Shenzhen Wuge Industrial Co., Ltd

Nickel titanium alloy wire is widely used in medical (such as orthodontic wire), aerospace, precision instruments and other fields due to its unique shape memory effect, superelasticity and biocompatibility. The molding process needs to balance material properties and dimensional accuracy. The following are the core production processes and key technical details:

1、 Raw material preparation and melting

1. Raw material ratio and purity

Main materials: high-purity nickel (Ni ≥ 99.9%) and titanium (Ti ≥ 99.5%), with an atomic ratio of Ni: Ti ≈ 55:45. The total amount of impurities (C, O, N, etc.) is ≤ 0.05% to avoid affecting the ductility of the wire material.   

Auxiliary materials: Trace elements (such as Fe to improve corrosion resistance) should be added according to the requirements, and the composition should be ensured to be uniform through vacuum induction melting (VIM) or electron beam melting (EBM).   

2. Smelting process

Under vacuum degree ≤ 10 ⁻ Pa or inert gas (Ar) protection, the raw material is heated to 1500-1700 ℃ and melted. Segregation is eliminated by electromagnetic stirring, and cast into ingots with a diameter of 50-150 mm. After cooling, the riser and shrinkage cavity need to be removed.   

2、 Ingot pretreatment and billet opening

1. Homogenization annealing

Objective: To eliminate dendrite segregation in ingots and refine grain size. Process: Keep at 900-1050 ℃ for 10-20 hours, cool in the furnace to ensure uniform plasticity during subsequent processing.   

2. Surface treatment and billet preparation

Mechanical polishing or acid washing (HF+HNO ∝ mixed solution) is used to remove oxide scale, and the ingot is cut into round bar blanks with a diameter of 10-30 mm for subsequent hot processing.   

3、 Hot processing forming (cutting stage)

1. Hot forging or hot rolling

Hot forging: suitable for large diameter billets (>10 mm), processed by forging hammer or rolling mill at 750-950 ℃ to a diameter of 5-10 mm, with a single deformation of ≤ 30%, to avoid cracking caused by low temperature.   

Hot rolling: used for medium diameter wire (3-8mm), using a multi roll hot rolling machine, temperature controlled at 800-900 ℃, rolling speed of 1-5 m/min, and requiring graphite lubrication to reduce surface scratches.   

2. Hot drawing (optional)

For billets with a diameter ≤ 5 mm, the hot drawing process can be used: the billet is heated to 600-800 ℃ and drawn to 2-5 mm through a hard alloy mold at a drawing speed of 0.5-2 m/s to reduce cold working deformation.   

Nickel titanium alloy wire

4、 Cold drawing forming (precision machining stage)

1. Principle of Cold Drawing Process

Apply tension to the wire material through a mold to reduce its diameter and increase its length, while improving its strength and surface finish. The equipment is a chain drawing machine or a rotary drawing machine, and the mold material is hard alloy or diamond (used for ultra-fine wires, diameter<0.1 mm).   

2. Key process parameters

Deformation amount of each pass: The deformation amount of a single cold drawing is ≤ 15% (to avoid excessive work hardening and fracture), multiple drawing is required, with annealing softening interspersed in between. For example, pulling from 5mm to 0.5mm may require 10-15 passes.   

Lubrication system: Using lime milk, animal fat, or specialized metal lubricants to reduce the friction coefficient (μ ≤ 0.1) and prevent surface abrasion of the wire material.   

Drawing speed: conventional wire (diameter>0.5 mm) speed 5-20 m/min; Ultra fine wire (diameter<0.1 mm) with a speed of ≤ 1 m/min, to avoid performance fluctuations caused by high-speed friction heat generation.   

3. Calibration and straightening

After drawing to the target diameter, the straightness (≤ 0.3 mm/m) and roundness (error ≤ 0.01 mm) of the wire are ensured through precision sizing molds (tolerance ± 0.005 mm) and mechanical straightening devices.   

5、 Heat treatment optimization performance

1. Intermediate annealing (during cold processing)

Purpose: To eliminate work hardening and restore plasticity. Process: Hold at 350-450 ℃ for 0.5-1 hour, air-cooled or water-cooled to reduce the hardness of the wire from HV 400 to below HV 200, facilitating subsequent drawing.   

2. Final time processing (key process)

Purpose: To activate shape memory effect or hyperelasticity and adjust phase transition temperature.   

Process parameters:

Temperature: 400-550 ℃ (such as commonly used medical orthodontic wires at 480-520 ℃), holding time of 0.5-2 hours, cooling method affects phase transition temperature (water cooling can increase Af point);   

Example: The Af point of medical silk needs to be controlled at around 37 ℃ to ensure rapid shape recovery at human body temperature, and the hyperelastic strain needs to be ≥ 8%.   

3. Annealing type selection

Stress relief annealing: Hold at 300-350 ℃ for 0.5 hours to eliminate residual stress, suitable for scenarios with high elasticity requirements (such as springs);   

Recrystallization annealing: Hold at 500-600 ℃ for 1 hour to refine the grain size and increase the elongation rate to over 20%.   

6、 Surface treatment and cleaning

1. Remove the oxide skin

Chemical acid washing: Soak in a mixture of 5%~10% HF+30%~50% HNO3 at room temperature for 3-5 minutes to remove the surface TiO ₂ oxide layer, and then rinse with deionized water;   

Electrolytic polishing: In a phosphoric acid sulfuric acid electrolyte, the surface smoothness is achieved by electrochemical action to Ra ≤ 0.1 μ m (medical grade requirement), reducing tissue irritation during biological implantation.   

2. Surface purification

Ultrasonic cleaning: Use ethanol or acetone to remove lubricant residues, and medical silk materials need to undergo additional sterilization treatment (such as gamma ray irradiation) to ensure biocompatibility.   

7、 Quality inspection and performance verification

1. Size and appearance inspection

Diameter: Laser caliper (accuracy ± 0.001 mm) is used, with ellipticity ≤ 0.005 mm;

Surface: Microscopic observation (50-100 times), no defects such as cracks, burrs, oxidation spots, etc. are allowed.   

2. Mechanical performance testing

Tensile test: Determine tensile strength (≥ 1000 MPa), elongation (≥ 15%), and hyperelastic cyclic stability (strain attenuation ≤ 5% after 1000 cycles);   

Bending test: Test the anti fatigue performance (such as orthodontic wires that need to pass 10 ⁶ bending cycles without fracture).   

3. Shape memory effect detection

DSC (Differential Scanning Calorimetry) is used to measure the temperature of Af and As, with the requirement that the Af of medical silk should be ≤ 37 ℃;   

Memory recovery rate test: After bending and deforming the wire, heat it to Af+10 ℃, and the recovery rate should be ≥ 95%.   

4. Metallographic analysis

Microstructure: Uniform grain size (≤ 15 μ m), without twinning or coarse grains;   

Phase composition: XRD detects the proportion of martensite phase to ensure that austenite phase dominates after aging (medical scenario).   

The forming process of nickel titanium alloy wire is centered on "hot processing billet opening+cold drawing precision forming+aging performance control", which requires precise control of deformation, temperature, and surface treatment to balance the strength, elasticity, and functional characteristics of the material. With the development of minimally invasive medical technology and intelligent devices, their processes are upgrading towards ultra-fine diameter and high uniformity, such as introducing electrospinning technology to prepare nanoscale nickel titanium fibers and expanding their applications in the field of biological scaffolds.