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• The influence of beam oscillating amplitudes on depositions was studied detailly. • O-LHAM with wide oscillating amplitudes was applied to fabricate Al-Mg-Sc alloy. • The thin wall with sound surface quality was achieved at the amplitude of 4 mm. • Full equiaxed grains were obtained when the amplitude beyond 2 mm. • The optimal mechanical performance was achieved at the amplitude of 4 mm. The oscillating laser-arc hybrid additive manufacturing was applied to fabricate Al-Mg-Sc alloy and the effect of beam oscillating amplitude on macro/micro-morphology and the mechanical properties were investigated in this study. With the oscillating amplitude increased from 0 to 6 mm, the stirring effect of laser beam on molten pool was firstly enhanced due to the expanded active region and then weakened because the laser spot located at the edge of pool. As a result, the improved forming quality, the reduced porosity and the refined microstructures were achieved when the amplitude reached 4 mm, and then negative effect occurred when the amplitude was 6 mm. Eventually, the optimal mechanical performance was achieved at the amplitude of 4 mm and the as-deposited tensile properties reached 353 MPa and 24 % for UTS, and EL in vertical direction, respectively, which were 13.5 % and 50 % higher than that of 0 mm. [ABSTRACT FROM AUTHOR]

Thick-section steel has been widely used in many heavy industries. Traditionally, very thick steel plates could be welded by using submerged arc welding and other welding processes. However, there were more or less drawbacks in these welding methods. Laser welding, a high-energy density welding method, is being considered for such structures to improve the production efficiency and reduce the residual stresses of the joints. In this study, butt joints with narrow gap were welded using a high-power CO laser. The effect of welding parameters including the relative position between the laser beam and the filler wire, welding speed, and the distance from the intersection of the beam and wire to root of the groove on the weld bead geometry and welding defects was studied. Additionally, high-speed photography was introduced in the experiment as an efficient method to record the total process of welding, especially the transfer of molten drop. The study found that when the beam was focused on the center of the groove, the filler wire could be melted successfully even though it would tremble slightly during welding process. The optimized distance from the intersection of the beam and wire to groove root was 3 mm. Later, butt weld joints of 70-mm-thick steel plate without lack of fusion can be obtained under optimized welding parameters. [ABSTRACT FROM AUTHOR]

This study aimed to explore the electrochemical properties and microstructure of friction stir welds to understand the correlation between their properties and processing. Friction stir welding is a promising solid-state joining process for high-strength aluminum alloys (AA). Although friction stir welding (FSW) eliminates the problems of fusion welding due to the fact that it is performed below T m , it causes severe plastic deformation in the material. Some AA welded by FSW exhibit relatively poor corrosion resistance. In this research, the corrosion resistance of such welds was enhanced through laser surface melting. A friction stir weld of AA 2219 was laser melted. The melt depth and microstructure were observed using optical and scanning electron microscopy. The melt zone exhibited epitaxially grown columnar grains. The redistribution of elemental composition was analyzed using energy-dispersive spectroscopy. The anticorrosion properties of both laser-melted and original welds were studied in aqueous 3.5% NaCl solution using cyclic potentiodynamic polarization. The results indicated a noticeable increase in the pitting corrosion resistance after the laser treatment on the surface. The repassivation potential was nobler than the corrosion potential after the laser treatment, confirming that the resistance to pitting growth improved. [ABSTRACT FROM AUTHOR]

The refined microstructure and enhanced mechanical properties of aluminum alloys containing Sc have attracted a great deal of attention in the recent past. In this work, a wire-based process of laser-arc hybrid additive manufacturing (LHAM) was developed and utilized to fabricate Al–Mg–Sc alloy thin wall deposits. The macro morphology, microstructural features and mechanical properties of LHAMed Al–Mg–Sc alloy were systematically investigated in comparison with wire and arc additive manufacturing (WAAM) under two processing conditions, i.e., same arc current (WAAM_SA) and same heat input (WAAM_SH). The surface quality of LHAMed deposits was significantly improved compared to that of WAAM_SA due to the stabilized effect of laser on arc behaviors. The deposits made by LHAM showed a refined and homogeneous microstructure compared to that of WAAM_SH under the condition of the same heat input, and a similar grain size with that of WAAM_SA even using a higher heat input. The microhardness results showed the LHAM had a higher and more stable microhardness distribution due to the homogeneous microstructure and less burning loss of magnesium. The as-deposited walls of LHAM and WAAM_SA showed a similar ultimate tensile strength (UTS) of more than 345 MPa for both horizontal and vertical directions, which is slightly higher than that of WAAM_SH. The as-deposited wall of LHAM also showed an exceptional elongation of about 26%, which is higher than that of both WAAM_SA and WAAM_SH samples, especially in the vertical direction, indicating less anisotropy in mechanical properties. • The Al–Mg–Sc alloy thin wall with high surface quality was obtained by LHAM. • The grain size and second phases of LHAM were refined in comparison with WAAM_SH. • The thin wall made by LHAM showed the minimum fluctuation of microhardness. • The enhanced tensile properties in both directions were achieved by LHAM. [ABSTRACT FROM AUTHOR]

