Induction Melting of NiTi Shape Memory Alloys – The Influence of the Commercial Crucible Graphite on Alloy Quality

✍ Scribed by J. Frenzel; K. Neuking; G. Eggeler

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 358 KB
• Volume: 35
• Category: Article
• ISSN: 0933-5137
• DOI: 10.1002/mawe.200400725

✦ Synopsis

Abstract

The present work studies the performance of three different kinds of commercial crucible graphite materials used during inductive melting of NiTi‐shape memory alloys (SMAs).

One low‐density extruded graphite and two isostatically pressed graphite‐materials with different porosity and purity were used for melting NiTi‐SMAs from pure Ni and Ti raw materials. The use of a low grade, lower density graphite crucible led to a high carbon contamination of the alloy. Much better results were obtained using graphite crucibles with higher density and higher purity. But in all cases, the first use of a graphite crucible leads to a higher carbon pickup than the second time of utilization. This can be explained by the formation of a protective TiC rich layer on the surface of the crucible during its first contact with the NiTi melt which acts as a diffusion‐barrier later on. Furthermore, attention has to be paid to the thermal conductivity of the graphite. Using a material with a low thermal conductivity may lead to the formation of cracks at the pouring‐lip of the crucible during casting. This may lead to spalling off of crucible material and can enhance the risk of non‐metallic inclusions in the alloy.