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The effect of cooling rate on phase transformation, microstructures, and mechanical properties of TC4 titanium alloy deposited metal by laser surfacing welding with filler wire was investigated by in situ observation with a high-temperature laser confocal microscope, XRD, SEM, and through microhardness test. The results showed that primary α phase began to appear at grain boundaries when the deposited metal was heated to 890.5°C, and microstructures were all composed of coarse β columnar grains after the temperature was raised to 1 190.2°C; with the increase of cooling rate during the cooling process, temperature for starting new phase transformation decreased from 943.7°C to 811.1°C, and temperature for ending the transformation increased gradually from 708.0°C to 736.2°C; when cooled to room temperature at a rate of 1°C/s, microstructures consisted of α phase; at a rate of 15°C/s, acicular α′ martensite appeared in the microstructures; at 100°C/s, the microstructures were completely comprised by acicular α′ martensite; when the cooling rate was low, V element would be slightly precipitated from αgb into the matrix; when cooling turned faster, V element had no time to enter the matrix by diffusion, so it became uniformly distributed; as the cooling rate getting faster, lamellar microstructures became thinner gradually, and microhardness of the deposited metal was improved in a gradual manner. [ABSTRACT FROM AUTHOR]

Laser-MAG (metal active gas) hybrid welding of nickel-saving 08Cr19Mn6Ni3Cu2N stainless steel was carried out by using 98%Ar + 2%N2 and 95%Ar + 5%CO2 as shielding gases. The effect of different shielding gases on the microstructure and properties of the welded joints was investigated. The results showed that arc shrinkage was significant with the addition of nitrogen, weld spatter increased with the expansion of arc volume, and arc stability deteriorated. The ferrite content in the weld decreased by about 60%, the ferrite dendrite also gradually became finer, and the secondary dendrite arm was shorter. Only a small amount of δ and γ phases existed in the weld, and no precipitation of the σ phase and nitride was found. Observing four crystal planes, we found that size of the austenite grains decreased with the addition of nitrogen. The average tensile strength of the welded joints decreased from 712 MPa to 704 MPa, but with improved corrosion resistance, the pitting corrosion rate increased from 19.45 g·m2/h to 18.72 g·m2/h, and the hardness of weld was slightly reduced. [ABSTRACT FROM AUTHOR]

In this paper, multi-stranded wires were used as the filling material for narrow gap oscillation laser wire filling welding of thick-walled titanium alloy. The effect of laser beam oscillation frequency and diameter on weld formation and welding defects was investigated. By using a high-speed camera, the melting process of the laser-melted multi-stranded wires was observed. The macrostructure and microstructure of the welded joint were analyzed using the optical microscope (OM), scanning electron microscope (SEM), electron back scattering diffraction (EBSD), and transmission electron microscope (TEM), and the mechanical property tests were also carried out. The results showed that the droplet was in the liquid bridge transition mode, while the oscillation diameter of the laser beam was 2 mm, and the frequency was 100 Hz. The obtained weld seam was continuous and stable, without pores and incomplete sidewall fusion. The base metal (BM) of the welded joint was composed of equiaxed α + β phase, the heat-affected zone (HAZ) was comprised of acicular α′ martensite and primary αp phase, and the welded metal (WM) consisted of coarse original β columnar grains and internal basketweave acicular α′ martensite. The microhardness of the WM was the highest. The average tensile strength of the welded joint reached 940 MPa. The tensile fracture occurred at BM. The fracture dimple was relatively deep, presenting the characteristics of ductile fracture. The average impact toughness of the welded joint was 12 J. The fracture dimple was shallow, appearing as a ductile fracture. [ABSTRACT FROM AUTHOR]