Wire-based additive manufacturing (AM) presents promising opportunities for making large components with high-price materials, such us aluminum alloys containing expensive rare earth element of scandium (Al-Sc alloy). However, wire-based AM of structurally sound and defect-free aluminum components is still challenging. In this work, an oscillating laser-arc hybrid additive manufacturing (O-LHAM) process was developed to fabricate thin-wall parts of Al-6Mg-0.3Sc alloy. To show the effect of oscillating laser beam on macro/micro-structure, porosity and mechanical properties, wire and arc additive manufacturing (WAAM) of Al-6Mg-0.3Sc alloy part was also performed for comparison. The results showed that the surface roughness of O-LHAM samples was reduced by 34.7% and the effective wall width was increased by 20% when maximum width of the wall was same with WAAM. The porosity was significantly mitigated and the microstructures was refined and homogenized due to the string effect of beam oscillating in the molten pool. The as-deposited wall of Al-Sc alloy showed an ultimate tensile strength of more than 350 MPa, reaching an exceptional level among the as-deposited aluminum alloy. The O-LHAM samples showed a higher elongation and lower anisotropy, which are resulted from the mitigated porosity and homogenized microstructure. [Display omitted] ● Al-6Mg-0.3Sc alloy thin-wall part was fabricated by O-LHAM. ● The surface quality of O-LHAM sample was improved in comparison with that of WAAM. ● The porosity was mitigated due to the stirring effect of oscillating laser beam. ● The microstructure was refined and homogenized in O-LHAM sample. ● The as-deposited Al-6Mg-0.3Sc alloy showed an UTS of more than 350 MPa. [ABSTRACT FROM AUTHOR]

In this work, TiC nanoparticles (TiCps) reinforced 7075 aluminium alloy was fabricated via an oscillating laser-arc hybrid additive manufacturing process. To show the beneficial effect of TiCps addition on microstructure and mechanical properties, a conventional 7075 aluminium alloy was also manufactured for comparison. Upon the addition of TiCps, remarkable grain refinement was achieved with a decrease in average grain size from 111.8 to 12.5 µm, and grain boundary segregation was alleviated. It was also found that many secondary phases precipitated at the grain interior in the deposit with TiCps. Owing to the refined microstructure and favourable precipitation, as-deposited 7075 aluminium alloy with TiCps showed enhanced tensile properties of ultimate tensile strengths of 346 and 355 MPa along scanning and build directions. [ABSTRACT FROM AUTHOR]

In this work, the asymmetric weld fusion geometry in oscillating laser beam welding (OLBW) of aluminium alloy was reported and investigated using a numerical approach. A multi-physics heat transfer and fluid flow model of OLBW was developed and validated with the corresponding experimental results. The weld fusion geometry, temperature fields, and fluid flow behaviours for four commonly used oscillation modes, line, circle, eight, and infinity, were calculated to analyse the origin of the asymmetric weld fusion geometry. The results show the asymmetry of local heat input along beam travelling path and the fluid flow pattern in the molten pool are the main factors that result in the asymmetric weld fusion geometry in OLBW. [ABSTRACT FROM AUTHOR]

Investigations were conducted on TB8 titanium alloy to assess the influence of aging heat treatment on microstructure and phase transformations induced by laser oscillating welding, and establish a correlation between tensile properties and microstructure. Low-temperature aging joint consists of ω phase and β phase, while high-temperature aging joint is comprised of α phase and β phase. The formation of poor and rich precipitation zone in weld area was due to segregation of major alloying elements like Al, Mo and Nb. Low-temperature aging deteriorates plasticity of joints, but high-temperature aging improved ambient-/elevated-temperature tensile properties. The effect of aging temperature and aging time were discussed in depth. After aging treatment at 450° Celsius for 1 h, intermediate ω phase in gathering place transformed to α phase, but dispersed ω phase still retained. With the increase of aging temperature and aging time, size of α precipitate increased. Note that there is a significant increase (85%) in tensile strength after high-temperature aging treatment and Portevin-Le Chatelier (PLC) effect can be avoided under thermo-mechanical loading conditions. It was found that after 1 h of aging treatment at 550° Celsius followed by air cooling, the welded joints owned the best tensile properties at both ambient and elevated temperature. [ABSTRACT FROM AUTHOR]

