A surface strengthening method for increasing thickness of gradient deformation layer on surface of titanium alloy

Abstract

The present application relates to a kind of surface strengthening methods for increasing the thickness of titanium alloy surface gradient deformation layer, using high-density electric pulse assisted ultrasonic surface roll strengthening technology, for the problem of surface plastic deformation layer thin caused by the limited work hardening ability of titanium alloy, by simultaneously applying high current density in the range of 5.09 A / mm² to 12.73 A / mm² direct current pulse in the process of ultrasonic roll, using the synergistic effect of electroplasticity and thermal plasticity, significantly enhance the plastic flow ability of titanium alloy in the deformation process. This method can significantly and efficiently increase the thickness of titanium alloy surface gradient plastic deformation layer while maintaining low surface roughness and high surface residual compressive stress of workpiece, far beyond the effect of conventional ultrasonic roll and low current density electric pulse assisted technology, thereby comprehensively improving the fatigue performance and wear resistance of titanium alloy parts.

Description

Technical Field

[0001] This invention relates to the field of metal surface strengthening technology, specifically to a surface strengthening method for increasing the thickness of the gradient deformation layer on the surface of titanium alloys. Background Technology

[0002] Titanium alloys are mainstream lightweight and high-strength materials, but their insufficient fatigue and friction properties limit their further applications. To enhance the performance of titanium alloys, ultrasonic surface rolling (USR) technology has been developed to improve their service performance. USR can harden the surface, introduce a compressive residual stress field, and reduce surface roughness, thus simultaneously improving the fatigue and tribological properties of titanium alloys.

[0003] However, titanium alloys have limited work hardening capabilities. During surface mechanical strengthening treatment, a thin hardened layer quickly forms on the sample surface due to severe deformation, which limits further improvement in surface properties.

[0004] To further enhance the benefits of ultrasonic surface rolling strengthening technology, researchers have introduced electrical pulses for assisted surface strengthening. Electrical pulse-assisted ultrasonic surface rolling strengthening technology can achieve lower surface roughness, higher surface hardness, and greater compressive residual stress. This is attributed to the fact that the electrical pulses allow for more complete surface deformation, without significantly affecting the thickness of the plastic deformation layer.

[0005] Currently, the density of the electrical pulses used is relatively low, and ultrasonic surface rolling strengthening technology under high current density assistance has not been developed. Summary of the Invention

[0006] To address the problems existing in the prior art, the purpose of this invention is to provide a surface strengthening method for increasing the thickness of the gradient deformation layer on the surface of titanium alloys, which can efficiently achieve a thick surface gradient deformation layer on titanium alloys while maintaining good overall surface properties.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A surface strengthening method for increasing the thickness of the gradient deformation layer on the surface of titanium alloys employs a composite processing method that couples high-density electric pulses with ultrasonic surface rolling strengthening. The pulse power supply is integrated into the ultrasonic surface rolling strengthening equipment to form a combined processing system. During the ultrasonic rolling process, high-current-density DC pulses are applied simultaneously. The synergistic effect of electroplasticity and thermoplasticity enhances the plastic flowability of the titanium alloy and increases the thickness of the surface gradient plastic deformation layer. The current density of the high-density electric pulses is controlled to be between 5.09 A / mm² and 12.73 A / mm².

[0008] Furthermore, the process parameters for ultrasonic surface rolling strengthening are: static pressure 800 N, feed speed 0.063 mm / rev, lathe speed 35 rev / min, and ultrasonic frequency 27 kHz.

[0009] Furthermore, the lathe speed for ultrasonic surface rolling strengthening was 35 rev / min, and the number of repeated processing cycles was 5.

[0010] Furthermore, the titanium alloy is a Ti6Al4V alloy.

[0011] Furthermore, the Ti6Al4V alloy was processed into rod-shaped samples with a diameter of 5 mm.

[0012] Furthermore, in the ultrasonic surface rolling strengthening process, silicon nitride ceramic balls are used as rolling tips.

[0013] Furthermore, the diameter of the silicon nitride ceramic sphere is 10 mm.

[0014] Furthermore, the pulse power supply is a DC pulse power supply with a pulse frequency of 600 Hz and a pulse amplitude of 10%.

[0015] Furthermore, the current of the high-density electrical pulse is adjustable from 0 to 400 A.

[0016] Furthermore, the current of the high-density electrical pulse is 100 A to 250 A.

