Modelling of billet shapes in spray forming using a scanning atomizer

Abstract

A numerical method is presented to predict and analyze the shape of a growing billet produced from the ‘spray forming’ which is a fairly new near-net shape manufacturing process. It is important to understand the mechanism of billet growth because one can obtain a billet with the desired final shape without secondary operations by accurate control of the process, and it can also serve as a base for heat transfer and deformation analyses. The shape of a growing billet is determined by the flow rate of the alloy melt, the mode of nozzle scanning which is due to cam profile, the initial position of the spray nozzle, scanning angle, and the withdrawal speed of the substrate. In the present study, a theoretical model was first established to predict the shape of the billet and next the effects of the most dominant processing conditions, such as withdrawal speed of the substrate and the cam profile, on the shape of the growing billet were studied. Process conditions were obtained to produce a billet with uniform diameter and flat top surface, and an ASP30 high speed steel billet was manufactured using the same process conditions established from the simulation.

Introduction

Spray forming is a fairly new near-net shape manufacturing process in which a bulk molten metal is converted to a spray of droplets and is deposited to a substrate to fabricate alloys of various shapes such as strips, tubes, and billets [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. In the spray forming processes, since droplets are made by atomizing gas and the nucleation occurs inside the individual droplets while they are propelled away from atomizer and fly onto the substrate, a preform of fine equiaxed grain structure sized 10–100 μm with essentially no macroscopic segregation can be obtained and therefore it allows alloy homogenizing heat treatments to be avoided or shortened [9], [11], [12], [13], [14], [15]. Spray forming offers potential benefits in comparison with conventional casting and powder metallurgy (PM) processes in that different deposit geometries can be produced by varying the substrate configuration and motion. It allows the multiple steps in PM operations of powder production, sieving, canning, degassing and consolidation to be reduced into a single integrated process, while maintaining microstructural characteristics of PM type in near-net shapes. By adding particulate ceramics into the gas stream during spraying, metal matrix composite (MMC) preforms with uniformly distributed ceramic particles can be produced, and furthermore fast solidification time during spray forming can prevent detrimental reactions which are often observed in casting processes for producing MMCs. In fact, those detrimental reactions at the metal–ceramic interface are caused by extended contact times between the molten matrix alloy and the particulate reinforcement in conventional casting and they deteriorate the mechanical properties of the final products. Moreover, a wide range of alloys such as Al-, Pb-, Cu-, Mg-, Ni-, Ti-, Co-, based alloys, and steels which are difficult to produce by other methods can be manufactured through spray forming [9], [11], [16].

However, spray forming has several drawbacks to overcome to be a competitive industrial manufacturing process. Among them material losses are the most notable ones and this is explained by the fact that the efficiency of feedstock conversion to final product is usually below 100%. Some spray droplets do not arrive at the target, and some bounce off from the surface of the preforms. Some portion of the products are removed by machining operations to obtain the desired final shape. Therefore the spray forming process must be designed such that the material loss is minimized.

Related with billet fabrication, a cam mechanism is utilized for scanning the gas atomizer and the purpose of scanning is to acquire uniform spray density and enthalpy across the preform surface by appropriate movement of the spray. Also, by designing the scanning cam profile properly preforms of various shapes can be produced. The rotating substrate where spray droplets are deposited is withdrawn vertically to keep constant distance between the gas atomizer and preform top surface and it leads to steady state deposition of spray droplets. Fig. 1 illustrates a typical spray forming process to produce a billet which is to be modeled by the present study.

Among the researchers who have pioneered modelling and analysis of spray forming process, we particularly mention the work of Grant et al., Annavarapu et al., and Mathur et al., [8], [9], [11], [12], [13], [16], [17], [20]. These and other authors [8], [9], [10], [11], [18], [19], [25] developed computer models to describe the in-flight dynamic and thermal histories of gas atomized droplets as a function of distance during spray forming. They also predicted solid fraction of each droplet by measuring gas velocities and droplet size distributions, and computed the solid fraction and temperatures of the preform top surface. Some research related to microstructural evolution of spray formed strip has been performed by Annavarapu et al., [12], [13], [21], [22], [23], and specifically, they investigated the effects of the flight distance, the substrate surface condition, the uniformity of droplet flux in the spray cone, and substrate motion upon the preform microstructures. Mathur et al., [8], [11], [24] developed a mathematical model to describe the interconnected processes of droplet-gas interactions in flight and subsequent droplet consolidation on the substrate. They demonstrated the importance of mathematical modelling to establish quantitative guidelines for optimizing the evolution of microstructures in droplet consolidation. The work of Cai and his colleagues [26], [27], [28] should also be noted. They carried out shape modeling in a manner similar to that of the present study though they did not include the scanning motion of the gas atomizer. They also presented experimental verification of the shape model and used the model to study the sticking efficiency.

The spray forming process is governed by many processing conditions, and therefore it is of importance to understand the effects of such conditions upon the process. It is essential to examine the preform growing mechanism because this provides useful information for heat transfer and deformation analyses, and microstructure control, especially when the shape of the preform is three dimensional, e.g. billets. Also, by accurate control of the process, the desired final shape without secondary cutting operations can be produced. The shape of a growing billet is determined by the flow rate of the alloy melt, the mode of nozzle scanning which is due to cam profile, the initial position of the spray nozzle, scanning angle, and the withdrawal speed of the substrate. In the present study, a theoretical model was first established to predict the shape of the billet and next the effects of the most dominant processing conditions, such as withdrawal speed of the substrate and the cam profile, on the shape of the growing billet were studied. Process conditions were obtained to produce a billet with uniform diameter and flat top surface, and an ASP30 high speed steel billet was manufactured using the same process conditions established from the simulation.