Weidong Huang

The influences of processing parameters on forming characterizations during laser rapid forming

Abstract Laser rapid forming experiments were carried out with 316L stainless steel and nickel-base alloy to investigate the influences of the processing parameters on forming characterizations systematically. It is found that the height of a single cladding layer was very important for fabrication accuracy and forming stability of components of laser rapid forming because it was affected by almost all processing parameters and was quite difficult to precisely control. For the system with lateral powder feeding pattern, the powder injection point was the most important factor to the height control of single layer. The variation of the width of single clad, which was mainly affected by laser power, spot diameter and scanning velocity, was similar to that in laser surface melting. The surface quality was another important characterization for laser rapid forming and was remarkably affected by oxidation and the powder adhesion. In order to improve the surface quality, the flow flux of shielding gas should be ≮10 l min −1 and the powder stream cannot be injected to the tail part of the melt pool. Based on the investigation, some metal components were fabricated.

Prediction of average spacing for constrained cellular/dendritic growth

Abstract Since 1990, it has been gradually understood that there exists an allowable range of cellular/dendritic spacings during unidirectional solidification. The lower and upper limits of the allowable range are very sharp at the same current growth condition, while the average spacing for cellular/dendritic arrays depends on the history of variation of solidification parameters. The present work puts forward a novel numerical model to predict the ultimate average spacing for constrained cellular and dendritic growth, according to the characteristics of unsteady-state evolution of cellular and dendritic arrays. Our analysis considering the history-dependent selection of the average spacing for cellular/dendritic arrays, we find — with no adjustable parameters — good agreement with experimental results obtained in different alloys and in different velocity histories.

Compressive deformation behavior of AZ31 magnesium alloy under quasi-static and dynamic loading

Abstract Compressive properties of AZ31 alloy were investigated at temperatures from room temperature to 543 K and at strain rates from 10 −3 to 2×10 4 s −1 . The results show that the compressive behavior and deformation mechanism of AZ31 depend largely on the temperature and strain rate. The flow stress increases with the increase of strain rate at fixed temperature, while decreases with the increase of deformation temperature at fixed strain rate. At low temperature and quasi-static condition, the true stress-true strain curve of AZ31 alloy can be divided into three stages (strain hardening, softening and stabilization) after yielding. However, at high temperature and high strain rate, the AZ31 alloy shows ideal elastic-plastic properties. It is therefore suggested that the change in loading conditions (temperature and strain rate) plays an important role in deformation mechanisms of AZ31 alloy.

Development of microstructures in laser surface remelting of DD2 single crystal

Abstract Detailed experiments were carried out to investigate the effects of crystallography on the microstructure evolution during the solidification of the laser molten pool with a DD2 single crystal specimen using a 5 kW CO2 laser. The experimental results showed that crystallographic effects can influence dendrite growth by favoring growth along a preferred crystallographic direction, and crystallographic effects are greater than that of heat flux during dendrite growth. On the same crystal face, the microstructures in the molten pool are very different when laser beam scanning along different crystal orientation. The same conclusions can be obtained by laser beam scanning along a fixed direction on different crystallographic surface. A growth velocity analysis was carried out during laser surface remelting processing based on a minimum velocity or minimum undercooling criterion and the relationships between dendrite growth velocities in the three directions, moving velocities of the solid/liquid interface and laser scanning velocities for the different crystallographic orientations were also set up. According to this analysis, the reconstructed and predicted molten pools concurred with the experimental results very well.

Effect of Substrate Surface Microstructure on Heterogeneous Nucleation Behavior

The heterogeneous nucleation behaviors of NH4Cl crystal on a rough chilling surface of aluminum immerged in NH4CI-H2O solution were experimentally analyzed and the relationship between the surface roughness and the nucleation site selection behaviors on polished aluminum substrate was investigated, and it was discovered that the number of nucleation sites decreases significantly with decreasing the roughness of the polished substrate. Further nucleation experiments were carried out on chemically etched aluminum substrate with regular micro-morphology on its surface. It has been shown that both the micro-morphology and the wettability vary with the substrate surface prepared by different etching process. The prepared surface with step-like structures has a strong wettability with NH4Cl-70 wt% H2O solution and the nucleation density of NH4Cl on the its surfaces is significantly higher than that of the reference surfaces, which shows that the geometrical morphology features have important effects on both the wettability and the nucleation behaviors.

Interfacial undercooling in solidification of colloidal suspensions: analyses with quantitative measurements.

