A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomised droplets as a function of distance during spray forming. Modifications of the model have been used to evaluate the relative importance of different physical assumptions in controlling droplet cooling and solidification behaviour, and ultimately the evolution of spray formed billet microstructures. Gas atomised droplet dynamic and thermal behaviour is strongly affected by the distribution of droplet diameters in the two phase spray and the momentum transfer between the droplets and the atomising gas, but is relatively unaffected by the details of the droplet solidification mechanism.

Modelling of droplet dynamic and thermal histories during spray forming—I. individual droplet behaviour

Spray forming is a novel method of rapidly manufacturing tools and dies for stamping and injection operations. The process sprays molten tool steel from a set of arc spray guns onto a ceramic former to build up a thick steel shell. The volumetric contraction that occurs as the steel cools is offset by a volumetric expansion taking place within the sprayed steel, which allows the dimensional accurate tools to be produced. To ensure that the required phase transformation takes place, the temperature of the steel is regulated during spraying. The sprayed metal acts both as a source of mass and a source of heat and by adjusting the rate at which metal is sprayed, the surface temperature profile over the surface of the steel shell can be controlled. The temperature profile is measured using a thermal imaging camera and regulated by adjusting the rate at which the guns spray the steel. Because the temperature is regulated by adjusting the feed rate to an actuator that is moving over the surface, this is an example of mobile control, which is a class of distributed parameter control. The paper describes the design of a time-varying H/sub 2/ controller for the process and presents results from the implementation of the controller on the spray forming process.

Control of temperature profile for a spray deposition process

Microstructural features of rapidly solidified powders and preforms of Al80Fe10V4Si6 alloy produced by spray forming process have been studied. The atomization and spray deposition were carried out using a confined gas atomization process and the microstructural features were characterized using scanning electron microscopy and transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The microstructure of a wide size range of atomized powders invariably revealed cellular and dendritic morphology. The extent of dendritic region and the dendritic arm spacing were observed to increase with powder particle size. The TEM investigations indicated the presence of ultrafine second-phase particles in the intercellular or interdendritic regions. In contrast, the spray deposits of the alloy showed considerable variation in microstructure and size and dispersion of the second-phase particles at specific distances from the deposit-substrate interface and the exterior regions of the deposit. Nevertheless, considerable homogeneity was observed in the microstructure toward the center of the spray deposit. The formation and distribution of a cubic phase α-Al(Fe,V)Si has been characterized in both atomized powders and spray deposits. A one-dimensional heat flow model has been used to analyze the evolution of microstructure during atomization and also during spray deposition processing of this alloy. The results indicate that thermal history of droplets in the spray on deposition surface and their solidification behavior considerably influence the micro-structural features of the spray deposits.

Microstructural features and heat flow analysis of atomized and spray-formed Al-Fe-V-Si alloy

The effect of process variables during spray forming of a commercial Babbit alloy containing Pb74–Sn12–Sb11.5–Cu1.25–NiO.75–Cd0.3–As0.2 on its microstructure and wear characteristics were investigated. Variation in atomization gas pressure from 0.6 to 1.2 MPa and nozzle to substrate distance from 0.2 to 0.4 m revealed considerable change in the nature of porosity and microstructural features of the spray deposits. The process variables during spray deposition were optimized to achieve microstructural homogeneity and refinement in second phase particles of this alloy. The wear study of both the spray formed and as-cast alloy under an applied load of 10 to 70 N and sliding velocity of 0.2 to 1.5 ms-1 indicated two distinct regimes of mild and severe wear. In both the regimes, the spray-formed alloy consistently indicated a low wear rate compared to that of the as-cast alloy. In addition, the mild wear regime of the spray-formed alloy was extended to higher load and sliding velocity. Wear characteristics of the spray formed alloy is discussed in light of its microstructural features induced during spray deposition processing.

Microstructural control by spray forming and wear characteristics of a Babbit alloy

This paper discusses the spray forming method for manufacturing alloys and metal matrix composites. In particular, attention is concentrated on available modelling techniques for the follow...

Modelling of Spray Forming

A Lagrangian computer model has been developed to track droplets and particles within a two-dimensional Eulerian framework of resolved gas velocities and temperatures, calculated to represent typical metal matrix composite (MMC) spray-forming conditions. The Eulerian conservation equations governing the two-phase flow were solved using PHOENICS software and the Lagrangian trajectories of 10,80 and 150 μm Al-4.5 wt% Cu droplets were generated as a function of various starting positions within the gas/metal impingement region to model the stochastic nature of gas atomisation. The temperature and solid fraction of droplets was calculated as they moved within the fast-flowing carrier gas and compared with those obtained under a simpler one dimensional exponentially decaying axial gas velocity.

S. Rogers

Papers

A Computer Model for Trajectories and Thermal Profiles of Atomised Droplets in Spray Forming