FREE-CUTTING TITANIUM MATERIAL AND PREPARATION PROCESS THEREOF

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/CN2025/111455, filed on Jul. 30, 2025, and claims priority of Chinese Patent Application No. 202411151740.5, filed on Aug. 21, 2024, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to the technical field of other metal processing, and in particular to a free-cutting titanium material and a preparation process thereof.

BACKGROUND

Among non-ferrous metals, titanium exhibits particularly excellent performance; due to its advantages such as high specific strength, wear resistance, high temperature resistance, and corrosion resistance, titanium is regarded as an important strategic material; and therefore, titanium is widely used in aerospace, marine engineering, automotive industry, and other fields. However, poor machinability of titanium materials exhibited during mechanical processing has long been a major bottleneck restricting its engineering application.

Specifically, poor machinability of titanium materials is mainly attributed to the following reasons: titanium has high strength, high hardness, and poor thermal conductivity; and a serious hardening phenomenon occurs during mechanical processing. Additionally, due to high chemical activity, titanium reacts with various gases in the air during processing to easily form a so-called “structure hardening layer” on a surface thereof; and consequently, a cutting force exerted on a cutting tool during titanium material cutting is extremely large, thereby causing severe cutting tool wear and frequent cutting tool chipping. Moreover, titanium has low thermal conductivity, and the thermal conductivity of titanium is only one-fourth of thermal conductivity of iron; and therefore, almost all the heat generated during titanium material cutting remains on a cutting edge of the cutting tool and cannot be dissipated, which results in a high temperature in a cutting zone and serious cutting tool wear. Furthermore, titanium has a small elastic modulus, thereby leading to significant deformation and easy rebound during processing, which not only further aggravates cutting tool wear but also seriously compromises the surface precision of processed parts. Furthermore, a friction coefficient between titanium and the cutting tool is large, and a built-up edge of the cutting tool is easily formed during cutting, such that a large amount of heat generated during titanium material cutting concentrates on a blade and aggravates cutting tool wear.

In summary, the above unfavorable factors result in poor machinability of titanium materials, which not only increases the titanium material processing cost but also fails to guarantee the precision of parts, thereby significantly limiting the commercial promotion and application of titanium materials.

For technical solutions in the prior art, the technical problem of difficulty in titanium material cutting is solved usually through the following two approaches: improving titanium material processing conditions and alloying titanium materials. The improving titanium material processing conditions refers to enhancing machinability of titanium materials by developing high-performance cutting tool materials and cutting fluids. The titanium material alloying refers to enhancing machinability of a titanium material by regulating its microstructure. It should be noted that in certain specific application fields of titanium materials, alloying is the only option to enhance machinability of titanium materials.

Based on this, Chinese Patent Application No. CN102605212A discloses a free-cutting titanium alloy and a preparation method thereof. A free-cutting titanium alloy is prepared by mixing Ti, Cu, and chromium (Cr) powders in a certain ratio according to a powder metallurgy method, and the prepared titanium alloy has the advantages of high strength, low elastic modulus, and excellent machinability. The main technical means for enhancing machinability includes adding Cu with high thermal conductivity to a titanium material, and thereby a free-cutting Ti2Cu phase is formed.

Additionally, another Chinese Patent Application No. CN102719701A further discloses a free-cutting titanium alloy and a preparation method thereof. A titanium alloy with excellent machinability is prepared by a vacuum consumable melting method, and the titanium alloy has good fatigue strength and hot workability. The titanium alloy prepared thereby includes titanium (Ti), carbon (C), iron (Fe), nitrogen (N), hydrogen (H), aluminium (Al), vanadium (V), oxygen (O), and rare earth (RE) elements; and free-cutting components are added, and the free-cutting components include bismuth (Bi), tin (Sn), tellurium (Te), phosphorus (P), nickel (Ni), sulfur (S), and other elements.

Furthermore, another Chinese Patent Application No. CN108097739A further discloses a method for processing a free-cutting TC4 alloy wire. First, a TC4 ingot with a nickel content of 0.5%-2% is prepared by using a sponge titanium wire, an industrial pure aluminum wire, and a titanium-nickel alloy wire; and then, a TC4 wire is prepared through forging, rolling, drawing, and low-temperature drawing. After corresponding continuous annealing (heat treatment) of the TC4 wire, a free-cutting TC4 titanium alloy wire is obtained. A second-phase particle Ti2Ni is formed in a matrix of the free-cutting wire; an internal structure thereof is refined through large-deformation drawing and continuous annealing; and therefore, compared to an ordinary TC4 wire, the free-cutting wire exhibits more excellent machinability.

Furthermore, another Chinese Patent Application No. CN115652141A further discloses a method for preparing a low-cost free-cutting antibacterial titanium alloy and a titanium alloy faucet. A free-cutting antibacterial titanium alloy is prepared by the vacuum consumable melting method, and specifically includes the following chemical components: C≤0.05%, H≤0.015%, N≤0.05%, O≤0.2%, Al:10.0-12.0%, Fe:6.0-10.0%, manganese (Mn):1.0-3.0%, molybdenum (Mo):0.1-0.5%, Cu:4.0-6.0%, Bi:0.5-1.0%, Sn:0.6-1.5%, and a balance being Ti and unavoidable impurities. A titanium alloy faucet is successfully prepared based on the above titanium alloy through a negative pressure casting method, and due to the synergistic effect of a brittle Ti3Al compound, dispersed bismuth particles, and other elements, the titanium alloy faucet demonstrates a better free-cutting technical effect than an ordinary titanium alloy ZTiAl6V4.

