding Ti in filler metal compared with that of the welds without Ti. But the degree of addition of Ti (2–4 wt%) is critical when the susceptibility of cracking is to be suppressed. If the level is allowed to exceed 4 wt%, more low-melting point eutectics (β-TiCu₄ and TiCu₂) will be formed in the welds, and cracking susceptibility will be increased again. Results of mechanical properties tests show that although adding Ti increases the hardness and strength of the weld compared to the base metal, the impact ductility and the plastic properties are not decreased significantly.     

Metallurgical characterization of pulsed current gas tungsten arc,friction stir and laser beam welded AZ31B magnesium alloy joints

This paper reports the influences of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on mechanical and metallurgical properties of AZ31B magnesium alloy. Optical microscopy, scanning electron microscopy, transmission electron microscopy and X-Ray diffraction technique were used to evaluate the metallurgical characteristics of welded joints. LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints due to the formation of finer grains in weld region, higher fusion zone hardness, the absence of heat affected zone, presence of uniformly distributed finer precipitates in weld region.     

Fatigue performance of pulsed current gas tungsten arc,friction stir and laser beam welded AZ31B magnesium alloy joints

This paper reports the influence of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on fatigue properties of AZ31B magnesium alloy. Fatigue experiment was conducted using servo hydraulic controlled fatigue testing machine. Fatigue strength, fatigue notch factor and notch sensitivity factor were evaluated. The LBW joints showed higher fatigue strength compared to FSW and PCGTAW joints. The formation of very fine grains in weld region, higher fusion zone hardness, uniformly distributed finer precipitates are the main reasons for superior fatigue performance of LBW joints compared to PCGTAW and FSW joints.     

激光焊对Fe-Mn-Si系形状记忆合金焊缝区组织和性能的影响

研究了激光焊对Fe-Mn-Si系形状记忆合金焊缝区组织和性能的影响,结果表明,激光焊焊缝区组织显著细化,大大提高基体的强度,同时,由于激光焊焊缝过冷度大,容易产生较多的层错,这些层错有利于应力诱发ε马氏体形核,从而显著提高焊缝区的形状记忆效应,和母材相比,提高幅度达40%~178%左右;在模拟油田介质情况下,激光焊焊缝耐腐蚀性能高于母材,这是因为,激光焊焊缝组织细小均匀,在腐蚀过程中产生钝化现象,同时也降低了腐蚀电池的电位差,从而降低了合金的腐蚀速度,提高耐腐蚀性能。     

Rapid prototyping of 5356-aluminum alloy based on variable polarity gas tungsten arc welding: process control and microstructure

This paper concentrates on rapid prototyping of a 5356-aluminum alloy based on a new deposition process of variable polarity gas tungsten arc welding (VPGTAW), and describes the microstructure and geometrical properties of the deposited layers. The wettability and distortion tendency of the deposited layers is effectively improved by preheating the substrate up to 118°C, monitoring the arc-length, and adjusting the arc current during the deposition process. The relationships between the geometry of the deposited layers and the welding parameters are developed. The surface roughness of the deposited parts is found to be in the order of 2 μm. The deposited layers exhibit equiaxed dendrites at the top layer, fine equiaxed grains at the middle, and bottom of a deposited wall together with some precipitates distributed at the grain boundary regions, and coarse columnar grains at the bonding zone between the deposited wall and the substrate. The residual microstructure such as grain size and distribution of precipitates is highly dependent on the related locations in the deposited wall. The deposited samples possess a maximum hardness at the top layer and exhibit a slight decreasing trend towards the middle and bottom of the fabricated part due to the heat effects of the material that occurs during the deposition. By understanding these relationships between parameters and their effect on the process output, the process can be used more effectively and the quality be improved as well.     

Chemical segregation of solute elements in ultrasonically gas atomized aluminium alloy powders

A new experimental approach to the evaluation of chemical segregation of solute elements in ultrasonically gas atomized aluminium-alloy powders using X-ray spectral data of scanning electron microprobe analyser is described. The experimentally obtained chemical segregation data is compared with the conventional method of quantitative analysis and with theoretical predictions as determined from Scheil’s approach to the evaluation of elemental segregation during the solidification process. A comparison of experimental and theoretical predictions confirms the validity of the experimental approach in the estimation of solute segregation levels and also suggests that the solidification conditions considered for estimation of microchemical segregation can appropriately be applied to ultrasonically gas atomized powders.     

Mechanical assessment of autogenous gas tungsten arc weldments of a super alpha-2 alloy

Butt welding of a 2-mm-thick super alpha-2 alloy (Ti-23Al-9Nb-2Mo-0.9Si) was carried out employing autogenous gas tungsten arc technique. Process parameters were adjusted to achieve full-penetration weld. Only β-Ti-based phase was detected in the fusion zone; however, minor quantities of a Ti₃Al-based structure were found at heat input higher than 518 J mm⁻¹. No cracking was observed within the fusion zone and its associated heat-affected zones. Subgrain boundaries and acicular transformation products were observed within the HAZ grains. The acicular transformation products are believed to be responsible for the higher microhardness value of the HAZ compared to those of the fusion zone and base material. It was found that the fusion zone and HAZ had lower strength and ductility than the base material. Fracture occurred within the HAZ, which is attributed to its higher microhardness value and acicular transformed microstructure. The maximum weld tensile strength achieved was about 90% of that of the base material.     

Metastable liquid phase separation in tungsten inert gas and electron beam copper/stainless-steel welds

The microstructure of Tungsten Inert Gas (TIG) and electron beam copper/stainless-steel welds were investigated using scanning electron microscopy. The relatively high cooling rates entailed in the welding result in high bulk supercooling, causing two microstructural effects: (i) melt separation into two liquids, iron-rich L1, and copper-poor L2, while each solidifies by a path dictated by the stable phase boundary; (ii) enhanced solute trapping of Cu in the -Fe phase, and Fe in the -Cu phase.     