• A new welding method of combining oscillating laser with vacuum environment were carried out for a deeper sound weld. • With the increase of the oscillating amplitude, the penetration depth first increases and then decreases. • The mechanism by which beam oscillation increases grain size in a vacuum environment has been elucidated. • The tensile strength of the joints welded by OLBW under vacuum all achieved 90% of the base metal. Beam oscillation in the laser welding process has demonstrated numerous advantages in suppressing spatters, eliminating pores, and reducing gap sensitivity, particularly for joining thin plates. This study presents the welding of thick section aluminum alloy using laser beam oscillation in a vacuum environment. During vacuum laser welding, the effect of beam oscillation variables, which include oscillating path, amplitude, and frequency, on weld seam morphology, microstructures, and mechanical properties was symmetrically evaluated. The findings indicate that in a vacuum environment, beam oscillation with a small amplitude can optimize the contour of the weld sidewall and increase the penetration depth, which is different from that observed in an atmospheric environment, where the melting penetration depth decreases with increasing oscillating amplitude. The deep penetration weld seam in a vacuum environment can achieve an extremely small size of 19 ∼ 30 µm. The addition of small oscillations causes a slight increase of the average grain size. The stirring effect of beam oscillation was weakened because of the minimized weld pool size under vacuum. This increased grain size is mainly attributed to the changes in the size of the molten pool caused by the oscillation cause changes in solidification parameters which accelerate grain growth. However, increasing the oscillation frequency can greatly increase the strength of the stirring effect and the refinement ability of small-sized grains. On further increasing the oscillation frequency to 200 Hz, the grown grains are refined. The results showed the tensile strength of the vacuum laser weld joints with and without beam oscillation can achieved a value of about 90 % of the base metal, which is superior to the strength of the joints welded by atmospheric pressure laser welding and can meet the vast majority of welding needs. [ABSTRACT FROM AUTHOR]

• Subatmospheric environment was used in AZ31 Mg alloy laser welding. • The effect of ambient pressure on the keyhole and weld pool behaviour in laser welding of AZ31 Mg alloy were investigated. • Vacuum laser welding was used to join 10 mm-thick AZ31 Mg alloy plates in a bottom-locking configuration. • Defect-free deep penetration magnesium weld was achieved by vacuum laser welding. Laser welding of magnesium alloy is still challenging because of the frequently occurred defects. In this work, a promising vacuum laser welding technique was used to join AZ31 magnesium alloy. The effect of ambient pressure on weld surface appearances and weld fusion cross sections was symmetrically investigated. It was found that the penetration depth increased and the weld width became narrower with the descending of ambient pressure from 101 kPa to 10 kPa. However, the penetration depth did not increase and even decreased when the ambient pressure further decreased to 1 kPa and 0.1 kPa. In addition, hump defects occurred at lower ambient pressure of 1 kPa and 0.1 kPa. The weld pool and keyhole behaviors were in-site observed using a high-speed camera at various ambient pressures. The results showed that unstable keyhole and turbulent flow were found in the molten pool at 101 kPa. The keyhole became more stable as the ambient pressure decreased from 101 kPa to 10 kPa. When the ambient pressure further decreased to 1 kPa and 0.1 kPa, the liquid metal thickness between keyhole wall and weld pool periphery became very narrow, which sometimes resulted in unstable absorption of the laser energy. Finally, a 10 mm-thick bottom-locking weld was made under vacuum. The average grain diameter of the weld in vacuum is 28.4 μm, which is much lower than 46 μm in atmosphere. The tensile results showed that the average ultimate tensile strength and the elongation of the joints welded in vacuum at 10 kPa was 8.8 % and 44.0 % higher than that of the joints welded in atmosphere respectively. The enhanced mechanical properties were attributed to the reduced porosity and refined microstrustures of the weld joint made under vacuum. [ABSTRACT FROM AUTHOR]

• The hot cracks are eliminated in the LPBF-processed ZK60. • The grain size is significantly reduced after the powders modified by Y. • The grain boundaries are strengthened by the I - phase. • The cracks in LPBF-processed ZK60 samples are attributed to the Mg 7 Zn 3. The hot cracks of laser powder bed fusion (LPBF) processed ZK60 components were eliminated by adding rare earth element Y. The cause of the cracks in the samples and the suppression mechanism were analyzed in detail. The cracks in ZK60 LPBF samples were mainly caused by the low-melting eutectic phase Mg 7 Zn 3 at the grain boundaries. The average grain size decreased from 7.2 μm to 2.0 μm after adding Y. The I-phase with excellent thermal stability strengthened the grain boundaries. The elimination of cracks was mainly attributed to the I-phase at the grain boundary and the grain refinement. [ABSTRACT FROM AUTHOR]