[0017] In summary, the present invention has the following advantages: During high-density electrical pulse-assisted ultrasonic surface rolling strengthening, the Ti6Al4V alloy surface strengthened by electrical pulse-assisted ultrasonic rolling exhibits a greater degree of plastic deformation than that strengthened by ultrasonic rolling, thanks to the combined effects of thermoplasticity and electroplasticity. It was observed that as the current density increased, the surface plastic deformation layer of the Ti6Al4V alloy also increased significantly. The microhardness near the surface was slightly lower than that of the ultrasonically rolled sample, indicating a shift of the maximum hardened area to the subsurface, while the surface residual stress was significantly increased. The Ti6Al4V alloy treated with this technique exhibited a significantly thicker surface plastic deformation layer than those obtained by current ultrasonic rolling strengthening techniques and low-density electrical pulse-assisted ultrasonic rolling strengthening techniques. In summary, the Ti6Al4V alloy sample with a surface gradient structure obtained in this study maintained a lower surface roughness, similar surface hardness, and higher surface compressive residual stress. Attached Figure Description

[0018] Figure 1 This is a microstructure diagram of the Ti6Al4V alloy used in this embodiment; Figure 2 This is a schematic diagram of the technical principle of the present invention; Figure 3 The surface morphology of Ti6Al4V alloy strengthened by high-density electrical pulse-assisted ultrasonic rolling; Figure 4 High-density electrical pulse-assisted ultrasonic rolling is used to enhance the surface roughness of Ti6Al4V alloy. Figure 5 A surface plastic deformation layer for Ti6Al4V alloy strengthened by high-density electrical pulse-assisted ultrasonic rolling; Figure 6 A surface hardening layer for Ti6Al4V alloy strengthened by high-density electrical pulse-assisted ultrasonic rolling; Figure 7 To improve the surface residual stress distribution of Ti6Al4V alloy by high-density electrical pulse-assisted ultrasonic rolling; Figure 8 This study investigates the relationship between the thickness of the plastic deformation layer and the number of processing steps in surface ultrasonic rolling strengthening of Ti6Al4V alloys in different research.

[0019] In the picture: 1-Ultrasonic enhancement equipment; 2-Lathe fixture; 3-DC pulse power supply. Detailed Implementation

[0020] The present invention will now be described in further detail.

[0021] A surface strengthening method for increasing the thickness of the gradient deformation layer on the surface of titanium alloys employs a composite processing method that couples high-density electric pulses with ultrasonic surface rolling strengthening. The pulse power supply is integrated into the ultrasonic surface rolling strengthening equipment to form a combined processing system. During the ultrasonic rolling process, high-current-density DC pulses are applied simultaneously. The synergistic effect of electroplasticity and thermoplasticity enhances the plastic flowability of the titanium alloy and increases the thickness of the surface gradient plastic deformation layer. The current density of the high-density electric pulses is controlled to be between 5.09 A / mm² and 12.73 A / mm².

[0022] This study selected annealed Ti6Al4V alloy with a dual-state microstructure as the target material, and machined it into test bars with a diameter of 5 mm using a lathe. In this embodiment, a self-made device, electrical pulse-assisted ultrasonic rolling strengthening (EPC-A) was used. The parameters of the ultrasonic rolling strengthening device 1 are as follows: static pressure 800 N, feed rate 0.063 mm / rev, lathe speed 35 rev / min, ultrasonic frequency 27 kHz, and repeated treatment 5 times. Silicon nitride balls with a diameter of 10 mm and a hardness of 78 HRC were used as the rolling head. The EPC-A assisted ultrasonic rolling strengthening parameters were the same as those for ultrasonic rolling strengthening. The selected pulse power supply was a DC pulse power supply 3 with a pulse frequency of 600 Hz, a pulse amplitude of 10%, and pulse currents of 100 A, 150 A, 200 A, and 250 A, corresponding to amplitude current densities of 5.09 A / mm². 2 7.64 A / mm 2 10.19 A / mm 2 and 12.73 A / mm 2 After electro-pulse assisted ultrasonic rolling strengthening, the thickness of the surface plastic deformation layer was significantly increased while maintaining the good comprehensive surface properties of Ti6Al4V alloy. Specifically, the surface roughness of the sample was significantly reduced, the compressive residual stress of the sample was enhanced, and the surface plastic deformation layer and surface hardening layer of the sample were significantly increased.

[0023] like Figure 1 As shown, in the annealed Ti6Al4V alloy used in this embodiment, the equiaxed grain structure is the primary α phase, while the lamellar structure is the secondary α phase formed by the transformation of the β phase.