Microstructural characterization and grain refinement of AA6082 gas tungsten arc welds by scandium modified fillers

The refinement in weld metal grain size and shape results in both improved mechanical properties (ductility and toughness) as well as a significant improvement in weldability. In the present study, the influence of scandium (Sc) additions to the fillers on the structure and mechanical properties of AA6082 gas tungsten arc (GTA) weldments were investigated. Controlled amounts of scandium as grain refiner were introduced into the molten pool of AA6082 by pre-deposited cast inserts (AA4043 and AA5356) by GTA welding. Full penetration GTA welds were prepared using alternating current (AC). It was observed that grain size decreased with increasing amounts of scandium. The grain refinement is mainly caused by the Al₃Sc particles, which act as heterogeneous nucleation of α-Al grains. It has been shown that welds prepared with AA5356 cast insert exhibited high strength and ductility when compared with other welds. The observed grain refinement was shown to result in an appreciable increase in fusion zone hardness, strength and ductility. Post-weld aging treatment resulted in improved tensile strength and hardness of the weldments and this aging response could be attributed to the weld dilution from the base metal. The slow diffusion of Sc in Al matrix and stability of Al₃Sc precipitates at elevated temperatures were suggested to be responsible for the improved high temperature yield strength of welds made from Sc modified fillers.     

Effect of heat input on the microstructure and mechanical properties of gas tungsten arc welded AISI 304 stainless steel joints

Influence of heat input on the microstructure and mechanical properties of gas tungsten arc welded 304 stainless steel (SS) joints was studied. Three heat input combinations designated as low heat (2.563 kJ/mm), medium heat (2.784 kJ/mm) and high heat (3.017 kJ/mm) were selected from the operating window of the gas tungsten arc welding process (GTAW) and weld joints made using these combinations were subjected to microstructural evaluations and tensile testing so as to analyze the effect of thermal arc energy on the microstructure and mechanical properties of these joints. The results of this investigation indicate that the joints made using low heat input exhibited higher ultimate tensile strength (UTS) than those welded with medium and high heat input. Significant grain coarsening was observed in the heat affected zone (HAZ) of all the joints and it was found that the extent of grain coarsening in the heat affected zone increased with increase in the heat input. For the joints investigated in this study it was also found that average dendrite length and inter-dendritic spacing in the weld zone increases with increase in the heat input which is the main reason for the observable changes in the tensile properties of the weld joints welded with different arc energy inputs.     

Microstructures and mechanical properties of the fusion zone of 316L-316LN stainless steel multi-pass gas tungsten arc welded joint

Journal of Materials Science - In this study, the microstructure and hardness of the fusion zone of 316L-316LN stainless steel multi-pass gas tungsten arc welding (GTAW) weld joint were...     

Analysis of the microstructure obtained by using unidirectional solidification,tungsten inert gas weld and laser surface melt traversing techniques in Al-Mn alloys

A major challenge to solidification theory over nearly three decades has been the understanding, prediction and control of rapidly solidified microstructures. The present paper reports results of systematic and controlled conditions of rapid solidification in Al-Mn alloys, which involved measurement of undercooling, solute concentration and cell spacing for solidification front velocities, which were increased progressively, to the level needed for partitionless solidification into a microsegregation-free solid which, in principle, can be crystalline, quasicrystalline or amorphous. Comparison of the measurements with predictions of theoretical modelling give an encouraging level of agreement.Nomenclature A constant = ²/P²D² - A constant = k(ab)^1/2 - B constant = mC₀p_c/D[1–pI_v(P)] - B constant - C G(Km^–1) - C _EU eutectic composition (at %, wt %) - C ₀ alloy concentration (at %, wt %) - C _L ^* tip concentration in liquid (at %, wt.%) - C _S ^* tip concentration in solid (at %, wt %) - D diffusion coefficient in liquid (m²s^–1) - G température gradient (Km^–1) - I _V(P) Ivantsov function (P exp(P)E₁(P)) - P solute Péclet number = V_SR/2D - R tip radius (m) - T _EU eutectic temperature (K) - T _F melting point of pure substance (K) - T _G arrest growth temperature (K) - T _L liquidus temperature (K) - V _ab absolute stability velocity (ms^–1) - V _s solidification front velocity (ms^–1) - a material constant - b material constant - k distribution coefficient (C_S/C_L) - k constant - m liquidus slope (K/at %, K/wt %) - n exponent - p complementary distribution coefficient (1–k) - Gibbs-Thomson coefficient (/s_f) (Km) - s _f entropy of fusion per mole (J mol^–1K^–1) - T ₀ liquidus-solidus range at C₀(T_S–T_L) (K) - ₁ cell spacing (m) - solid/liquid interface energy - 3.1416 - _c constant = 1–(2k/[1+(2/P)²]^1/2–1+2k)     

Study on the temperature measurement of AZ31B magnesium alloy in gas tungsten arc welding

This paper is based on the Finite Element Analysis (FEA) to study the AZ31B Magnesium Alloy welding temperature filed, using a convenient, non-contact and fast response measured temperature method—Infrared Radiation (IR), the welding temperature field of AZ31B magnesium alloy plate in Gas Tungsten Arc Welding (GTAW) is measured by IR, the isothermal map of magnesium alloy plate is measured using IR device. The cooling curves are measured by thermocouple. Experiments and simulations by FEA are carried out to investigate the welding temperature field. The simulated results showed good agreement with the experiment ones.