In the welding process of thick-walled titanium alloys, the selection of the wire type is one of the critical factors affecting the welding quality. In this paper, flux-cored and cable wires were used as filler materials in the welding of thick-walled titanium alloys. The macrostructure, microstructure, texture, and grain size of both welded joints were compared by employing an optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM), and the tensile and impact properties were also evaluated. The comparison result showed that the fusion zone microstructure of both welded joints was dominated by a basketweave structure composed of interwoven acicular α′ martensite, whereas the microstructure of flux-cored wire welded joints was finer, and the degree of anisotropy was low. The strength of both welded joints was higher than that of the base metal, ensuring that fracture occurred in the base metal area during tension. The Charpy impact energy of the flux-cored wire welded joint was 16.7% higher than that of the cable wire welded joint, indicating that the welded joint obtained with the flux-cored wire performed better in the welding process of thick-walled titanium alloys. [ABSTRACT FROM AUTHOR]

The welding solidification crack is a common welding defect occurring in the narrow gap laser welded high-strength steel joint, which poses a detrimental effect in reducing the weld quality. However, the relationship between the weld geometric characteristics and the metallurgical characteristics with the solidification crack of the narrow gap laser weld is still unclear. In this study, a 20-mm-thick 10Ni5CrMoV high-strength steel with a root gap of 3 mm and groove angle of 1º was used to analyze the single-pass welding process of narrow gap laser welding using high-frequency beam oscillation. The correlation between the weld form factor and the welding solidification crack was discussed, alongside with the clarification on the influence of the welding process parameter on the welding solidification crack, the aim of which was to analyze the causes for the formation of welding solidification crack and explore the effective ways to suppress it. The results showed that the influence of welding process parameters on welding solidification crack could be attributed to change in the form factor of the weld and weld metal solidification crystallization characteristics. The smaller the weld form factor, the larger the area of the opposite crystal growth in the upper part of the weld cross section, and the higher the tendency toward solidification crack formation. Furthermore, the higher welding speed was also likely to have columnar grains with a larger angle of crystallization direction with the centerline. This resulted in a higher tendency of solidification crack formation. Under the test conditions used in this study, a weld form factor φ = 0.8 was considered effective for controlling welding solidification crack. [ABSTRACT FROM AUTHOR]

A 20 mm thick TC4 titanium alloy plate was welded by ultranarrow gap laser welding with filler wire with Ar and He as shielding gas, respectively. A characterization analysis of the microstructures and mechanical properties of the welded joint was conducted with OM, SEM, XRD, and EBSD and through the microhardness test and tensile test. The results showed that HAZ of the welded joint formed with Ar as shielding gas was much wider than that with He, and weld microstructure composition with the two shielding gases was basically consistent; phase boundary of the weld metal obtained with Ar was clearer, with a larger misorientation between the laths; α′ martensite lath in weld metal prepared with He showed obvious preferred orientation distribution, and α′ martensite microstructure was much finer; the misorientation of α′ phase grain boundary of weld microstructure prepared with Ar was slightly less distributed in high angle grain boundary than that with He; tensile property of the welded joint prepared with He was better than that with Ar; the hardness of each zone of welded joint prepared with He was less fluctuated and the hardness value measured was slightly higher than that with Ar. [ABSTRACT FROM AUTHOR]

Wire and arc additive manufacturing (WAAM) is an efficient technique for fabricating large and complex components that are applied in the manufacturing industry. In this study, anisotropic mechanical properties of a low-carbon high-strength steel component fabricated by WAAM were investigated via mechanical testing, and the transversal and longitudinal deformation behavior of the component were studied using the digital image correlation (DIC) method. Additionally, the features of microstructure, texture, and fracture mode of the inter-layer area and deposited area were also investigated to reveal the mechanism of anisotropy. The results showed the mechanical properties of longitudinal specimens were inferior to that of the transversal specimens. Several strain concentration zones in the longitudinal specimen were relevant to the inter-layer characteristics observed from the fracture surface and macrostructure, which was confirmed by the strain evolution recorded by DIC. The inter-layer areas were proved to be the weak link in the deposited component by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) analysis results, including various phase composition, phase morphology, misorientation angle, grain size, Schmid factor, and texture. Finally, based on the fractography analysis, anisotropy resulted from inter-layer zones is also confirmed via the comparison of transversal and longitudinal fracture morphology. [ABSTRACT FROM AUTHOR]

