Preparation method of NiTi alloy MIM part

Abstract

The invention relates to a NiTi alloy MIM (metal injection molding) piece preparation method which comprises the following steps: S1, powder preparation: performing ball-milling mechanical mixing on Ni, Ti, Zr and Hf metal powder according to the mass ratio of 58: 38: 2: 2 to obtain mixed powder; the mixed powder is smelted into an ingot, then the ingot is smelted three times in a consumable electrode vacuum furnace, the obtained cast ingot is annealed, the annealed cast ingot is subjected to supersonic gas atomization to form alloy powder, and NiTi alloy powder with the powder particle size D90 smaller than or equal to 20 micrometers is screened out; s2, feed preparation; s3, injection molding; s5, degreasing is conducted; and S6, sintering. Through introduction of Zr and Hf elements, decomposition of a Ni4Ti3 phase can be effectively prevented, and the hardness and fatigue resistance of the NiTi alloy MIM part are effectively improved.

Description

Technical Field

[0001] The invention relates to the field of metal materials and powder injection molding, and in particular to a method for preparing a NiTi alloy MIM part. Background Art

[0002] 60NiTi alloy (Ni:Ti mass fraction ratio is 60:40) has the advantages of low elastic modulus, high hardness, excellent corrosion resistance, non-magnetic, light weight and conductivity, and is considered to be the new aerospace bearing material with the greatest application potential. As early as the 1950s, 60NiTi alloy was successfully developed by the U.S. Naval Ordnance Laboratory, but it was not until after 2000 that its excellent comprehensive properties were gradually discovered by the National Aeronautics and Space Administration (NASA). At present, the most typical application scenario of 60NiTi is the corrosion-resistant bearings for the water purification system of the International Space Station. In recent years, with the rapid development of my country's aerospace technology, the attention and research on 60NiTi alloy are increasing significantly.

[0003] The main components of 60NiTi alloy are Ni4Ti3 phase and B2 phase, which will coarsen during high-temperature service and transform into more stable Ni3Ti2 phase and Ni3Ti phase, resulting in reduced hardness of the alloy. Based on this, when it is necessary to obtain 60NiTi alloy parts with complex structures by powder injection molding, since the 60NiTi alloy powder will undergo a high-temperature service process, the problem of reduced hardness is prone to occur when preparing 60NiTi alloy MIM parts by powder injection molding technology. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing a NiTi alloy MIM part, which can effectively prevent the decomposition of the Ni4Ti3 phase by introducing Zr and Hf elements, and effectively improve the hardness and fatigue resistance of the NiTi alloy MIM part.

[0005] The technical solution to achieve the purpose of the present invention is: the present invention comprises the following steps: S1. Powder preparation: four metal powders of Ni, Ti, Zr and Hf are mechanically mixed by ball milling at a mass ratio of 58:38:2:2 to obtain a mixed powder; the mixed powder is smelted into an ingot, and then smelted three times in a vacuum consumable furnace, the obtained ingot is annealed, and the annealed ingot is then subjected to supersonic gas atomization to form an alloy powder, and a NiTi alloy powder with a powder particle size of D90≤20 microns is screened out; S2, feed material preparation: mixing NiTi alloy powder and binder to prepare feed material; S3, injection molding: using injection molding technology to prepare the green body of NiTi alloy composite material; S4, degreasing: degreasing the green body of the NiTi alloy composite material to form a degreased body; S5. Sintering: Sintering the debinded blank to obtain a sintered part.

[0006] Furthermore, the ball milling mechanical mixing step in the above step S1 is: A. Choose a stainless steel ball mill with double vents; B. Pretreatment: First, take out the stainless steel ball mill and let it stand and cool for 1 hour; use an inflatable pillow to ventilate the stainless steel ball mill. When ventilating, use a rubber tube to connect the inflatable pillow and one vent of the stainless steel ball mill. The other vent of the stainless steel ball mill is connected to the balloon. By squeezing the inflatable pillow, the gas is introduced into the stainless steel ball mill. The gas in the stainless steel ball mill is discharged from the other vent until the balloon swells; each time, the degree of balloon inflation is controlled to control the amount of gas introduced; the gas in the inflatable pillow is filled with N2:O2=9:1~9.5:0.5, and the oxygen content shall not exceed 10%; C. Add four metal powders of Ni, Ti, Zr and Hf into a stainless steel ball mill according to a mass ratio of 58:38:2:2; perform high-energy ball milling under a protective gas atmosphere, the ball milling medium is a zirconia ball with a diameter of 3 mm to 10 mm, the ball milling speed is 300 rpm to 400 rpm, the ball milling time is 15 hours to 20 hours, and finally obtain a mixed powder; D. After taking out the mixed powder, repeat steps A to C to obtain an amount of mixed powder sufficient for subsequent preparation.

