Simulations of Temperatures, Residual Stresses, and Porosity Measurements in Spray Formed Super Alloys Tubes

Abstract

Spray Forming is a technology to produce near net shape components and preforms. A simulation tool for the temperature distribution which incorporates all necessary sub steps (metal deposition on the substrate, heat transfer across the surface by convection and radiation) is developed to calculate residual stresses during the different process steps. The resulting temperature distribution was used to calculate the stresses during all process stages. Thermal histories of temperatures at certain positions will be shown. The dependency of the residual stress on the thermal history of the material was examined. Mainly at the interface substrate/deposit, a region with elevated porosity was observed. Porosity measurements will be presented. Introduction Spray forming combines two distinct processes. In the first step, liquid metal is atomized into a spray cone of droplets. The impinging droplets form a near net shape product on a substrate in the second step. In contrast to spray formed billets, (in which the surface temperature is mostly constant) the substrate surface temperature just ahead of the spray cone is a strong function of time. Therefore, the obtained material density or the porosity is a function of the surface temperature. Porosity measurements of super alloy rings were reported in [1]. The results show that at temperatures above 1100°C, low porosity is measured. High convection coefficients in the vicinity of the spray cone cause a temperature increase for initially cold substrates and a temperature drop for initially hot substrates. Mathematical models for the temperature distributions in the spray formed tubes were created [2, 3, 4, 5, 6]. A detailed analysis of heat exchange phenomena on small time scales is given by [3]. The temperature distribution in substrate and deposit act as a load for the developing residual stresses. Due to the difficulties in simulating stresses in spray formed deposits, only a few papers have been presented [7, 8, 9]. Mechanical properties for IN718 are reported for temperatures up to 1100°C [10]. The uncertainties in the mechanical properties cover a small temperature range, from 150K to the solidus temperature of the material. The resulting stresses are mainly affected by thermal strains. Specifically, they depend on the heating and cooling rates during the spray process, and later on post spray conditions. Therefore, the knowledge of the temperature distribution during the spray forming process, as well as the behavior of temperature during heat treatment and cooling processes is of great importance in the understanding of the development of residual stresses.