Search

Due to the mirror image symmetry of laser mirror welding heat sources, the post-weld deformation is observably lower than that of conventional welding methods. However, the mechanism of gravitational effects on the coupled keyhole molten pool at different welding positions is still unclear, which results in the tough control of welding defects, hindering the application of laser mirror welding in precision manufacturing of aerospace structural parts. In this study, the gravity effects on keyhole behaviors in laser mirror welding of 2219 aluminum alloy were investigated by establishing a computational fluid dynamics (CFD) model. The keyhole morphology, coupling characteristics, and its dynamic evolution as well as force conditions were analyzed at length by employing the calibrated model. The results showed that gravity prominently affects the fluid velocity in the molten pool, which determines the keyhole characteristics. The keyhole surface flow, which enlarges the keyhole diameter, is impeded by gravity, resulting in weak keyhole coupling. The keyhole area on the weld cross-section before coupling is less than 2 mm2 and is negatively correlated with the gravitational component on this section. The keyhole area after coupling is tightly relevant to the keyhole diameter. The increase or decrease in the keyhole diameter is contingent on the resultant direction of gravity, fluid pressure, and vapor recoil pressure. Additionally, the keyhole coupling and dynamic behaviors in the welding direction are primarily affected by the surface tension gradient, while slightly affected by gravity. [ABSTRACT FROM AUTHOR]

Laser melting deposition (LMD) is an advanced repairing and remanufacturing technology that involves complex thermodynamic and kinetic characteristics in a molten pool. A thorough understanding of the thermal-fluid dynamic behavior of molten metal is crucial for investigating the distribution of reinforced particles and the characterization of solidification. This study proposes a three-dimensional transient thermal-fluid coupling model for describing thermal-fluid dynamic behavior and particle migration behavior within a molten pool during LMD. The simulated morphology of the deposition layer and temperature agree well with the experiments. The simulation results indicate that Marangoni convection induced by temperature gradient dominates the convection mode. The fluid velocity distribution along the scanning direction of the molten metal is showing M shape. Meanwhile, the dimension of molten pool increases slightly, and the Marangoni convection becomes more vigorous with higher heat input. The Marangoni motion tends to drive the TiC particle migration from the center to both lateral sides, resulting in a more uniformly distributed of TiC particle in deposition layer. Furthermore, the solidification parameters at solid–liquid interface are calculated, which used to assess the solidification microstructure in deposition layer. [ABSTRACT FROM AUTHOR]

Wire arc additive manufacturing (WAAM) is widely studied due to its high deposition rate. However, WAAM samples have poor forming quality compared with other additive manufacturing technologies. The quality of sample formation is directly related to the time required for subsequent mechanical processing. Hence, cycle time can further be reduced by controlling the forming quality. Additionally, better forming quality results in less material wastage. Therefore, the forming quality of samples has always been the focus of WAAM. In this paper, the droplet transfer process of the WAAM is shot with a high-speed camera. This article attempts to explain the WAAM forming process from the perspective of droplet transfer and evaluate the influence mechanism of droplet transfer on the side surface forming quality. High-speed photography is used to extract the molten pool size and droplet transfer frequency. The study also explored the variation of these factors with wire feed speed and number of layers. A two-dimensional droplet transfer model is established based on the results of high-speed photography to study the droplet transfer process. Next, the 3D sample model is built to obtain the roughness of deposition samples. Finally, the relationship between the forming quality and droplet transfer process is studied through the fitting planes. The relationship model of wire feeding speed, molten pool length/droplet transfer frequency, and roughness is obtained. The roughness of the side surface increases with higher wire feed speeds when the wire feed speed is 5.5–6.5 m/min. However, at a wire feed speed of 7.0 m/min, the roughness of the side surface suddenly decreases. The droplet transfer motion rule is then used to explain this change in forming quality. [ABSTRACT FROM AUTHOR]

Friction stir additive manufacturing technology has emerged as an efficient solid-state process option that provides superior connectivity for lightweight structural material with equiaxed microstructures and outstanding mechanical properties. In this paper, a multilayer ring-shaped build made from 6061-T6 aluminum alloy using friction stir additive manufacturing (FSAM) was characterized. The rotating torque exerted on the friction tool by the surrounding material was measured as it moves to the middle and the end of the ring. Additionally, the correlation between the microstructure and the rotating torque was investigated. The results revealed a significant decrease in rotating torque when the stir tool returned to its original position in the additive zone, leading to a reduction in grain size within the additive zone. Underneath the bottom of the stir zone, there was an overlapping interface filled with deformed grains. Along the horizontal direction, the highest hardness existed in the base material region, while the lowest value appeared in the mixed area of the thermal–mechanical affected zone and heat-affected zone. The hardness value of the stir zone was found to be intermediate between the base material region and the mixed area of the thermal–mechanical affected zone and heat-affected zone. The highest and lowest hardness values along the tool-axial direction were observed at the top of the stir zone and the overlapping interface below it. [ABSTRACT FROM AUTHOR]

Laser welding is a feasible process for joining the additive manufactured (AMed) parts to meet the manufacturing demand of specific large-scale components. Microstructure and property evolution as well as weldability of the laser welding of wire arc additive manufactured (WAAMed) Al-Cu alloy are investigated. Results indicate that the WAAMed Al-Cu alloy has excellent laser weldability and the joint is nearly free of defect. The width of the heat-affected-zone (HAZ) and equiaxed crystal zone (EQZ) as well as the grain size of the fusion zone (FZ) will increase with higher laser power and heat input. The joint with laser power of 3500 W has the highest tensile strength and elongation, reaching 203.48 MPa and 4.13%. The result from electron backscatter diffraction (EBSD) test indicates that the texture intensity will affect tensile properties. The tensile strength and elongation of the laser welded WAAM sample perpendicular to the deposition direction are higher than that parallel to the deposition direction. The microhardness value of HAZ is higher than that of FZ and BM due to 163 HV. The feasibility of laser welding of WAAMed samples is validated, and process parameters are found for hybrid manufacturing of large-scale components. [ABSTRACT FROM AUTHOR]