[0024] Figure 2 This is a schematic diagram illustrating the technical principle of electrical pulse-assisted ultrasonic rolling strengthening. The equipment used for ultrasonic rolling strengthening integrates a lathe, an ultrasonic generator, a rolling device, and a high-pressure gas cylinder. During the strengthening process, the workpiece is clamped on the lathe, the ultrasonic generator produces ultrasonic vibrations which are transmitted to the rolling device, and the high-pressure gas cylinder applies static pressure to the rolling device. The ultrasonic rolling strengthening process is completed as the lathe rotates. Electrical pulse-assisted ultrasonic rolling strengthening is a processing method that integrates direct current (DC) pulse power supply into the ultrasonic rolling strengthening equipment. The DC pulse power supply contacts the lathe fixture 2 through conductive carbon brushes, and the workpiece is clamped on the lathe to form a closed circuit. During the ultrasonic rolling process, the workpiece is subjected to the DC pulse current, and the surface integrity is improved by relying on the "electroplastic" effect of the pulse current.

[0025] Figure 3 The surface morphology of Ti6Al4V alloy strengthened by high-density electrical pulse-assisted ultrasonic rolling. Figure 3In the images, (a) shows the surface morphology of a Ti6Al4V alloy rod after turning, with obvious machining marks on the sample surface. (b) shows the surface morphology after USR treatment; the regular machining features on the sample surface have basically disappeared, with only slight traces remaining. (c)-(f) show the sample surface after high-density pulsed current assisted USR treatment; machining marks and small surface defects are eliminated, and the sample surface has a high degree of smoothness. This indicates that this embodiment fully utilizes the "electroplasticity" of the electric pulse under high-density electric pulse action and avoids the negative impact of thermal effects on the surface morphology. It can be considered that under the assistance of this current density, HEUSR treatment has the potential to further improve the sample surface morphology. To avoid overheating and surface morphology deterioration, even higher density pulsed currents were not applied to the USR treatment process.

[0026] Figure 4 High-density electrical pulse-assisted ultrasonic rolling (USR) was used to enhance the surface roughness of Ti6Al4V alloy. The machined sample surface exhibited alternating peaks and valleys with deep valleys. After USR treatment, the surface roughness Ra decreased from 0.996 µm to 0.228 µm, a reduction of 77%. The Ra of the HEUSR-1 sample with a 100 A pulse current decreased to 0.169 µm, demonstrating the significant filling effect of the electrical pulse on the surface valleys. As the pulse current increased to 150, 200, and 250 A, the corresponding Ra of the HEUSR samples increased to 0.190, 0.237, and 0.362 µm, respectively, indicating that surface valleys gradually became more apparent. Overall, the HEUSR treatment significantly improved the surface finish of the samples.

[0027] Figure 5This describes the surface plastic deformation layer of Ti6Al4V alloy strengthened by high-density electrical pulse-assisted ultrasonic rolling (HEUSR). After simple ultrasonic rolling (USR), the surface plastic deformation layer of the sample is approximately 150 µm thick, with a grain plastic flow deflection angle of 51º. The plastic flow direction differs between the surface and subsurface layers, a phenomenon likely caused by the deviation in machining direction between turning and rolling. After HEUSR treatment with pulsed currents ranging from 100 to 250 A, the grain deflection angle of the sample increases sequentially with increasing current to 59º, 64º, 68º, and 73º, while the thickness of the plastic deformation layer increases simultaneously to 230, 270, 330, and 380 µm. The HEUSR-4 sample under 250 A shows a 124% increase in deformation layer thickness and a 43% increase in grain deflection angle compared to the USR sample. Magnified observation of the surface layer shows that HEUSR-1 still retains the turning plastic deformation zone, while the turning deformation zone of HEUSR-4 completely disappears, forming a microstructure of equiaxed crystals on the surface and fibrous grains on the subsurface. The formation of these equiaxed crystals is presumably related to the dynamic recovery and recrystallization behavior during the treatment process, and HEUSR treatment can significantly enhance the degree of plastic deformation on the sample surface compared to USR.

[0028] Figure 6 This is a surface hardening layer for Ti6Al4V alloy strengthened by high-density electrical pulse-assisted ultrasonic rolling (HEUSR). After USR treatment, the microhardness at 25 μm on the sample surface increased to 396.7 HV, and the hardened layer depth was approximately 350 μm. Although the hardness at this location was higher than at 50 μm, the dispersion was greater, and the uniformity of plastic deformation was poor. After HEUSR treatment with pulse currents ranging from 100 to 250 A, the near-surface hardness of the sample showed a continuous decreasing trend. The location of the maximum hardness gradually shifted deeper with increasing current, moving from 25 μm to 50, 75, and 150 μm respectively. At 250 A, the hardness at 25 μm on the HEUSR-4 sample decreased to 368 HV. Overall, HEUSR treatment significantly increased the depth of the hardened layer on the sample surface, and the location of the maximum microhardness shifted deeper into the matrix with increasing pulse current.

