Investigation on microstructure and properties of laser solid formed low expansion Invar 36 alloy

Abstract

This study elucidated the microstructure, defect formation, mechanical properties and thermal expansion properties of the laser solid formed (LSF) Invar 36. Its macrostructure is composed of irregular columnar dendritic grains grown epitaxially from the bottom up, and the average dendrite spacing gradually decreased as the scanning speed increased. The overlapping curved and horizontal bright white bands (planar growth region) alternated in the neighboring deposited layers. At a scanning speed of 500 mm min−1, cracks were absent, but when the scanning speed was significantly increased or decreased, cracks begin to form. Crack formation at extreme scanning speeds can be attributed to the segregation of impurity elements at the grain boundary and the formation of internal stresses during LSF. When the scanning speed increased from 400 mm min−1 to 800 mm min−1, the tensile strength increased by 22.1% and the elongation increased by 52.1%, and the fracture mechanism was also altered, from dimple plus cleavage fracture to dimple fracture. The coefficient of thermal expansion (CTE) of Invar 36 prepared by LSF is on par with those prepared via conventional processing method, which can be attributed to the presence of Ni in the alloy affecting the overall CTE of the alloy.

Keywords

• Laser solid forming
• Microstructure
• Crack
• Mechanical properties
• Coefficient of thermal expansion