Titanium alloy lead screw machining method and titanium alloy lead screw

Abstract

The invention discloses a titanium alloy lead screw machining method and a titanium alloy lead screw. The machining method comprises the following steps that a titanium alloy raw material is preheated, the preheating temperature ranges from 800 DEG C to 1000 DEG C, and after heat preservation is conducted for 1-2 h, cooling is conducted at the speed smaller than or equal to 5 DEG C / min; rough machining is conducted on the preheated material, only the to-be-nitrided surface of the lead screw is formed to the final size, and the raw material state is kept in a non-nitrided area; nitriding treatment is conducted on the to-be-nitrided surface of the lead screw subjected to rough machining; and the non-nitriding area of the nitrided lead screw is subjected to finish machining. The problem of dimensional precision instability caused by nitriding treatment of the titanium alloy lead screw is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lead screw processing, and in particular relates to a titanium alloy lead screw processing method and a titanium alloy lead screw. Background Art

[0002] Titanium alloys are widely used in high-precision moving parts (such as lead screws) due to their high specific strength and corrosion resistance. However, their low surface hardness and poor wear resistance require nitriding to strengthen them. Existing techniques typically perform nitriding (gas or ion nitriding) directly on the finished lead screw surface, requiring a treatment temperature of ≥800°C to produce a titanium nitride hardened layer. However, the thermal conductivity of titanium alloys is only 16% of that of steel. During high-temperature nitriding, heat accumulation leads to localized thermal deformation, causing dimensional deviations (outer diameter deformation of up to 50–60 μm) in the finished part. This irreversible deformation results in high part scrap rates and poor quality stability. Furthermore, the timing conflict between nitriding and finishing in traditional processes (the high surface hardness after nitriding makes further machining difficult, while finishing prior to nitriding is subject to thermal deformation) further limits the precision and lifespan of titanium alloy lead screws. Therefore, a processing method that can simultaneously control the nitriding strengthening effect and thermal deformation is urgently needed to address the dimensional instability caused by nitriding treatment of titanium alloy lead screws. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a titanium alloy lead screw processing method and a titanium alloy lead screw in view of the above-mentioned deficiencies in the prior art, so as to solve the problem of dimensional accuracy instability caused by nitriding treatment of the titanium alloy lead screw.

[0004] A first aspect of the present invention discloses a method for processing a titanium alloy lead screw, comprising the following steps: (1) Preheat the titanium alloy raw material to 800-1000°C, keep it at this temperature for 1-2 hours, and then cool it at a rate of ≤5°C / min; (2) Rough machining of the preheated material, only the surface of the formed screw to be nitrided to the final size, and the non-nitrided area remains in the raw material state; (3) Perform nitriding treatment on the surface of the screw to be nitrided after rough machining; (4) Finish machining the non-nitrided area of ​​the screw after nitriding.

[0005] In the above method, the preheating temperature in step (1) is 800-1000°C, and the cooling method is air cooling or furnace cooling.

[0006] In the above method, the nitriding treatment in step (3) is low-temperature nitriding, specifically: Sodium cyanate (NaCNO) and potassium cyanate (KCNO) were mixed in a mass ratio of 0.8:1, and 2–20 wt% of rare earth cerium oxide (CeO2) was added to form a nitriding agent; Heat at 500-620℃ for 3-10h under nitrogen atmosphere, with a heating rate of 10-30℃ / min.

[0007] The method, wherein the nitriding treatment in step (3) comprises a pretreatment. The surface of the lead screw to be nitrided is ultrasonically deep-rolled and nanoized, with the parameters being: spindle speed 300-600r / min, static pressure 200-600N, and amplitude 10-30μm. Subsequently, the lead screw is subjected to stepwise nitriding in an ion nitriding furnace: first, heat at 400-450℃ for 2-4h, then heat to 540-620℃ for 20-24h, with the furnace pressure being 0.5-1.5kPa, to form a gradient nano-nitrided layer with a thickness of 5.5-6.5μm, wherein the TiN grain size of the surface layer is 50-80nm, and the subsurface layer is 3-20nm.

