High speed steels are ferrous based alloys of the Fe–C–X multicomponent system where X represents a group of alloying elements comprising mainly Cr, W or Mo, V, and Co. These steels are mainly used...

Solidification of high speed steels

Laser-engineered net shaping (LENS) is a rapid direct manufacturing process. The LENS process can be analyzed as a sequence of discrete events, given that it is a layer-by-layer process. The thermal history associated with the LENS process involves numerous reheating cycles. In this article, the thermal behavior during laser deposition with LENS is simulated numerically by using the alternate-direction explicit (ADE) finite difference method (FDM). The simulation results showed that deposited material experiences a significant rapid quenching effect during the initial stages of deposition and can attain a very high cooling rate. With an increase in deposit thickness, the rapid quenching effect decreases and eventually disappears. The effects of the processing parameters on the thermal behavior of deposited materials were also simulated and analyzed. The objective of this study is to provide insight into the thermal history during the LENS process, where the ability to correlate process parameters to microstructural evolution is a motivating force.

Thermal Behavior and Microstructural Evolution during Laser Deposition with Laser-Engineered Net Shaping: Part I. Numerical Calculations

Gas-atomized 6061 aluminum powder was used as feedstock for deposition using a high pressure cold-spraying process. The microstructures of the as-received powder and cold spray processed (CSP) ultrafine-grained (UFG) 6061 depositions were characterized by different electron microscopy techniques. It was found that there is segregation of solute elements at the particle grain boundaries, which is increased after cold spraying (CS). Various microstructural features were observed in both directions (parallel and perpendicular) of the CSP layer, including low-angle grain boundaries, clustered-small-cell walls, and dislocation tangle zones. The results also indicated that a combination of different recrystallization mechanisms (i.e., continuous and geometrical) may contribute to the formation of nano and UFG structures during CS.

Microstructural evolution of 7075 Al gas atomized powder and high-pressure cold sprayed deposition

Solidification Study of Aluminum Alloys using Impulse Atomization: Part I: Heat Transfer Analysis of an Atomized Droplet

The allotropic, martensitic phase transformation (hcp → fcc) in cobalt was investigated by differential scanning calorimetry (DSC) upon isochronal annealing at heating rates in the range from 10 K min−1 to 40 K min−1. The microstructural evolution was traced by optical microscopy and X-ray diffractometry. The kinetics of the phase transformation from hcp to fcc Co upon isochronal annealing was described on the basis of a modular phase transformation model. Appropriate model descriptions for athermal nucleation and thermally activated, anisotropic interface controlled growth tailored to the martensitic phase transformation of Co were implemented into the modular model. Fitting of this model of phase transformation kinetics to simultaneously all isochronal DSC runs yielded values for the energy of the interface separating the hcp and fcc Co phase and the activation energy for growth.

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Kinetics of the allotropic hcp–fcc phase transformation in cobalt

In order to predict gas and droplet velocities, droplet temperature, and fractional solidification with flight distance during spray forming, the Newtonian heat transfer formulation has been coupled with the classical heterogeneous nucleation and the specific solidification process. It has been demonstrated that the thermal profile of the droplet in flight is significantly affected by process parameters such as droplet size, initial gas velocity, undercooling, and superheat. With increasing droplet size or initial gas velocity, the onset and completion of solidification are shifted to greater flight distances and the solidification process also extends over a wider range of flight distances. It has been found that the corresponding solid fractions formed during recalesced, segregated, and eutectic solidifications are linearly related to the degree of undercooling and that those solid fractions are insensitive to droplet size, initial gas velocity and superheat.