The stress corrosion resistance of 30 mm-thick narrow-gap rotating laser welding (NG-RLW) 5A06 aluminum (Al) joint was investigated by slow strain rate testing (SSRT). The results show that the average ultimate tensile strength (UTS) of the joints in 3.5% NaCl solution is about 11% lower than that in air, and its elongation is only half of that in air. The stress corrosion index I SSRT of the joint is ∼ 0. 4 9 8 , which is at the middle level of the range of I SSRT . Moreover, the failure positions of the SSRT specimens in air are weld metal (WM), while the failure positions in the corrosive medium are base metal (BM). This indicates that the stress corrosion resistance of BM is slightly lower than that of WM due to the different microstructures. Many dimples are formed on the fracture surface of the specimen in air, which is a typical ductile fracture. But the fracture surface shows the characteristics of ductile and brittle mixture with obvious corrosion products. This result indicates that the NG-RLW 5A06 Al joint has a tendency to brittle fracture in 3.5% NaCl solution. [ABSTRACT FROM AUTHOR]

• The oscillating laser with a cycloid scan path could restrain the formation of pore defects in aluminum alloy laser filler welding. • The increase in oscillating amplitude and frequency could inhibit the formation of pore defects. • The stirring effect on the molten pool of the oscillating beam made the droplet enter the pool instead of making a spatter. • The oscillating laser avoided the plasma in the groove to a certain extent and increased the energy on the base metal. • The defect-free multilayer welding seam of 18 mm thick was realized by oscillating laser with a cycloid path. In order to reduce porosity and improve welding quality during laser welding with narrow gap of 5A06 aluminum alloy, a laser oscillation method was applied in this study. By comparing the welds with different scan paths, it was found that the laser oscillation with cycloid path could restrain the formation of pore defects, improve the surface appearance, and enhance the stability of the molten pool. EBSD results under different scanning paths described the effect of laser oscillation on the microstructure of the weld. The influence of oscillation amplitude and frequency on welding quality was studied as well. The pore defect could be eliminated when the amplitude was greater than 2 mm and the frequency was greater than 150 Hz. Dynamic information of the molten pool and plasma was obtained through a high-speed camera system. It was found that the oscillating laser could partly avoid the influence of the plasma on laser energy attenuation. Finally, a multi-layer welding experiment was carried out using the optimized oscillated laser parameters. [ABSTRACT FROM AUTHOR]

The problem of anisotropy in the microstructure and mechanical property of the fabrications limits the application and development of wire and arc additive manufacturing (WAAM). By introducing inter-layer ultrasonic impact (UI) strengthening in the low carbon steel WAAM process, the anisotropy can be effectively improved. After being treated by UI, the typical columnar microstructure with obvious direction is transformed into a uniform and fine equiaxed microstructure. Digital image correlation results show that the stress concentration in the tensile process is significantly improved. The anisotropy of tensile strength and yield strength is reduced from 4.2% to 1.6% and 10.1%–2.3%, respectively. Scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) are applied to investigate this phenomenon. The results indicate that UI can break the constraint for dislocations to move, promoting dislocation merging and annihilating. And then, numerous substructures are formed, partially recrystallized by the subsequently deposited layers' thermal effect. The transformation can block the columnar microstructure growth and divide the columnar microstructure into the cellular or equiaxed microstructure. In addition, the results show that inter-layer ultrasonic impact strengthening can result in local misorientation reduction, grain refinement (average grain size reduction from 2.18 μm to 1.65 μm), and texture orientation strength weakening. [ABSTRACT FROM AUTHOR]