Numerical investigation of alternative process conditions for influencing the thermal history of spray deposited billets

In spray forming, multiple atomizers may be used simultaneously to increase the throughput and to spray large size billets. In addition, the advantage of the use of multiple sprays is better control of the distribution of the incoming mass and enthalpy fluxes on the deposit surface. By this, the thermal condition in the top layer of the billet is controlled to ensure steady-state growth and cooling of the deposit. The interaction of two overlapping sprays is described in this paper. The mass flux and the enthalpy distribution have been experimentally investigated for different spray conditions and overlapping angles of the two sprays. Starting from a geometric model, a three-dimensional simulation of the preform geometry and incoming enthalpy flux on the top layer of the deposit has been conducted. The most relevant process parameters and physical properties are taken into account in this model. The model results are in good agreement with experimental preform shapes.

Analysis of deposit growth in spray forming with multiple atomizers

Spray forming is a casting process in which the molten metal is directly converted to a solid bulk with unique characteristics. When processed under optimum conditions, spray formed materials typically present microstructures composed of refined polygonal (non-dendritic) grains, uniformly distributed with low levels of micro- and macro-segregation. This set of characteristics is achieved regardless of the alloy system, making spray forming an attractive process to produce alloys where processing by conventional casting techniques is complicated. This chapter is dedicated to presenting the mechanisms that take place when the atomized droplets arrive at the deposit surface, and how the spray-formed microstructures evolve during deposition. It will be seen that spray forming is a self grain-refining casting process and cannot be considered a rapid solidification technique. Section 7.6 will address the main differences between the microstructural evolution in spray forming and other spray deposition or “thermal spray” processes. These processes include plasma spraying, high velocity oxy-fuel, wire arc spraying, detonation gun spraying, etc. In this way, it will show why spray forming is such a unique process. This chapter is also dedicated to presenting how the porosity—an intrinsic feature of spray-formed microstructures—is generated and how the processing parameters affect its type, size and distribution. Furthermore, the generation of other defects related to the solidification and/or to the cooling of the spray formed product after deposition—such as residual stresses and hot cracks—and their influence on the product quality and material properties will be presented. Finally, this chapter will also discuss the effect of the atomization gas (Ar, N2 or He) on the final product quality in terms of porosity and chemical composition of steels, superalloys, and copper alloys.

Microstructural Evolution in Spray Forming

During the production of large diameter spray formed copper and alloy steel billets, hot cracks were observed at the head with residual melt present during crack initiation. Temperature simulations were carried out to investigate the hot cracking phenomenon in the spray forming process. A commercial finite element program was used for the calculations with boundary conditions and assumptions based on experimental results. The temperature history of the billet was calculated during and after spraying. It is shown that different spray parameters influence the radial thermal gradient, especially at the top of the billet; reduced heat input at the top of the billet decreases the thermal gradient. In the experiments hot cracks are reduced or avoided with a lower heat input.

S. Hansmann

Papers

Thermal cracking in large diameter spray formed billets