Heat Affected Zone Interface Research Articles

Inhomogeneity of microstructure in friction stir welded TC17 alloy joint and its effects on mechanical behavior

Friction stir butt welding was carried out to assemble 6 mm-thick TC17 titanium alloy plates with a W–Re weld tool. Inhomogeneous microstructures were obtained in stir zone (SZ) and heat affected zone (HAZ) of the weldment. A series of tests were conducted to reveal the influence of inhomogeneity of microstructure on mechanical behavior. Comparative refined β phase without precipitating secondary metastable microstructure generated a soft SZ and favored on the ductility of weldment. Strain localization during tension was promoted by the interface of HAZ and SZ that formed by thermal-mechanical coupling effects. Coincidentally, fatal fatigue cracks were verified to nucleate around the same region, and presented in a stress dependent mode.

Damage Model Determination for Predicting Creep Rupture Time of 2 1/4Cr–1Mo Steel Weld Joints

In a creep damage analysis for a weld joint (WJ) using a damage model, it is important to select a damage model that properly reflects complex conditions of stress generated in the interface between the base metal and the heat-affected zone (HAZ). For 2 1/4Cr–1Mo steel, we examined what type of scalar stress is appropriate to properly reflects the conditions of stress generated around the HAZ for accurately predicting creep rupture time, in the framework of the time exhaustion rule, which evaluates damage as a function of stress. For a specimen of a round rod WJ through which an HAZ penetrates obliquely, we compared actual tests and finite element method (FEM) analysis, in temperature-stress ranges from 120 to 160 MPa at 823 K, and from 80 to 100 MPa at 873 K. Using FEM calculations, we evaluated the rupture time based on the time exhaustion rule, considering three types of scalar stress: the maximum principal stress, the equivalent stress, and the Huddleston stress. Comparing with the tests, we found that the scalar stress that gives an appropriate rupture time is the Huddleston stress. In this stress, the damage accumulation due to the creep deformation in the base metal (BM)-HAZ interface and the cavity formation due to the hydrostatic pressure are taken into account. Analyses of the stress distribution inside and outside of the HAZ and in the BM-HAZ interface indicated that, for the 2 1/4Cr–1Mo steel, the cavity formation due to the tensile hydrostatic pressure is also important when we evaluate damage. We optimized a parameter b, which regulates the balance in the Huddleston stress, so that the observed results are reproduced with the determination coefficient of R2 = 0.948, and we obtained the value of b as b = 0.34.

Role of heat-treatment and filler on structure-property relationship of dissimilar welded joint of P22 and F69 steel

This paper investigates the microstructural features and mechanical behavior of the dissimilar welded joint (DWJ) of lean duplex A 182 F 69 steel and 2.25Cr-1Mo (P22) steel. The multi-pass DWJ has been prepared using the Gas Tungsten Arc Welding (GTAW) process employing the Inconel 617 filler material. The DWJ has been subjected to post-weld heat treatment (PWHT) in a temperature range of 760 °C (PW 1)-810 °C (PW 2). The tensile testing and metallographic testing sample have been cross-sectioned from the DWJ for different heat treatment conditions of the welded joint. Each zone of the DWJ was characterized using the optical and scanning electron microscope (SEM). Cellular and columnar dendrites have been observed near the interface of weld fusion zone (WFZ) and heat-affected zone (HAZ) while equiaxed structure in center of the WFZ. The WFZ showed the austenitic structure with dendritic core and inter-dendritic boundaries. The inter-dendritic boundaries showed the presence of secondary phase particles, mainly enriched with Cr, Mo and Ti. Unmixed zone and compositional variation across the WFZ and HAZ interface have been observed on both the sides of DWJ. For as-welded condition, tensile fracture surface revealed the mixed mode of the fracture with tensile strength higher than that of the base P22 and F69 steel. The fracture location was in the center of the WFZ. For PW 1 condition, the crack initiation zone showed the mixed-mode while the final fracture showed the brittle fracture with cleavage facets. In PW 1, the fracture has been observed in HAZ of P22 side with strength lower than that of the base metal of F69. In PW 2 condition, complete brittle mode of the fracture has been observed and fracture surface showed the cleavage facets with river marking.

Investigations on multi-run metal made of HSLA steel – Heterogeneous microstructure and mechanical properties

Multi-run high strength low alloy (HSLA) steel plates welded by manual metal arc (MMA) process were investigated. Heterogeneous weld metal microstructure was studied accompanied with its effect on the mechanical properties of the joint. Microstructural studies revealed: (1) in the last run, there were intergranular acicular ferrite and pro-eutectoid ferrite, (2) in previous weld runs, there were refined grains with precipitated particles and (3) in the interlayer region of previous runs, there were some heterogeneous ferrite grains, grain boundary discontinuities and precipitated microalloying elements. In the last run, in fusion zone weld and heat affected zone interface, the solidification is a phenomenon of an epitaxial growth type structure. However, in previous weld runs, there was neither epitaxial growth nor typical refined grain structure. The mechanical properties varied in different parts of the welded plate due to the metallurgical characteristics of weld metal in comparison with the production procedure of HSLA steels and their metallurgical parameters.