The bottom-locking joint is widely used in the aerospace manufacturing field as an imperative connection structure. The laser welding experiment for the Ti-6Al-4V titanium alloy bottom-locking joint is carried out in the present paper. Meanwhile, porosity in the weld seam including the process porosity and metallurgical porosity is investigated. Furthermore, it is novel that the distribution, element characteristics, and formation mechanism of process porosities and metallurgical porosities in the bottom-locking joint are comprehensively analysed based on the porosity morphology, microstructure, and element analysis of the resultant bottom-locking joints. The process porosity, which mainly appeared in the weld seam centre, is formed by the combined effect of two mechanisms which closely related to the dynamic behaviour of the molten pool and the stability of the keyhole. Besides, a large number of micropores with a diameter of 2–5 µm are observed on the surface of the process porosity, whose formation is related to the evaporation of elements. Moreover, the tensile stress generated during the solidification and shrinkage of the molten metal is the main factor leading to microcracks in the micropores. The nucleation characteristics of bubbles cause metallurgical porosities to be mainly distributed at the bottom of the weld seam. [ABSTRACT FROM AUTHOR]

In order to meet the shape control requirements of "dead flat" transverse thickness profile of electrical steel sheet in cold rolling process, the 3D finite element model (FEM) for roll stacks and strip of 6-high tandem cold rolling mills (TCMs) was built with the developed Edge Drop Control Work Rolls for Non-shifting (EDW-N). The efficiency curves of the work roll bending (WRB), the intermediate roll bending (IRB), and the intermediate roll shifting (IRS) in cold rolling process are obtained for transverse thickness profile control performance. The control strategy of multi-stand and multi-variable profile and flatness control actuators, i.e., WRB, IRB, and IRS of stands 1 ~ 5 in 6-high TCMs is proposed based on the GA-PSO hybrid algorithm with better optimization ability. The results show that the control strategy can fully exploit the shape control ability and achieve the high precision control for transverse thickness profile of 6-high TCMs. The industrial application on the production gives remarkable results that the rate of the measured strip crown less than 7 μm increased from 38.58 to 67.74% for electrical steel sheet in the 1420-mm 6-high TCMs. The control strategy has applied to the production successfully. [ABSTRACT FROM AUTHOR]

Laser direct joining of carbon fiber-reinforced thermoplastic composites (CFRTP) to aluminum alloy with diverse surface micro-patterns was performed. The influence of micro-pattern type on interface characteristic and mechanical property of CFRTP/aluminum alloy laser bonding joint was concerned especially. Prior to the laser joining, two different micro-patterns including groove structure and circular hole structure were fabricated on the metal surface using nanosecond pulsed laser. Then, the appearance of joint and the morphology at joining interface were observed by optical microscope and scanning electron microscope. Moreover, the tensile shear test was conducted to obtain the fracture load and fracture surface morphology. The results indicated that the cross section of the groove structure was a trapezoid, while that of the circular hole structure was a triangle with a large ratio of depth to diameter. In spite of the more pores existed at the interface of joints with circular hole structure, this type of micro-pattern strengthened the mechanical anchoring effect between CFRTP and aluminum alloy more significantly. With the change of micro-pattern from groove structure to circular hole structure, the maximum tensile load of laser bonding joint increased from 1616.40 to 2623.40 N, and the main failure mode transformed from interface failure to cohesive failure. The micro-pattern fabricated on the metal surface before the laser joining process is an effective approach to obtain the larger bonding area and higher mechanical strength of laser bonding joint. [ABSTRACT FROM AUTHOR]

As the material experiences non-uniform thermal expansion and contraction under different thermal behavior during laser melting deposition (LMD), the microstructure of the components will be affected, which also causes changes in mechanical properties. It is essential to understand the thermal behavior of the melt pool during LMD to improve the microstructure and joint quality. In this study, two 80-mm-thick Ti6Al4V plates are successfully joined using three different scanning speeds (10, 15, and 20 mm/s). The effects of thermal behavior on the grain morphology and dimension of deposition area, equiaxed crystal zone (EQZ), and the substrate at different scanning speeds are quantitatively analyzed. The results show that the fine equiaxed crystals are formed near the joint boundary due to the high-temperature gradient and cooling rate. The growth direction of fine columnar crystals continuously changes from the joint boundary to the center of the deposition area due to the change of heat-extraction direction. However, the epitaxial growth direction of the coarse columnar crystal is the same as the previous deposition layer at the center of the deposition area. Given the effect of high heat accumulation and low-temperature gradient during LMD, the dimension of columnar crystal is coarsen significantly at the center of the deposition area. [ABSTRACT FROM AUTHOR]

In this investigation, a sinusoid modulated pulse gas metal arc welding (SP-GMAW) method based on a current waveform control method and on the welding mechanism of pulsed gas metal arc welding (P-GMAW) was proposed. This method achieved smooth control over the welding current. The process formulas were simplified. Then, the control parameters were discussed and optimized. Finally, three comparison experiments were conducted. The results indicated that the SP-GMAW method, as an improved double-pulsed gas metal arc welding (DP-GMAW) method, had the following features: The SP-GMAW minimized porosity rate and generated palpable weld ripples in contrast with the P-GMAW. The SP-GMAW produced the finest fusion zone grain, highest fusion zone microhardness, and highest tensile strength among three welding methods. The feasibility and superiority of the method for welding aluminum sheet were verified. [ABSTRACT FROM AUTHOR]