Shape memory alloys (SMAs) are materials that can regain their original shape after being deformed when heated to a specific temperature. Nickel-titanium alloy, discovered in the 1960s, is the most widely used and practical SMA. It has found applications in aerospace, electronics, and medicine, among other fields.
To reduce costs and facilitate production, research on memory alloy capillary tube processing focuses on existing methods. Special considerations are given to the nature of capillary tube processing when selecting process routes. Controlling impurity content is important for improving process ductility in nickel-chromium alloy billets. Cold working and annealing are commonly used as heat treatment methods to ensure tubing material surface quality.
Raw materials: High-purity titanium and nickel metals
Processing equipment: Powder metallurgy equipment, precision cutting machine, cold drawing machine, welding equipment, surface treatment equipment, and cleaning equipment. This equipment can be selected and configured according to specific cost requirements and process flow.
Main Processes:
High-frequency vacuum induction melting → hot forging and hot rolling → machining and punching to produce extrusion billet → medium temperature hydrostatic extrusion to produce pipe billet → annealing → multiple passes of rolling with intermediate annealing → multiple passes of long core rod stretching → finished product.
Results and Discussion:
- Material Selection: The nickel-titanium capillary tubes are mainly used in cardiac stents, so the recovery temperature needs to be within the normal range of the human body. The shape memory effect of iron-nickel memory alloys is influenced by material composition, impurity content, processing, and heat treatment. Among these factors, the composition factor has the most significant impact on the recovery temperature.
- Hydrostatic extrusion: It is beneficial for processing challenging materials for two reasons. Firstly, conventional extrusion methods can be used for shape memory alloy tube processing, but the resulting billets are larger due to mold and extrusion force considerations. Secondly, capillary tube processing has strict requirements for billets, and issues can arise during this stage. Hydrostatic extrusion utilizes high-pressure lubrication and three-way hydrostatic pressure to obtain optimal billets.
- Cold rolling: It is a common method for producing precision tubes. Currently, only this method can obtain cold-drawn billets suitable for tube diameter and wall thickness in China. The billets obtained from hydrostatic extrusion can undergo simple surface treatment before cold rolling.
- Long core rod drawing: It is preferred over hollow drawing as it maximizes plasticity, limits deformation, and prevents cracks and fractures. It is an ideal method for obtaining high-quality capillary tubes. Heat treatment and dimensional accuracy control are important during processing. Annealing is required between each pass due to the hardening rate, even though long-core rod drawing allows for a higher processing rate.
The research on the fabrication process of high-precision nickel-titanium shape memory alloy capillaries is feasible. The key challenge is to address surface cracks in the fabricated tubes. To ensure high-quality fabrication, the method of hot rolling billet reduction can be used for long-core rod stretching. Accurate control of cold working spring back is necessary for capillaries made of a nickel alloy.
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