Search

Metal/ceramic functionally graded materials (FGMs) have attracted significant attention due to their ability to combine the unique mechanical properties of metals with the high-temperature resistance of ceramics while mitigating the thermal stresses arising from the differences in their thermal expansion coefficients. In this study, the directed energy deposition (DED) technology is applied to manufacturing TiC/Ti6Al4V FGMs with TiC contenting up to 50 wt%. However, the variations in composition gradients and the complex thermal cycling processes lead to difficulties in the microstructure evolution and control. Therefore, a combined approach involving finite element (FE) simulations and experiments was employed to investigate the mechanisms underlying microstructural transformations under various gradient thermal cycles. The results showed that the morphology of TiC in the FGM from top to bottom is as follows: coarse dendritic TiC + unmelted TiC, granular primary TiC + underdeveloped dendritic TiC, chain-like eutectic TiC + granular eutectic TiC. Meanwhile, both α-Ti and β-Ti exhibited equiaxed transformation, with their sizes decreasing continuously with the gradient. This is due to the strong orientation relationship between TiC particles and the titanium matrix, allowing α-Ti and β-Ti to form non-uniform nucleation within the TiC phase. In addition, the chain-like eutectic TiC and dendritic primary TiC showed a certain degree of fragmentation and granulation after undergoing thermal cycling. The microstructural evolution mechanism in different gradient regions during the DED process under the coupling effect of thermal cycling and ceramic composition was clarified by combining the FE simulation results. The specimen at the bottom of FGMs exhibits strong and ductile (tensile strength: 1296 MPa, elongation: 7.17 %), which are mainly attributed to the Hall Petch strength and solid solution strengthening. The brittle phase of dendritic TiC and unmelted TiC under high gradients leads to decreased tensile properties. • Directed energy deposition technology can successfully prepare TiC/Ti6Al4V FGMs with TiC. • The influential mechanism of the TiC phase on the morphology of α-Ti and β-Ti is revealed. • The microhardness of the TiC/Ti6Al4V FGM shows a progressive increase, rising from 379.77 HV to 736.71 HV. [ABSTRACT FROM AUTHOR]

The uneven microstructure and element distribution between the inner-layer zone (INZ) and the inter-layer zone (ITZ) resulting from the thermal cycling effect during the directed energy deposition process with the electric arc energy source (DED-Arc) can lead to inferior mechanical properties. In order to address this problem, the ultrasonic treatment of the 2319 Al-Cu alloy during the DED-Arc process is investigated to control the multi-regional microstructure quantitatively. Therefore, the effect of ultrasonic vibration during the DED-Arc process on grain morphology, distribution of precipitates and mechanical properties in different regions are explored in detail. The formation mechanism of differential microstructure is elucidated by using an ultrasonic-heat-flow coupling simulation model for the DED-Arc process. The results indicate that ultrasonic vibration refines the average grain size, leading to 16% and 40% reductions at the molten pool center (MPC) and boundary (MPB), respectively. However, the grains in the semi-melting zone (SMZ) experience coarsening due to the reduced temperature gradient after ultrasonic treatment. The enhanced melt flow rate is considered responsible for both the homogenization of Cu and the fragmentation of the θ-Al 2 Cu phase. The higher subgrain boundary density in the ITZ leads to an average microhardness of approximately 60 HV, which is lower than that found in the INZ (72.3 HV). Meanwhile, ultrasonic treatment increases the average strength and elongation by 38.1% and 17.5%, respectively, mainly through precipitation strengthening, which results in a strength increment of 91.6 MPa. The slip directions tend to be consistent after ultrasonic vibration, which is conducive to crystal sliding and displays excellent ductility. [ABSTRACT FROM AUTHOR]

The microstructure under various conditions shows noticeable differences in the wire-laser directed energy deposition (DED) method using a coaxial head due to the intrinsic properties of aluminum alloy wire materials and the intricate heating process involved. The impacts of varied heat inputs and substrate preheating temperatures on solidification and microstructure evolution were explored in this study. A coupled model, including macroscopic finite element and microscopic cellular automata with interpolation, was employed to investigate the temperature field, thermal cycles, solidification parameters, and microstructure evolution under varied process parameters. This study revealed the transformation of four grain forms in the deposition layer, namely, lamellar grain, equiaxed grain, equiaxed fine grain, and columnar grain, through a comprehensive dynamic simulation of the microstructure. Quantitative research was conducted to determine the solidification region of equiaxed grains and the average length of columnar grain amid solidification under varied crystallization parameters. The findings demonstrated that under a lower heat input and preheating temperature, equiaxed fine grains are generated at the lower region of the deposition layer, while lamellar grains are formed above the equiaxed grains at the upper region of the deposition layer, which is similar to the morphology of a cast microstructure. The typical primary dendritic distance of columnar grain diminishes with the enlargement of the cooling rate and temperature gradient. The findings of this study offer a theoretical framework for regulating the microstructure of aluminum alloy deposition layers manufactured via wire-laser DED technology using a coaxial head. [Display omitted] • The effect of heat input on microstructure evolution was studied through CA-FE simulation and experimental comparison. • The transformation of four crystal forms was revealed through a comprehensive dynamic simulation of the microstructure. • The average length variation of columnar crystals was quantitatively analyzed under different crystallization parameters. [ABSTRACT FROM AUTHOR]

Laser additive manufacturing (LAM) with directed energy deposition (DED) as the predominant foundation is an efficient and flexible joining technology that makes the fabrication of components with large thicknesses and complex structures possible. Heat accumulation phenomena caused by successive layer deposition strongly affect the microstructure and properties of components created with LAM. This study presents the conditions for component joining of constant process parameters and laser power gradient reduction process parameters to compare and investigate the influence of heat accumulation on the uniformity of mechanical properties and microstructure. The effects of the thermal cycle curve in different zones during LAM on the microstructure and mechanical properties of the top, middle and bottom of the deposition zone (DZ) under various parameters were investigated through finite element simulation and experiment. Results indicated that the cooling speed at the top of the DZ is slow and the heat accumulation effect is high. The aspect ratio of the α phase at the top of the deposition zone is high and high heat accumulation deteriorates mechanical properties. The heat accumulation effect is weakened when the gradient process parameters are used for joining. The content and size of the α phase at different joint heights are further homogenized. The tensile strength difference at various positions decreases and is higher than the tensile strength of the joint under constant parameters. This study promotes the LAM technology and provides a control reference for heat accumulation and microstructure regulation of large-thickness Ti6Al4V titanium alloy joints fabricated by LAM. [Display omitted] • The effect of heat accumulation on the microstructure and mechanical properties of the TC4 titanium alloy joint was investigated. • The distribution characteristics of temperature field and the change rule of thermal cycle curve in different zones during the laser additive manufacturing process were explored. • The differences of microstructure and mechanical properties at the top, middle and bottom of the addition zone under gradient parameters and constant parameters were comparatively evaluated. [ABSTRACT FROM AUTHOR]