Interfacial undercooling in the complex solidification of colloidal suspensions is of significance and remains a puzzling problem. Two types of interfacial undercooling are supposed to be involved in the freezing of colloidal suspensions, i.e., solute constitutional supercooling (SCS) caused by additives in the solvent and particulate constitutional supercooling (PCS) caused by particles. However, quantitative identification of the interfacial undercooling in the solidification of colloidal suspensions, is still absent; thus, the question of which type of undercooling is dominant in this complex system remains unanswered. Here, we quantitatively measured the static and dynamic interface undercoolings of SCS and PCS in ideal and practical colloidal systems. We show that the interfacial undercooling primarily comes from SCS caused by the additives in the solvent, while PCS is minor. This finding implies that the thermodynamic effect of particles from the PCS is not the fundamental physical mechanism for pattern formation of cellular growth and lamellar structure in the solidification of colloidal suspensions, a general case of ice-templating method. Instead, the patterns in the ice-templating method can be controlled effectively by adjusting the additives.

On cellular spacing selection of Cu-Mn alloy under ultra-high temperature gradient and rapid solidification condition

Cellular spacing selection of Cu-27.3 wt pct Mn alloy has been investigated by laser surface rapid resolidification experiments. The experimental results show that there exists a wide distribution range in cellular spacing under ultra-high temperature gradient and rapid solidification conditions and the average spacing decrease with increase of the growth rate. The experimental results are compared with the current KGT model for rapid cellular/dendritic growth, and a reasonable agreement is found.

In situ observation the interface undercooling of freezing colloidal suspensions with differential visualization method

Interface undercooling is one of the most significant parameters in the solidification of colloidal suspensions. However, quantitative measurement of interface undercooling of colloidal suspensions is still a challenge. Here, a new experimental facility and gauging method are designed to directly reveal the interface undercooling on both static and dynamic cases. The interface undercooling is visualized through the discrepancy of solid/liquid interface positions between the suspensions and its solvent in a thermal gradient apparatus. The resolutions of the experimental facility and gauging method are proved to be 0.01 K. The high precision of the method comes from the principle of converting temperature measurement into distance measurement in the thermal gradient platform. Moreover, both static and dynamic interface undercoolings can be quantitatively measured.

Orientation selection of equiaxed dendritic growth by three-dimensional cellular automaton model

Abstract A three-dimensional (3-D) adaptive mesh refinement (AMR) cellular automata (CA) model is developed to simulate the equiaxed dendritic growth of pure substance. In order to reduce the mesh induced anisotropy by CA capture rules, a limited neighbor solid fraction (LNSF) method is presented. It is shown that the LNSF method reduced the mesh induced anisotropy based on the simulated morphologies for isotropic interface free energy. An expansion description using two interface free energy anisotropy parameters (e1, e2) is used in the present 3-D CA model. It is illustrated by present 3-D CA model that the positive e1 favors the dendritic growth with the 〈100〉 preferred directions, and negative e2 favors dendritic growth with the 〈110〉 preferred directions, which has a good agreement with the prediction of the spherical plot of the inverse of the interfacial stiffness. The dendritic growths with the orientation selection between 〈100〉 and 〈110〉 are also discussed using the different e1 with e2=−0.02. It is found that the simulated morphologies by present CA model are as expected from the minimum stiffness criterion.

Numerical simulation and experimental calibration of additive manufacturing by blown powder technology. Part I: thermal analysis

Purpose This paper aims to address the numerical simulation of additive manufacturing (AM) processes. The numerical results are compared with the experimental campaign carried out at State Key Laboratory of Solidification Processing laboratories, where a laser solid forming machine, also referred to as laser engineered net shaping, is used to fabricate metal parts directly from computer-aided design models. Ti-6Al-4V metal powder is injected into the molten pool created by a focused, high-energy laser beam and a layer of added material is sinterized according to the laser scanning pattern specified by the user. Design/methodology/approach The numerical model adopts an apropos finite element (FE) activation technology, which reproduces the same scanning pattern set for the numerical control system of the AM machine. This consists of a complex sequence of polylines, used to define the contour of the component, and hatches patterns to fill the inner section. The full sequence is given through the common layer interface format, a standard format for different manufacturing processes such as rapid prototyping, shape metal deposition or machining processes, among others. The result is a layer-by-layer metal deposition which can be used to build-up complex structures for components such as turbine blades, aircraft stiffeners, cooling systems or medical implants, among others. Findings Ad hoc FE framework for the numerical simulation of the AM process by metal deposition is introduced. Description of the calibration procedure adopted is presented. Originality/value The objectives of this paper are twofold: firstly, this work is intended to calibrate the software for the numerical simulation of the AM process, to achieve high accuracy. Secondly, the sensitivity of the numerical model to the process parameters and modeling data is analyzed.