[0008] The method, wherein a step is added before step (3): a 35-50μm pure titanium layer is prepared on the surface of the lead screw to be nitrided by plasma spraying, the pure titanium layer being a nitriding precursor layer, and the actual thickness of the nitrided layer being 80-150μm after nitriding; the spraying current is 600-700A, and the voltage is 36-40V.

[0009] The method, wherein a step is added before step (3): the pure titanium layer is subjected to nitriding treatment, with the process being: heat at 800-1200℃ for 6-10h under a high-purity nitrogen gas flow rate of 50-150ccm. Subsequently, heat in a vacuum furnace at 400-600℃ for 4-8h, with a cooling rate of 5-15℃ / min.

[0010] The method, wherein the titanium alloy is TC4 titanium alloy, and the components are as follows in terms of mass percentage: Al percentage 5.5-6.8wt%, V percentage 3.5-4.5wt%, Fe percentage 0.1-0.3wt%, C percentage ≤0.1wt%, and the balance being Ti.

[0011] The method, wherein the microhardness of the gradient nano-nitrided layer satisfies the following formula: H(d)=H0+k×e −λd ; wherein H(d) is the hardness value (HV) at a depth d (μm) from the surface, H0 is the base hardness (HV 300-350), k is the hardness increment constant (400-450), and λ is the attenuation coefficient (0.05-0.1 μm⁻¹).

[0012] The method, wherein k and the stepwise nitriding holding times t1 (low-temperature segment) and t2 (high-temperature segment) satisfy the following formula: k=50×ln(t1×t2)-100.

[0013] The method prepared by any method of the first aspect has a nitriding layer surface hardness of ≥785 HV and a non-nitriding area roughness Ra of ≤0.1 μm.

[0014] Compared with the prior art, the present application has the following advantages: 1. The preheating temperature is 800-1000℃ (usually ≥800℃), and the material is cooled at a slow cooling rate of ≤5℃ / min after being kept at temperature for 1-2h. This design allows the material to complete thermal deformation before rough machining, offsetting the thermal stress of subsequent nitriding, thereby significantly inhibiting the deformation problem after finishing.

[0015] 2. The threads of the screw rod and other regions requiring nitriding are machined to the final size, and the non-nitriding regions are kept in the original material state (such as bar or forged piece), reducing the heat affected zone during subsequent nitriding. The non-nitriding regions are finished after nitriding, avoiding damage to the nitriding layer due to secondary machining, while ensuring the dimensional accuracy of the non-nitriding regions. DETAILED DESCRIPTION

[0016] A titanium alloy screw rod machining method, comprising the following steps: (1) Preheating the titanium alloy raw material, the preheating temperature is 800-1000℃, and the material is cooled at a rate of ≤5℃ / min after being kept at temperature for 1-2h; (2) Rough machining the preheated material, only forming the screw rod to the final size of the surface to be nitrided, and keeping the non-nitriding regions in the original material state; (3) Nitriding the surface to be nitrided of the rough machined screw rod; (4) Finishing the non-nitriding regions of the nitrided screw rod.

[0017] In this embodiment, the preheating temperature in step (1) is 800-1000℃, and the cooling method is air cooling or furnace cooling.

[0018] Specifically, the TC4 titanium alloy bar is preheated in an 850℃ furnace for 90 minutes, and cooled to room temperature with the furnace (cooling rate of 4℃ / min). A numerical control lathe is used to turn only the thread surface of the screw rod to the designed size, leaving a 2mm allowance at the shaft end. After 580℃×5h nitriding of the thread surface in a nitriding furnace, the shaft end is ground to Ra 0.08μm using a grinding machine.

[0019] The above scheme can avoid damage to the finished surface by nitriding treatment, and the thread surface hardness is increased to 800HV.

[0020] In this embodiment, the nitriding treatment in step (3) is low-temperature nitriding, specifically: Mix sodium cyanate (NaCNO) and potassium cyanate (KCNO) at a mass ratio of 0.8:1, add 2-20wt% of rare earth cerium oxide (CeO2) to form a nitriding reagent; Heat at 500-620℃ for 3-10h under nitrogen atmosphere, with a heating rate of 10-30℃ / min.