[0007] Furthermore, the three-time smelting process in the above step S1 is as follows: the mixed powder is smelted into an ingot, and then the ingot is placed in a vacuum consumable furnace for heating and smelting, and homogenization smelting is performed for about 10 to 15 minutes to obtain a first-smelting ingot, and then the above operation is repeated twice to obtain a thrice-smelting ingot, and then the thrice-smelting ingot needs to be placed in a protective atmosphere, cooled to 400°C, and then air-cooled.

[0008] Furthermore, in the above step S1, the annealing temperature is 1000-1100° C., and the holding time is 24-36 hours.

[0009] Furthermore, the volume ratio of the binder in the above feed is 35%~45%; the components of the binder are: 75%~80% polyoxymethylene, 5%~10% polyethylene, 5%~10% polypropylene, 3%~5% lubricant and 1%~3% antioxidant.

[0010] Furthermore, it also includes step S6, heat treatment; the method of the heat treatment is: placing the sintered blank into a metal furnace for solution treatment, the solution temperature is 1000~1100℃, the insulation time is set to 1~2h, and the sample is water-cooled after the insulation is completed; then the sintered part after the solution is subjected to aging treatment, the temperature is 500~600℃, the insulation time is 2~3h, and air-cooling is performed after the insulation is completed.

[0011] Furthermore, in the above step S4, the green body of the NiTi alloy composite material is subjected to acid degreasing; the conditions for acid degreasing are: N2 atmosphere, ventilation flow rate of 3.5~4.5m3 / h, temperature of 50℃~200℃, time of 4 hours~8 hours; the acid degreasing medium is oxalic acid.

[0012] Furthermore, the technical parameters of sintering in the above step S5 are: the vacuum degree is 10-2Pa~10-4Pa, first the temperature is raised to 800℃ at 3℃ / min and kept for 1h for pre-alloying to discharge the gas in the matrix, and then the temperature is raised to 1300~1500℃ at 2℃ / min and the holding time is 5 hours to 6 hours.

[0013] Furthermore, the technical parameters of the injection molding in step S3 are: mold temperature of 80-100° C., feed heating temperature of 130-180° C., injection pressure of 80-120 MPa; and holding time of 2-3.5 s.

[0014] Furthermore, the sintering temperature T satisfies the following condition: 0.6TL≤T≤0.95TL, wherein TL is the theoretical melting point temperature of the metal or alloy.

[0015] The present invention has positive effects: the present invention can effectively prevent the decomposition of Ni4Ti3 phase by introducing Zr and Hf elements, and effectively improve the hardness and fatigue resistance of NiTi alloy MIM parts. Specifically, in nickel-titanium alloys, impurity elements such as sulfur and phosphorus are easily segregated at the grain boundaries to form low-melting-point inclusions, which affect the precipitation of Ni4Ti3 phases, thereby weakening the grain boundary strength. By adding zirconium elements to the alloy, zirconium elements will preferentially combine with impurities to form stable compounds with high melting points, avoiding the segregation of impurities at the grain boundaries, thereby purifying the grain boundaries, improving the grain boundary strength, and retaining more Ni4Ti3 phases. The Hf element is completely dissolved in the matrix phase and the second phase (Ni4Ti3), which promotes the precipitation of the second phase and significantly improves the high-temperature strength of the alloy. DETAILED DESCRIPTION