Solidification progress and heat transfer analysis of gas-atomized alloy droplets during spray forming

The solidified carbide morphology, the decomposition behavior of the M2C carbide, and the carbide distribution after forging of an Fe-1.28C-6.4W-5.0Mo-3.1V-4.1Cr-7.9Co (wt pct) high-speed steel prepared by spray forming have been investigated. The spray-formed microstructure has been characterized as a discontinuous network of plate-shaped M2C carbides and a uniform distribution of fine, spherical MC carbides. The metastable M2C carbides formed during solidification have been fully decomposed into MC and M6C carbides after sufficient annealing at high temperatures. Initially, the M6C carbides nucleate at M2C/austenite interfaces and proceed to grow. In the second stage, the MC carbides form either inside the M6C carbides or at the interfaces between M6C carbides. With this increasing degree of decomposition of the M2C carbide, the carbides become more uniformly distributed through hot forging, which produces a significant increase in ultimate bend strength. The decomposition treatment of M2C carbide has been found to be most important for obtaining a fine homogeneous carbide distribution after hot forging.

Solidification microstructure and M2C carbide decomposition in a spray-formed high-speed steel

It has been demonstrated that the activation energies of the solid state transformations obeying the modified Johnson-Mehl-Avrami/Austin-Rickett type kinetic equation can be determined from both the Kissinger method and the Ozawa method QRTm is lager than 5. The errors involved in the activation energy determined from the two methods have been presented as a function of Q/RTm is larger than 5. The errors involved in the effects of the kinetic exponents, c and n, and the heating rate θ and ym and Tm have been analyzed It has shown that the fraction ym is nearly independent of the heating rate while the peak temperature Tm is a very weak function of the exponents n and c. The effects of the kinetic parameters and the heating rate on the (yvs T) and (dy/dt vs T) curves are also discussed. Moreover, procedures for the simultaneous determination of the exponents c and n from a single DSC trace are provided. This kinetic model has been applied to the analyses of the non-isothermal aging kinetics of three CuZnAl shape memory alloys. The comparison between isothermal and non-isothermal results further supports the validity of this kinetic equation for the description of heterogeneous solid state transformations.

A transformation kinetic model and its application to CuZnAl shape memory alloys—II. Non-isothermal conditions

In order to clarify the relationship between the extent of the distortion from the ideal structure and the nature of ordering (type, degree), a rigid hard sphere model for the M18R structure is developed by introducing the modified Bragg-Williams-Gorski-type order parameters The ide- ally close-packed 9R or 18R structures are produced from the disordered or D03 ordered parent phases, while the modified 9R and 18R structures are produced from the B2 ordered and L21 ordered parent phases, respectively The extent of the distortion in the close-packed plane in- creases with increasing degree of B2 order or with decreasing degree of L21 order Related experiments based on the variation of X-ray diffraction pattern for the stabilization of Cu- Zn-Al shape memory alloy have confirmed the validity of the theoretical development

Effects of ordering type and degree on monoclinic distortion of 18R-type martensite in Cu-Zn-Al alloys

Isothermal and nonisothermal aging behaviors of five Cu/(13 to 28)Zn/(3 to 9)A1 (weight percent) shape memory alloys have been investigated. The electrical resistivity during isothermal aging from 180 °C to 340 °C increases with aging time at each aging temperature. The time to reach a constant fraction transformed during the isothermal aging increases with increasing Al content or decreasing Zn content in the Cu-Zn-Al alloys, except alloy D (Cu-18.2Zn-7.2Al). This alloy exhibits a double aging behavior by the earlier formation of the α1 bainite and the subsequent formation of the cubic γ2 phase from the β1 matrix phase. The apparent activation energy of alloy D varies with the fraction transformed by the double aging behavior. However, other alloys A, B, C, and E exhibit isokinetic behavior. The apparent activation energies estimated in the nonisothermal aging from 100 °C to 500 °C are found to be about equal to those obtained in the isothermal aging.

Eon-Sik Lee

Papers

Solidification progress and heat transfer analysis of gas-atomized alloy droplets during spray forming

Solidification microstructure and M2C carbide decomposition in a spray-formed high-speed steel

A transformation kinetic model and its application to CuZnAl shape memory alloys—II. Non-isothermal conditions

Effects of ordering type and degree on monoclinic distortion of 18R-type martensite in Cu-Zn-Al alloys

Composition dependence of aging kinetics in some Cu-Zn-Al shape memory alloys