Search

Metal fabrications experience complex physical metallurgical processes during additive manufacturing, leading to residual stress and coarse microstructure with directional growth. It significantly affects the comprehensive performance of the fabrications, which limits the application of additive manufacturing. Ultrasonic impact treatment (UIT), as a strengthening means to assist additive manufacturing, can effectively improve the stress state and refine the microstructure and the comprehensive performance. This paper introduces the effect of UIT on AM metal fabrications on microstructure morphology, stress distribution, surface roughness, internal defects, and comprehensive performance to gain a deeper understanding of the role of UIT on additively manufactured metal fabrications, which is based on the working principle and effect of process parameters. In addition, the strengthening mechanism of UIT in additive manufacturing is described from the perspective of surface plastic deformation and substructure formation, providing support for the shape and property control of metal fabrications in the process of additive manufacturing assisted by UIT. Finally, the issues that need to be studied in depth on UIT in additive manufacturing are summarized, and an outlook on future research directions is taken. [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]

A self-developed Ti-Al-V-Mo system titanium alloy flux-cored wire was used to join the Ti64 titanium alloy plate via laser welding with filler wire. The microstructure and properties of the obtained welded joint were investigated. The results showed the WM (welded metal) of the welded joint consisted mainly of acicular α' martensite, and the HAZ (heat affected zone) was comprised of a primary αp phase, Widmanstatten, and a few α' martensite and a residual β phase. The strength and elongation of the welded joint after breaking are equivalent to that of the BM (base metal). The tensile fracture, presenting as a microvoid coalescence ductile fracture, was encompassed by massive shear lips with deep and uniform dimples. The overall microhardness of the welded joint was sequenced as WM > HAZ > BM. In the WM, large-angle grain boundaries with intragranular misorientation greater than 15° accounted for about 84%. By XRD, it was discovered the welded joint was mainly composed of the α' martensite, with a modest amount of extremely weak multi-angle α phase diffraction peak. The test results showed the designed welding method of titanium flux-cored wire and laser wire filling is suitable for high-quality welding of titanium alloy plate. [ABSTRACT FROM AUTHOR]

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]

The behavior and emission spectrum of hybridwelding arc plasmas are investigated in low-power YAG laser--metal active gas (MAG) arc hybrid-welding process, comparing with simple MAG welding case. The electron temperature and electron density of local arc plasma, deducing, respectively, from the Boltzmann plot method and Stark broadening mechanism, are analyzed in this paper. The conditions of local thermodynamic equilibrium (LTE) of arc plasma and self-absorption of emission spectral lines are discussed. The results indicate that laser-induced attraction and constriction of hybrid arc root are generated in low-power laser hybrid-welding process, and they make the synergy effect between laser and MAG arc increase. Comparing with simple MAG welding case, a zone above laser-heated spot with higher electron temperature and greater electron density is seen in hybrid-welding process. The maximal values of electron temperature and electron density of hybrid arc in this zone are about 1.56 [+ or -] 0.09 x [10.sup.4] K and 1.61 [+ or -] 0.16 x [10.sup.17] [cm.sup.-3], respectively. The welding arc plasmas are in the state of LTE, and none of the lines selected from emission spectrum of arc plasma is affected by self-absorption. Index Terms--Arc attraction and constriction, electron density, electron temperature, emission spectrum, local thermodynamic equilibrium (LTE), YAG laser-metal active gas (MAG) hybrid welding.

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]

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]

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]

Wire and arc additive manufacturing (WAAM) is a novel technique for fabricating large and complex components applied in the manufacturing industry. In this study, a low-carbon high-strength steel component deposited by WAAM for use in ship building was obtained. Its microstructure and mechanical properties as well as fracture mechanisms were investigated. The results showed that the microstructure consisted of an equiaxed zone, columnar zone, and inter-layer zone, while the phases formed in different parts of the deposited component were different due to various thermal cycles and cooling rates. The microhardness of the bottom and top varied from 290 HV to 260 HV, caused by temperature gradients and an inhomogeneous microstructure. Additionally, the tensile properties in transversal and longitudinal orientations show anisotropy characteristics, which was further investigated using a digital image correlation (DIC) method. This experimental fact indicated that the longitudinal tensile property has an inferior performance and tends to cause stress concentrations in the inter-layer areas due to the inclusion of more inter-layer zones. Furthermore, electron backscattered diffraction (EBSD) was applied to analyze the difference in Taylor factor between the inter-layer area and deposited area. The standard deviation of the Taylor factor in the inter-layer area was determined to be 0.907, which was larger than that in the deposited area (0.865), indicating nonuniform deformation and local stress concentration occurred in inter-layer area. Finally, as observed from the fracture morphology on the fractured surface of the sample, anisotropy was also approved by the comparison of the transversal and longitudinal tensile specimens. [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]