Design and optimisation of a multi-powder feed system for the HVOF deposition process

Currently no specific method exists for the deposition of High Velocity Oxy-Fuel (HVOF) thermal spray functionally graded coatings. This paper investigates the design and optimisation of a multi-powder HVOF thermal spray device in order to deposit aluminium/tool-steel functionally graded coatings. A multi-powder feed device concept was developed. The concept was based on a stand alone two powder chamber device which integrates with common hopper systems to allow the mixing of two powders during thermal spray deposition. This concept was verified by simulation the design of this device for multi-powder flow using Finite Element Analysis (FEA) to arrive at the optimum dual feed design. The FEA model predicted the mixing and flow of two powders of various ratiosʹ of mass flow rate and velocity based on an optimum designed shape and pressure ratiosʹ of nitrogen gas in the chamber to pick-up shaft of 2.25:1. This yielded the best results in terms of carrying the powders from the mixing zone into the nitrogen gas flow path, inside the pick-up shaft and on towards the HVOF gun. Post finite element analysis the device was manufactured for the utilisation within the HVOF process. Optimisation tests of the device included; powder flow bench tests and HVOF thermal spraying of graded deposits. The results revealed a calibration graph for the two powders in question and the compositional variation across the deposit during functionally graded deposition. The composition of the graded deposits were close to that anticipated hence this showed the suitability of the newly designed multi-powder deposition system in mixing two powders for the purpose of producing HVOF graded coatings.