[0029] Figure 7The residual stress distribution on the surface of Ti6Al4V alloy strengthened by high-density electrical pulse-assisted ultrasonic rolling is shown. As shown in (a), the surface compressive stress of the original machined sample was -277.5 MPa, which increased to -558.5 MPa after USR treatment, 2.01 times that of the machined sample. After HEUSR treatment, the surface compressive stress of the sample first increased and then decreased with the increase of pulse current. At 100 A, HEUSR-1 reached a peak of -733 MPa (2.64 times that of the machined sample), and at 250 A, HEUSR-4 decreased to -549.5 MPa, close to that of the USR sample. As shown in (b), the gradient distribution shows that the residual compressive stress in the surface region of HEUSR-3 increased most significantly. The increase weakened with increasing depth but continued to exist, and the compressive stress decay trend decreased with depth.

[0030] Figure 8 This study examines the relationship between the thickness of the plastic deformation layer and the number of processing passes in ultrasonically rolled (HEUSR) strengthening of Ti6Al4V alloys in different studies. Under USR treatment, the thickness of the plastic deformation layer on the sample surface increases with the number of processing passes, a trend corroborated by previous EUSR studies. Furthermore, the applied electrical pulses in these studies did not significantly increase the thickness of the plastic deformation layer, likely due to the low current density used, which failed to effectively control the deformation layer thickness. In contrast, this study, using high-density electrical pulses, significantly increased the thickness of the plastic deformation layer on the sample surface. The deformation layer thickness after only 5 HEUSR passes was far higher than in other studies. Notably, the deformation layer thickness of HEUSR-3 and HEUSR-4 samples even exceeded the effects of 25 or 30 passes in other studies, confirming that HEUSR strengthening technology can efficiently construct a plastic deformation layer on the surface of Ti6Al4V alloys.

[0031] This invention achieves a significant synergistic effect in the surface modification of titanium alloys by organically coupling high-density pulsed current in the range of 5.09 A / mm² to 12.73 A / mm² with ultrasonic surface rolling strengthening technology. Specifically, the high-density pulsed current not only directly promotes dislocation movement and reduces instantaneous deformation resistance through electroplasticity, but also softens the material matrix through the thermoplasticity it induces. These two processes mutually induce and deeply couple, fundamentally overcoming the plastic flow bottleneck caused by the limited work hardening capacity of titanium alloys. Experimental data show that under this synergistic mechanism, far superior effects to conventional ultrasonic rolling and low-current-density pulsed current-assisted techniques can be achieved with very few processing cycles, while simultaneously increasing residual compressive stress and maintaining excellent surface finish. This simultaneous leap in deformation layer depth, surface integrity, and processing efficiency fully demonstrates the unexpected synergistic effect between high-density pulsed current and ultrasonic rolling.

[0032] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A surface strengthening method for increasing the thickness of the gradient deformation layer on the surface of titanium alloys, characterized in that: A composite processing method coupling high-density electric pulses and ultrasonic surface rolling is adopted. The pulse power supply is integrated into the ultrasonic surface rolling equipment to form a combined processing system. During the ultrasonic rolling process, high current density DC pulses are applied simultaneously. The synergistic effect of electroplasticity and thermoplasticity is used to enhance the plastic flow capacity of titanium alloys and increase the thickness of the surface gradient plastic deformation layer. The current density of the high-density electric pulses is controlled from 5.09 A / mm² to 12.73 A / mm².

2. The surface strengthening method according to claim 1, characterized in that, The process parameters for ultrasonic surface rolling strengthening are: static pressure 800 N, feed rate 0.063 mm / rev, lathe speed 35 rev / min, and ultrasonic frequency 27 kHz.

3. The surface strengthening method according to claim 2, characterized in that, The lathe speed for ultrasonic surface rolling strengthening was 35 rev / min, and the number of repeated processing cycles was 5.

4. The surface strengthening method according to claim 1, characterized in that, The titanium alloy is a Ti6Al4V alloy.

5. The surface strengthening method according to claim 4, characterized in that, Ti6Al4V alloy was processed into rod-shaped specimens with a diameter of 5 mm.

6. The surface strengthening method according to claim 1, characterized in that, In the ultrasonic surface rolling strengthening process, silicon nitride ceramic balls are used as rolling tips.

7. The surface strengthening method according to claim 6, characterized in that, The diameter of the silicon nitride ceramic ball is 10 mm.

8. The surface strengthening method according to claim 1, characterized in that, The pulse power supply is a DC pulse power supply with a pulse frequency of 600 Hz and a pulse amplitude of 10%.

9. The surface strengthening method according to claim 1, characterized in that, The current adjustable range of the high-density electrical pulse is 0 to 400 A.

10. The surface strengthening method according to claim 9, characterized in that, The current of the high-density electrical pulse is 100 A to 250 A.

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