Constitutive equation for the hot deformation behavior of Csf/AZ91D composites and its validity for numerical simulation

The flow stress behavior of 10 vol. % short carbon fibers reinforced AZ91D composites (C-sf/AZ91D) were investigated by hot compression test. The results show the flow stress reach the peak value at small strain and then decrease monotonically until the end of the large strain, which exhibits an obvious dynamic strain softening. The decrease of stress level with deformation temperature increasing or strain rate decreasing can be represented by Zener-Hollomon parameter in a hyperbolic sine equation. By considering the effect of strain on material constants, a modified viscoplastic constitutive equation was established to characterize the dependence of flow stress on the deformation temperature, strain, and strain rate. The stress-strain values calculated by the constitutive equation are in consistent with the experimental results. Applying the constitutive equation, the plastic deformation of C-sf/AZ91D) composites during the hot compression process were analyzed by finite element simulation. The calculated punch force-stroke curves agree well with the measured ones. The results confirmed that the established constitutive equation can accurately describe the hot plastic deformation behavior of C-sf/AZ91D composites, and can be used for the finite element analysis on the hot forming process. (C) 2016 Elsevier Ltd. All rights reserved

Insights into the impact and solidification of metal droplets in ground-based investigation of droplet deposition 3D printing under microgravity

Droplet deposition 3D printing is an additive manufacturing technique offering a great potential for metal parts fabrication in space, of which some preliminary testing is usually performed on ground in the early research. The previous work mimicked the anti-gravity deposition of molten metal droplet through manipulating it into perpendicularly depositing on a vertical substrate. However, the ground-based simulation of droplet deposition 3D printing under microgravity remains an elusive goal, since the spreading and receding processes are still affected by gravity. To address this issue, the prevailing physical mechanisms of gravity effect on droplet impact and solidification are urgent to be defined. Here, we present the studies on the impact dynamics and transient solidification of the molten metal droplet deposited on vertical substrates through numerical modeling and experiments. It is observed that the spreading and retraction of the droplet are asymmetric, besides its solidification shape tilts to gravitational direction. The formation mechanisms of these undesired behaviors are further demonstrated. The results show that the asymmetrical spreading, retraction and solidification shape of the droplet originate from the interaction of gravity and solidification. Moreover, the tilt of the solidified droplet has a correlation with the critical process parameters, i.e. impact velocity, temperatures of droplet and substrate. With a larger impact inertia and a lower solidification rate, the undesired solidification shape can be effectively eliminated. This work provides a foundation for the further investigation of the ground-based physical simulation of outer space droplet deposition 3D printing

Effect of the surface morphology of solidified droplet on remelting between neighboring aluminum droplets

Good metallurgical bonding between neighboring droplets is essential in droplet-based 3D printing. However, although the mechanism of remelting has clearly been mastered, cold laps are still common internal defects of formed parts in uniform aluminum droplets deposition manufacturing, which is due to the overlook of the surface morphologies of solidified droplets. Here, for the first time, the blocking effect of ripples and solidification angles on the fusion between droplets is revealed. To investigate the detailed process of remelting, a 3D numerical model was developed, basing on the volume of fluid (VOF) method. Experiments and simulations show that the remelting process between neighboring droplets can be divided into two stages according to the transient contact between the second droplet and the substrate. In the first stage, a non-intuitive result is observed that cold laps can also be formed even if the remelting conditions are satisfied in theory. Ripples on the surface of previously-deposited droplet block its direct contact with the new-coming droplet. In the second stage, cold laps on bottom surface are formed due to incomplete filling of liquid metal when the solidification angle is greater than 90°. Furthermore, these cold laps are difficult to be completely avoided by improving the temperature parameters. To address this problem, a novel strategy of decreasing the thermal conductivity coefficient of the substrate is proposed. This method effectively promotes remelting between droplets by eliminating ripples and decreasing solidification angles

Geometry control of closed contour forming in uniform micro metal droplet deposition manufacturing