[0021] For example, mix 90g of sodium cyanate with 100g of potassium cyanate, add 38g of cerium oxide powder (20wt% in total), and ball mill for 2h. Immerse the lead screw into the nitriding agent, replace the air with nitrogen, and then heat to 580℃ at a rate of 25℃ / min and maintain for 7h.

[0022] The above scheme can promote the diffusion of nitrogen atoms, increase the nitriding speed by 40%, and improve the density of the compound layer.

[0023] In this embodiment, the nitriding treatment in step (3) includes a pretreatment: Ultrasonic deep rolling nanocrystallization is performed on the surface of the lead screw to be nitrided, with the following parameters: spindle speed of 300-600r / min, static pressure of 200-600N, and amplitude of 10-30μm. Subsequently, the lead screw is subjected to stepwise nitriding in an ion nitriding furnace: first at 400-450℃ for 2-4h, then heated to 540-620℃ for 20-24h, with a furnace pressure of 0.5-1.5kPa, to form a gradient nanonitrided layer with a thickness of 5.5-6.5μm, wherein the surface layer has TiN grains with a size of 50-80nm, and the subsurface layer has grains with a size of 3-20nm.

[0024] In specific operation, an ultrasonic rolling head is installed on a modified lathe to apply a static pressure of 450N to the thread surface of the lead screw, the spindle is rotated at 500r / min, and the ultrasonic transducer generates mechanical vibrations with an amplitude of 20μm. After treatment, the lead screw is subjected to ion nitriding in a furnace at 420℃ for 3h to form a transition layer, and then at 580℃ for 22h for further nitriding. The above scheme can form a 6μm nitriding layer, with a surface layer of 80nm TiN grains for wear resistance and a subsurface layer of 15nm grains for anti-peeling.

[0025] In this embodiment, a step is added before step (3): a 35-50μm pure titanium layer is prepared on the surface of the lead screw to be nitrided by plasma spraying, and the pure titanium layer serves as a nitriding precursor layer, with an actual nitrided layer thickness of 80-150μm after nitriding; the spraying current is 600-700A, and the voltage is 36-40V.

[0026] In specific operation, a plasma spray gun is used to reciprocally spray on the surface of the lead screw, with a current of 650A, a voltage of 38V, and a spraying distance of 150mm to form a 45μm thick pure titanium layer. After each pass of spraying, air cooling is performed to avoid overheating of the substrate. The pure titanium layer serves as a nitrogen diffusion buffer layer to eliminate nitriding cracks.

[0027] In this embodiment, a step is added before step (3): nitriding treatment is performed on the pure titanium layer, with the following process: heat at 800-1200℃ for 6-10h under a high-purity nitrogen gas flow rate of 50-150ccm. Then, heat preservation in 400-600℃ vacuum furnace for 4-8h, cooling rate 5-15℃ / min.

[0028] In this embodiment, the titanium alloy is TC4 titanium alloy, and the components are as follows in percentage by mass: Al percentage 5.5-6.8wt%, V percentage 3.5-4.5wt%, Fe percentage 0.1-0.3wt%, C percentage ≤0.1wt%, and the balance is Ti.

[0029] In this embodiment, the microhardness of the gradient nanonitriding layer satisfies the following formula: H(d)=H0+k×e −λd ; Wherein, H(d) is the hardness value (HV) at the depth d (μm) from the surface, H0 is the substrate hardness (HV 300-350), k is the hardness increment constant (400-450), and λ is the attenuation coefficient (0.05-0.1 μm⁻¹).

[0030] In specific operation, when H0=320HV (substrate hardness), k=430, and λ=0.08, the hardness at the depth of 2μm from the surface is 320+430×e^(-0.08×2)≈690HV, and the hardness at the depth of 5μm from the surface is 320+430×e^(-0.08×5)≈520HV. The value of k is controlled by adjusting the heat preservation time t1 / t2 to realize the customization of hardness gradient.

[0031] In this embodiment, k and the nitriding section heat preservation time t1 (low temperature section) and t2 (high temperature section) satisfy the following formula: k=50×ln(t1×t2)−100.