Repair Welding of Defects in 2219 Al Alloy Friction Stir Welded Joints

The primary and secondary friction stir repair welding experiments had been untaken for the lack of penetration and cavity of 2219 aluminum alloy joints welded by friction stir welding. The results show that the original defects can be removed with proper welding process parameters and thus we can achieve joints with smooth appearance and good properties. But the area of joint softened zone is increased dramatically by the increase of repair welding times. The tensile properties of primary repaired joints are satisfactory and better than that of secondary repaired for both lack of penetration and cavity defects. In the original joints, fractures all are started in the defects which led to a low ductility. However, after repair, joints fractured show a ductile fracture character at the interface of heat-affected zone and thermal-mechanical affected zone in retreating side.

Effects of weld cross-section profiles and microstructure on properties of pulsed Nd:YAG laser welding of Ti6Al4V sheet

In this study, the influence of weld cross-sectional profiles and microstructure, under different welding conditions, was investigated on the mechanical properties of pulsed Nd:YAG laser-welded joint of Ti6A14V alloy. The V- and H-shaped weld cross-section profiles were obtained under low and high heat input, respectively. The microstructure in the fusion zone (FZ) of V- and H-shaped welded joint consisted of a fine acicular martensitic α′ solidification structure within the prior-β grains, responsible for the maximum hardness in the FZ. Tensile tests revealed that the V-shaped welded joint fractured at the near heat-affected zone (HAZ)/base metal (BM) interface, but the H-shaped welded joint fractured in the BM. Furthermore, the tensile strength of the V-shaped welded joint was lower than that of the H-shaped welded joint. This was attributed to the fact that the heterogeneity of strain distribution at the interface of HAZ and BM of the V-shaped welded joint was greater than that of the H-shaped welded joint during tensile process.

Fatigue behavior of friction stir spot welded AZ31 Mg alloy sheet joints

The fatigue behavior of friction stir spot welded (FSSW) AZ31 magnesium alloy sheet joints was investigated by tension–compression of fatigue test. The results suggest that all the fatigue failures occur at the stir zone of the FSSW AZ31 sheet joints, and all cracks initiate at the stir zone outer edge between the upper and lower sheet. When the cycle force equals 1 kN, the crack propagates along the interface of heat-affected zone and thermo-mechanical zone, simultaneously across the direction of force; while the cycle force equals 3 kN, the crack propagates along the diameter of stir zone and shear failure occurs finally. Moreover, the transverse microsections indicate that there is a tongue-like region at the outer edge of stir zone between the two AZ31 sheets, and the direction of tongue-like region is toward outside of the stirred zone and all fatigue cracks initiate at the tongue-like region.

Evolution of microstructures and mechanical properties of AZ31B magnesium alloy weldment with active oxide fluxes and GTAW process

In this study, the effect of active oxide fluxes with gas tungsten arc welding on the microstructure and mechanical properties of AZ31B magnesium alloy weldment was investigated. The gas tungsten arc welding process through a flux spray layer was applied to an AZ31B magnesium alloy sheet to produce a bead-on-plate specimen. Oxide (TiO2, SiO2, Fe2O3, Al2O3, and ZrO2) powders were used as the activating fluxes. The macrographs and micrographs of the weld beads were examined using an optical microscope and a scanning electron microscope. The specimens with SiO2 and Fe2O3 fluxes had high depth-to-width ratio welds, followed by those with TiO2 and ZrO2 fluxes and while that with Al2O3 flux had the low ratio weld. The use of 70 A welding current for the specimens with different fluxes produced complete penetration, whereas the specimen without any flux required a 90 A welding current to produce complete penetration. The weld bead microstructure was affected by the activating fluxes, which created different thermal effects that changed the convection direction and promoted the formation of various precipitates in the fusion zone during solidification. Three types of precipitates were found in the fusion zones, that is, a long layer-shaped TiAlMg precipitate with TiO2 flux, a spherical AlMgZn precipitate with Al2O3 flux, and an oval-shaped MgAlMn precipitate with all types of fluxes. The mechanical properties of AZ31B magnesium alloy were measured by tensile testing in the rolling direction. Fractures occurred in the fusion zone near the heat-affected zone interface of specimens welded with TiO2 flux, revealing a brittle fracture with trans-granular cleavage facets and a large number of small, bright dimples at the center. Such brittle fractures also occurred in the fusion zone of specimens welded with Al2O3, ZrO2, SiO2, and Fe2O3 fluxes. Similarly, the specimens welded with Al2O3 exhibited a brittle fracture with trans-granular facets, whereas the other specimens revealed a brittle fracture with inter-granular cleavage facets.

Effect of pre-strain on the strength of spot-welds

This investigation primarily aims to reveal the effect of pre-strain of base metals on the strength of spot-welds. Strength of spot-welds have been determined using tensile-shear specimens made on as-received and uniaxially pre-strained sheets of extra interstitial free (EIF) and high strength interstitial free (HIF) commercial steels used for automobile applications. These experiments are supplemented by characterization of microstructure, substructure, hardness profiles along the welds and post-failure fractographic examinations. The major results infer that: (i) the strength of spot-welds increases with increasing pre-strain, (ii) the location of failure is commonly at the interface of heat-affected zone and base metal except for the spot-welds on as-received HIF steel, where failure occurs at the base metal, and (iii) the remnant dislocation density around the spot-weld increases with increasing pre-strain. The increase in strength of the spot-welds of pre-strained specimens has been attributed to the amount of remnant dislocation density around the weld zone. The location of failure has been discussed in terms of microstructure, substructure characterized by TEM and hardness profile along the weldment.