Influence of laser absorption by water- and gas-atomised powder feedstock on Laser Metal Deposition of AISI 431 stainless steel

Abstract

This research explores the importance of the powder feedstock characteristics on the manufacturing of martensitic stainless steel elements by Laser Metal Deposition (LMD). Water- and gas-atomised (WA and GA respectively) powders are the most common feedstock material used in LMD. The atomisation process medium influences powder properties including particle size distribution, inner and outer morphology, and microstructure. In this work, commercial WA and GA powders of AISI 431 stainless steel alloy were used to produce LMD coatings under identical deposition parameters. It was found that the powder processing route significantly affected the powder’s morphology, and this led to large changes in the laser energy absorbance during LMD. Consequently, significant changes in the microstructure of the deposited layer were observed. The parts manufactured with WA powder presented a duplex structure composed of martensite and ferrite, while a predominantly martensitic structure was verified in the GA deposition. The microstructural differences led to a wear rate reduction by a factor of 35 and a gain of 150 mV in pitting potential in the GA material. Though, the application of GA powder increased the production cost by 53% due to its lower deposition efficiency and higher retail price. The marked differences have all been attributed to the characteristics of the starting powder feedstock. This work highlights that the properties of the powder feedstock are a critical parameter in determining the properties of the LMD material, and emphasises the importance of the powder processing route on the energy absorption during LMD. • Water-atomised powder absorbs more laser energy compared to gas-atomised powder. • Higher laser energy absorbance resulted in a duplex microstructure for the water-atomised AISI 431 stainless steel. • Lower laser energy absorbance resulted in a fine martensitic microstructure with improved wear and corrosion performance for the gas-atomised AISI 431 stainless steel.