Micro metal droplet deposition manufacturing shows great potential applications in many industrial areas such as micro circuits printing, thin-wall metal parts, porous metal parts, and heterogeneous material parts. However, excessive overlapping of metal droplets in corners deteriorates the quality of printed parts. To solve this problem, the droplet center-to-center distance must always keep uniform and be in an ideal range. First, reasons of excessive overlapping in corners are analyzed and a mathematical model is proposed. Then droplet center-to-center distance is optimized and compensated according to corner angle of contour lines and number total of droplets so that the distance between adjacent droplets is proper. The coordinate of rearranged droplets is obtained by calculation. To verify this method, uniform solder droplets were ejected and deposited on a flat substrate. A series of deposition experiments were carried out. Some triangle contours were formed. The results show that quality of the formed parts had been significantly improved by using the proposed method

Numerical evaluation of the influence of porosity on bending properties of 2D carbon/carbon composites

Numerical simulation with progressive damage criterion is implemented to investigate the effect of porosity on the bending properties of 2D cross-ply carbon/carbon (C/C) composites. The mechanical properties of Pyrocarbon matrix regarding the change of porosity are calculated by using Mori-Tanaka approach. Combining with the stiffness degradation scheme, the ultimate bending strengths are calculated in Abaqus though a user-defined subroutine (USDFLD). Delamination is modelled by inserting cohesive elements between two adjacent plies. A good agreement is obtained when the FEM results are compared to three-point bending experiments. The FEM results show that the bending strength decreases greatly with the increase of porosity. When the porosity reaches up to 18%, the bending strength is decreased by 57%. The major fracture behaviors are interlamination delamination and continuous crack damage in 90° plies. With the increase of porosity, more severe interlamination delamination will be slightly aggravated. In addition, the increase of porosity will also accelerate the damage in 90° plies

Direct fabrication of unsupported inclined aluminum pillars based on uniform micro droplets deposition

In order to investigate forming directly complex parts without support materials or structures by uniform micro droplets deposition technique, the present work focus on fabricating the unsupported inclined aluminum pillars through offset deposition. An experimental system is developed to produce and deposit uniform molten aluminum droplets. A model is introduced to describe the inclined angle of the droplet deposition at different offset ratios. A one dimensional heat transfer model is proposed to help select the initial temperature parameters of the impinging droplet and the previous solidified droplet to ensure that the fusion occurs. No melting, partial melting and excessive melting region at different offset ratios are determined. The correspondence between offset ratio and inclined angle is considered to be a simple cosine function, and the hypothesis is verified by experiments. The influence of deposition error on an inclined angle of pillars is studied. Internal microstructure of droplet fusion is observed in order to ensure good metallurgical bonding. All of these studies show the feasibility of fabricating directly unsupported inclined aluminum pillars in the limited angle range by using uniform micro droplets

Modeling of transverse welds formation during liquid–solid extrusion directly following vacuum infiltration of magnesium matrix composite

Liquid–solid extrusion directly following vacuum infiltration (LSEVI) is an infiltration–extrusion integrated forming technique, and transverse weld between upper residual magnesium alloy and magnesium matrix composites is a common internal defect, which can severely reduce the yield of composite products. To improve current understanding on the mechanism of transverse welding phenomenon, a thermo-mechanical numerical model of LSEVI for magnesium matrix composites was developed. The formation of transverse weld during extrusion was visualized using finite element simulation method, and the formation mechanism was discussed from the aspect of velocity field using a point tracking technique. The simulation results were verified by the experimental results in term of weld shape

Metal droplet printing of tube with high-quality inner surface via helical printing trajectory and soluble support

Metal droplet-based 3D printing provides unique advantages for fabricating micro complex parts. Especially, assisted by soluble support, structures with high-quality inner surfaces can be directly printed without post-processing, which is very promising for the fabrication of waveguides and antenna horns. Here, a spatially distributed equidistant helical deposition strategy with only one-step positioning was proposed to improve the inner surface quality of the parts, and a five-axis motion stage was designed for matching its motion planning. The influence mechanisms of key process parameters on the forming quality were investigated. The droplet positioning errors, the aggregation behaviour, and the hole-defects formation on sloping surfaces were analyzed. As a proof-of-concept, a horn-structured tube was directly printed via the proposed printing method, which possessed both high-quality cavity surfaces and high density. This work paves the route towards the efficient additive manufacturing of metal tubes with high-quality inner surfaces