[0016] The present invention comprises the following steps: S1. Powder preparation: four metal powders of Ni, Ti, Zr and Hf are mechanically mixed by ball milling at a mass ratio of 58:38:2:2 to obtain a mixed powder; the mixed powder is smelted into an ingot, and then smelted three times in a vacuum consumable furnace, the obtained ingot is annealed, and the annealed ingot is then subjected to supersonic gas atomization to form an alloy powder, and a NiTi alloy powder with a powder particle size of D90≤20 microns is screened out; The ball milling mechanical mixing steps are as follows: A. Choose a stainless steel ball mill with double vents; B. Pretreatment: First, take out the stainless steel ball mill and let it stand and cool for 1 hour; use an inflatable pillow to ventilate the stainless steel ball mill. When ventilating, use a rubber tube to connect the inflatable pillow and one vent of the stainless steel ball mill. The other vent of the stainless steel ball mill is connected to the balloon. By squeezing the inflatable pillow, the gas is introduced into the stainless steel ball mill. The gas in the stainless steel ball mill is discharged from the other vent until the balloon swells; each time, the degree of balloon inflation is controlled to control the amount of gas introduced; the gas in the inflatable pillow is filled with N2:O2=9:1~9.5:0.5, and the oxygen content shall not exceed 10%; C. Add four metal powders of Ni, Ti, Zr and Hf into a stainless steel ball mill according to a mass ratio of 58:38:2:2; perform high-energy ball milling under a protective gas atmosphere, the ball milling medium is a zirconia ball with a diameter of 3 mm to 10 mm, the ball milling speed is 300 rpm to 400 rpm, the ball milling time is 15 hours to 20 hours, and finally obtain a mixed powder; D. After taking out the mixed powder, repeat steps A to C to obtain an amount of mixed powder sufficient for subsequent preparation.

[0017] The three-time smelting process is as follows: the mixed powder is smelted into an ingot, and then the ingot is placed in a vacuum consumable furnace for heating and smelting, and homogenization smelting is performed for about 10 to 15 minutes to obtain a first-smelting ingot, and then the above operation is repeated twice to obtain a third-smelting ingot, and then the third-smelting ingot needs to be placed in a protective atmosphere to cool to 400°C and then air-cooled; S2, feed preparation: NiTi alloy powder and binder are mixed to prepare feed; wherein the ratio of NiTi alloy powder to binder is 1.6; NiTi alloy powder accounts for 66% of the feed by volume, and the rest is binder; the components of the binder include 60% to 70% of polyoxymethylene, 12% to 15% of polyethylene, 15% to 20% of polypropylene and 3% to 5% of stearic acid in terms of mass percentage; S3, placing the feed material in an injection molding machine, and injecting it into a green body of the NiTi alloy composite material through a mold; the injection parameters are: mold temperature of 80-100° C., feed material heating temperature of 130-180° C., injection pressure of 80-120 MPa; holding time of 2-3.5 s; S4. Degreasing the green body of the NiTi alloy composite material to obtain a degreased body; the specific technical parameters of the degreasing are: using N2 atmosphere, the ventilation volume is 4m3 / h, the temperature is 100℃~150℃, the time is 6 hours, the acid flow rate is 2g / min; the acid degreasing medium is oxalic acid; S5. The degreased blank is subjected to high vacuum sintering to obtain a sintered blank. The specific technical parameters of the sintering are: high vacuum sintering is performed in a metal cavity sintering furnace, the pressure in the furnace is ≤1.0×10-3Pa, the sintering temperature is 550°C, and the insulation time is 10h.

[0018] S6. The sintered parts are heat treated in the following ways: the solution temperature is 1100°C, the holding time is 2h, and the sample is water-cooled after the holding period. Then, the sintered parts are aged at 550°C for 3h after the solution treatment, and air-cooled after the holding period.

[0019] The density of the sintered parts after heat treatment is 99.5%, the tensile strength is 700MPa, and the yield strength is 630MPa.

