Effect of scan speed on grain and microstructural morphology for laser additive manufacturing of 304 stainless steel

Abstract

The laser scanning parameters used in laser additive manufacturing (LAM) can impact the growth direction of the columnar grains produced during rapid solidification. This growth direction affects the mechanical properties of the manufactured parts after cladding. The effective use of a high laser power and a rapid scanning speed for LAM requires an accurate analysis of the relationships between the laser scanning process parameters and the grain growth direction and microstructural morphology of the scanned material. An experimental study was conducted to determine the macromorphology, microstructural morphology, and grain growth direction of 304 stainless steel material obtained during the laser scanning process at different scan speeds. The impact of the scan speed on different regions in the cladding layer (the clad zone (CZ), the heat affected zone (HAZ), and the dilution zone (DZ)) was determined, as well as on the direction of grain growth, the grain morphology and the grain size (which are the microstructures of the cladded materials), the degree of elemental mixing during laser scanning (which are changes in material composition of cladded material), and the microscopic hardness of the CZ (which is one of the mechanical properties of cladded material). With increasing scan speed, the CZ, HAZ, DZ, and dilution rate (DR) of the material gradually decreased, and grain growth gradually oriented towards the building direction of the cladding layer. At a 16 mm·s−1 scan speed, the angle between the grain growth direction and the scan direction was 84°. Changing the scan speed from 4 to 16 mm·s−1 caused the columnar grain size to gradually decrease from 13.3 to 9.2 μm and the corresponding microhardness to gradually increase.

Keywords

• laser additive manufacturing
• stainless steel
• laser scan speed
• the grain growth
• microstructural morphology
• dilution rate
• microhardness