Heat-affected zone analysis for laser-cut and micro-electrical discharge machined nitinol

As the demands for implantable nitinol micro-devices proliferate, so does the need for improvements in strength, fatigue, radiopacity and biocompatibility. Heat-affected zone (HAZ) is a really critical factor in thermal processes as it directly influences the need for finishing processes depending on the intended application. Finishing processes like electropolishing and buffing are expensive hence increasing the cost of production. For nitinol, the HAZ could affect shape memory and/or superelasticity and ultimately part performance. Even though micro-electrical discharge machining (µEDM) and Laser cutting are both thermal processes, their nature of ablation is dissimilar, and so are the involved performance control parameters. Owing to its higher material removal rate, laser cutting has been more predominantly used to machine nitinol but as the demand for better surface finish and closer dimensional tolerances increases, this advantage could be negated and thereby favoring µEDM.

This study analyses both the surface and subsurface of nitinol tubes machined by µEDM and laser cutting and reports on the effect of varying open circuit voltage on the size and distribution of the heat-affected zone in µEDM. This is then compared to HAZ from laser-cut surfaces with an aim of making a case for µEDM as a suitable standalone process for manufacturing nitinol medical micro-parts.

Key words: nitinol, heat-affected zone, µEDM, laser