[0020] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a NiTi alloy MIM part, characterized in that The following steps are involved: S1. Powder preparation: four metal powders of Ni, Ti, Zr and Hf are mechanically mixed by ball milling at a mass ratio of 58:38:2:2 to obtain a mixed powder; the mixed powder is smelted into an ingot, and then smelted three times in a vacuum consumable furnace, the obtained ingot is annealed, and the annealed ingot is then subjected to supersonic gas atomization to form an alloy powder, and a NiTi alloy powder with a powder particle size of D90≤20 microns is screened out; S2, feed material preparation: mixing NiTi alloy powder and binder to prepare feed material; S3, injection molding: using injection molding technology to prepare the green body of NiTi alloy composite material; S4, degreasing: degreasing the green body of the NiTi alloy composite material to form a degreased body; S5. Sintering: Sintering the debinded blank to obtain a sintered part.

2. A method for preparing a NiTi alloy MIM part according to claim 1, characterized in that: The ball milling mechanical mixing step in step S1 is: A. Choose a stainless steel ball mill with double vents; B. Pretreatment: First, take out the stainless steel ball mill and let it stand and cool for 1 hour; use an inflatable pillow to ventilate the stainless steel ball mill. When ventilating, use a rubber tube to connect the inflatable pillow and one vent of the stainless steel ball mill. The other vent of the stainless steel ball mill is connected to the balloon. By squeezing the inflatable pillow, the gas is introduced into the stainless steel ball mill. The gas in the stainless steel ball mill is discharged from the other vent until the balloon swells; each time, the degree of balloon inflation is controlled to control the amount of gas introduced; the gas in the inflatable pillow is filled with N2:O2=9:1~9.5:0.5, and the oxygen content shall not exceed 10%; C. Add four metal powders of Ni, Ti, Zr and Hf into a stainless steel ball mill according to a mass ratio of 58:38:2:2; perform high-energy ball milling under a protective gas atmosphere, the ball milling medium is a zirconia ball with a diameter of 3 mm to 10 mm, the ball milling speed is 300 rpm to 400 rpm, the ball milling time is 15 hours to 20 hours, and finally obtain a mixed powder; D. After taking out the mixed powder, repeat steps A to C to obtain an amount of mixed powder sufficient for subsequent preparation.

3. A method for preparing a NiTi alloy MIM part according to claim 1, characterized in that: The three-time smelting process in step S1 is as follows: the mixed powder is smelted into an ingot, and then the ingot is placed in a vacuum consumable furnace for heating and smelting, and homogenization smelting is performed for about 10 to 15 minutes to obtain a first-smelting ingot, and then the above operation is repeated twice to obtain a third-smelting ingot, and then the third-smelting ingot needs to be placed in a protective atmosphere, cooled to 400°C, and then air-cooled.

4. The method for preparing a NiTi alloy MIM part according to claim 1, characterized in that: In step S1, the annealing temperature is 1000-1100° C., and the holding time is 24-36 hours.

5. The method for preparing a NiTi alloy MIM part according to claim 1, characterized in that: The volume ratio of the binder in the feed is 35%-45%; the components of the binder are: 75%-80% polyoxymethylene, 5%-10% polyethylene, 5%-10% polypropylene, 3%-5% lubricant and 1%-3% antioxidant.

6. The method for preparing a NiTi alloy MIM part according to claim 1, characterized in that: The method also includes step S6, heat treatment; the heat treatment method is: placing the sintered blank into a metal furnace for solution treatment, the solution temperature is 1000-1100°C, the insulation time is set to 1-2h, and the sample is water-cooled after the insulation is completed; then the sintered part after the solution is subjected to aging treatment, the temperature is 500-600°C, the insulation time is 2-3h, and air-cooling is performed after the insulation is completed.

7. The method for preparing a NiTi alloy MIM part according to claim 1, characterized in that: In the step S4, the green body of the NiTi alloy composite material is degreased by acid; the conditions for acid degreasing are: N2 atmosphere, ventilation flow rate of 3.5-4.5m3 / h, temperature of 50°C-200°C, time of 4 hours-8 hours; the acid degreasing medium is oxalic acid.

8. The method for preparing a NiTi alloy MIM part according to claim 1, characterized in that: The technical parameters of sintering in step S5 are: vacuum degree is 10-2Pa~10-4Pa, firstly, the temperature is raised to 800℃ at 3℃ / min and kept for 1h for pre-alloying to discharge the gas in the matrix, then the temperature is raised to 1300~1500℃ at 2℃ / min and the holding time is 5h~6h.