Analytical Heat Transfer Model to Predict Friction Surfacing Process Parameters

Abstract

The objective of this research is to develop a low-order heat transfer model that can rapidly predict the deposition temperature during friction surfacing. This is motivated by the need for tools that can help select process parameters that will result in a uniform deposit with a strong bond to the substrate beneath it. Friction surfacing (FS) is a solid-state metal additive manufacturing technique that can deposit any similar and dissimilar metal alloy on existing CNC machine tools: i.e., no special tooling or machine is required. It involves rotating a consumable tool (solid rod), plunging it into a surface and traversing while continuing to plunge. When the correct process parameters are selected, in our experience, a 1-2 mm thick deposit is created on the surface. This is a hot-working process where friction and plastic deformation play a key role in generating heat and shearing / moving material, all while keeping the process temperature below the solidus temperature of the materials. In this work, a steady-state, zero-dimensional analytical model is developed for friction surfacing to predict the rubbing interface temperature, which will assist in process planning. The model considers the contributions of friction and plasticity as sources of heat input, conduction into the consumable rod and substrate, as well as advection of material into the flash and deposit. A comprehensive description of the analytical heat transfer model is provided, where a thermal resistance network is utilized to represent the heat transfer processes both into and out of the control volume. The process temperature profiles for varying parameters, including rotational speed, travel speed, and single-layer versus multi-layer deposits, are systematically investigated, with experimental and numerical results for single-layer deposits showing a maximum difference between measured and numerical of 128 K (for interface temperature of 1429 K) for 150 mm/min feed rate at 2000 RPM, whereas the minimum difference was 39 K (for interface temperature of 1584 K) for feed rate of 120 mm/min at 4000 RPM. The multi-layer deposit analysis is based solely on numerical studies. The effects of spindle speed, travel speed, and single- and multi-layer deposits are explored. A sensitivity analysis of the model is performed and finds that heat power

input, axial force, and temperature-dependent friction coefficient have the greatest influence on the results. Spindle speed and traverse feed rate, which are process parameters directly controlled by the operator in feed control mode, change the predicted temperature by only 32 K and 13 K, respectively, when varied by ± 10%. To maintain the same temperature for Layer 1 as predicted for Layer 5, the spindle speed must be increased by 50%. This adjustment compensates for the temperature differences between the two layers. Future work will include conducting multi-layer tests to test the hypothesis that when friction surfacing in feed control mode, maintaining a constant deposition temperature will results in a uniform deposit morphology.