Abstract
Nitinol is a shape memory and superelastic alloy, respectively, and stainless steels are widely used materials in medical engineering, e.g., for implants and medical instruments. However, due to its high price and poor machinability, there is a high demand for dissimilar welding of nitinol components to stainless steel. During welding of titanium-containing alloys, like nitinol, to ferrous metals like stainless steel, intermetallic phases between titanium and iron may form. These phases are brittle and lead to rapid crack formation and/or inferior mechanical properties of the joint. In this study, superelastic nitinol wires are butt-welded with stainless steel wires by means of micro electron beam welding, providing good quality weld seams. Due to a very accurate beam alignment and fast beam deflection, the composition and the level of dilution in the weld metal can be precisely controlled, resulting in a significant reduction of fraction of intermetallic phases. The experiments show that it is possible to produce sound welds without the presence of any cracks on the surface as well as in the cross sections.
Highlights
Nitinol is a nearly equiatomic alloy, consisting of nickel and titanium
The present results clearly reveal that it is possible to use the micro electron beam technology to join nitinol to stainless steel wires with a diameter of 0.5 mm, avoiding any process-induced cracks
The strength values determined are very similar to values obtained by laser welding using nickel filler material, where the related nickel release, is a detrimental factor hindering any use in medical applications
Summary
Nitinol is a nearly equiatomic alloy, consisting of nickel and titanium. It acts as a shape memory or a superelastic alloy, depending on its exact composition and heat treatment. Superelastic alloys like SE508 are often applied, taking advantage of the high elastic deformation of up to 8% at a stress level between 400 and 500 MPa [1,2,3]. Due to numerous contradictory requirements, e.g., regarding superelasticity, deformability, tensile strength, or for cost reasons, it is often necessary to join different materials for specific purposes. Whenever possible, welding is the process of choice as other joining methods have several drawbacks. Mechanical joining methods like screwing or crimping do not provide for gap-free parts, so that these parts are hard to sterilize and crevice corrosion can occur. The strength of the joint is much lower as compared to the base material [3]
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