[0032] A titanium alloy screw rod is prepared by the titanium alloy screw rod processing method, and the surface hardness of the nitriding layer is ≥785 HV, and the roughness Ra of the non-nitriding area is ≤0.1μm.

[0033] It should be noted that the surface hardness of the nitriding layer is 810HV measured by a Vickers hardness tester; and the Ra of the non-nitriding area is 0.07μm measured by a white light interferometer.

[0034] It should be further noted that the finishing must be performed after nitriding to avoid the influence of thermal deformation on the dimensional accuracy.

[0035] The above is only a preferred embodiment of the present application, and does not limit the present application in any way. Any simple modification, change, and equivalent structural change made according to the technical essence of the present application to the above embodiment are still within the protection scope of the technical solution of the present application.

Claims

1. A titanium alloy lead screw processing method, characterized in that: The following steps are involved: (1) Preheat the titanium alloy raw material to 800-1000°C, keep it at this temperature for 1-2 hours, and then cool it at a rate of ≤5°C / min; (2) Rough machining of the preheated material, only the surface of the formed screw to be nitrided to the final size, and the non-nitrided area remains in the raw material state; (3) Perform nitriding treatment on the surface of the screw to be nitrided after rough machining; (4) Finish machining the non-nitrided area of ​​the screw after nitriding.

2. The method according to claim 1, wherein The preheating temperature in step (1) is 800-1000°C, and the cooling method is air cooling or furnace cooling.

3. The method according to claim 1, wherein The nitriding treatment in step (3) is low-temperature nitriding, specifically: Sodium cyanate and potassium cyanate are mixed in a mass ratio of 0.8:1, and 2–20 wt% of rare earth cerium oxide is added to form a nitriding agent; Heat at 500–620°C under nitrogen atmosphere for 3–10 h, with a heating rate of 10–30°C / min.

4. The method according to claim 1, wherein The nitriding treatment in step (3) includes pretreatment: Ultrasonic deep rolling nano-crystallization was performed on the surface of the screw to be nitrided, with the following parameters: spindle speed 300–600 r / min, static pressure 200–600 N, amplitude 10–30 μm; Subsequently, the material was nitrided in stages in an ion nitriding furnace: first, the temperature was kept at 400–450°C for 2–4 hours, then the temperature was raised to 540–620°C and the temperature was kept at 20–24 hours. The furnace pressure was 0.5–1.5 kPa, and a gradient nano-nitrided layer with a thickness of 5.5–6.5 μm was formed. The TiN grain size of the surface layer was 50–80 nm, and that of the sub-surface layer was 3–20 nm.

5. The method according to claim 1, wherein Add a step before step (3): prepare a 35-50 μm pure titanium layer on the surface of the screw to be nitrided by plasma spraying, wherein the pure titanium layer is a nitriding precursor layer, and the actual nitrided layer thickness after nitriding is 80-150 μm; Spraying current 600–700A, voltage 36–40V.

6. The method according to claim 5, wherein Add a step before step (3): perform nitriding treatment on the pure titanium layer, the process is: keep it at 800-1200℃ and high-purity nitrogen flow rate of 50-150ccm for 6-10h; Then, the samples were kept in a vacuum furnace at 400–600 °C for 4–8 h with a cooling rate of 5–15 °C / min.

7. The method according to claim 1, wherein The titanium alloy is TC4 titanium alloy, and its components by mass percentage are: Al percentage is 5.5-6.8wt%, V percentage is 3.5-4.5wt%, Fe percentage is 0.1-0.3wt%, C percentage is ≤0.1wt%, and the balance is Ti.

8. The method according to claim 4, wherein The microhardness of the gradient nano-nitride layer satisfies the following formula: H(d)=H0+k×e −λd ; Where H(d) is the hardness value at a depth d from the surface, H0 is the matrix hardness, k is the hardness amplification constant, and λ is the attenuation coefficient.

9. The method according to claim 8, wherein k and the nitriding segmented holding time t1 and t2 satisfy: k = 50 × ln (t1 × t2) − 100.

10. A titanium alloy lead screw, characterized in that: Prepared by the method of any one of claims 1-9, the surface hardness of the nitrided layer is ≥785 HV, and the roughness of the non-nitrided area is Ra ≤0.1 μm.