However, the technical solutions disclosed in prior art patents still have the technical problems of failing to significantly enhance machinability of titanium materials or failing to make titanium materials simultaneously possess the advantages of excellent machinability, low cost, and simple process.

SUMMARY

In view of the above, it is necessary to provide a free-cutting titanium material and a preparation process thereof so as to solve the technical problems and further enhance machinability of the titanium material.

A free-cutting titanium material includes a matrix component group and a free-cutting component group, and at least one free-cutting component group is added to the matrix component group;

• • the matrix component group includes the following in percentage by mass: iron (Fe): 0-1.0%; nitrogen (N): 0-0.08%; hydrogen (H): 0-0.02%; oxygen (O): 0-0.50%; aluminium (Al): 0-8.0%; vanadium (V): 0-15.0%, and a balance being titanium and unavoidable impurities;
  • the free-cutting component group respectively refers to: a rare earth (RE) and sulfur (S) group, a copper (Cu) and S group, a zirconium (Zr) group, a carbon (C) group, a boron (B) group, or a magnesium (Mg) group;
  • in the RE and S group, RE is an element with the atomic number of 57-71 or a rare earth alloy composed of elements with the atomic number of 57-71; in percentage by mass, an addition amount of RE accounts for 0-4.0%, and an addition amount of S accounts for 0-1.0%; a mass ratio of RE to S satisfies: RE:S=n:1, where n=1-4;
  • in the Cu and S group, Cu is added in the form of Cu powder or copper alloy; in percentage by mass, an addition amount of Cu accounts for 0-4.5%, and an addition amount of S accounts for 0-1.0%; a mass relationship between Cu and S satisfies: Cu=3S+m, where m=0-1.5%;
  • an addition amount of the Zr group accounts for 0-3.0% in percentage by mass; in the Zr group, Zr is added in the form of zirconium-titanium alloy, and a mass relationship between Zr and titanium (Ti) satisfies: Zr=0.048Ti−p, where p=0-2.5%;
  • an addition amount of the C group accounts for 0-1.0% in percentage by mass; in the C group, C is added in the form of graphite or graphene;
  • an addition amount of the B group accounts for 0-0.5% in percentage by mass; in the B group, B is added in the form of pure boron or ferroboron alloy; and
  • an addition amount of the Mg group accounts for 0-1.0% in percentage by mass, in the Mg group, Mg is added in the form of magnesium-iron alloy or magnesium-aluminum alloy, and a mass relationship between Mg and O satisfies: Mg=4O+q, where q=0-0.05%.

Further, in another embodiment, the matrix component group includes the following in percentage by mass: Fe: 0-0.500%; N: 0-0.050%; H: 0-0.015%; O: 0-0.400%; Al: 0-4.000%; V: 0-3.000%; and a balance being titanium and unavoidable impurities.

Further, in another embodiment, in the RE and S group, in percentage by mass, an addition amount of RE accounts for 0.5-2.4%, and an addition amount of S accounts for 0.1-1.0%; and a mass ratio of RE to S satisfies: RE:S=n1:1; where n1=1.25-3.00.

Further, in another embodiment, in the Cu and S group, in percentage by mass, an addition amount of Cu accounts for 0.5-2.5%, and an addition amount of S accounts for 0.1-0.8%; and a mass relationship between Cu and S satisfies: Cu=3S+m1, where m1=0.1-0.9%.

Further, in another embodiment, in percentage by mass, a relationship between an addition amount of the Zr group and an addition amount of Ti satisfies: Zr=0.048Ti−p1, where p1=0-1.3%.

Further, in another embodiment, an addition amount of the C group accounts for 0.40-0.65% in percentage by mass, and an addition amount of the B group accounts for 0.002-0.030% in percentage by mass.

Further, in another embodiment, in the Mg group, in percentage by mass, a relationship between an addition amount of Mg and an addition amount of O satisfies: Mg=4O+q1, where q1=0-0.03%.

Specifically, in the above free-cutting titanium material, RE is added in the form of block-like or powder-like rare earth and rare earth alloy during raw material mixing, where one element in rare earth may be added alone, or several elements in rare earth may be added in combination.

Specifically, in the above free-cutting titanium material, an additive medium of S includes: sublimed sulfur (sulfur powder), an ferrosulfur alloy (FeS2, FeS, or the like), MoS2, TiS2, Cu2S, and the like.

Furthermore, in a specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 95.5%, lanthanum (La): 0.5%, S: 0.3%, Al: 1.6%, V: 2.0%, Fe: 0.15%, and unavoidable impurities; and a mass ratio of La (RE) to S is 1.67:1.

Furthermore, in another specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 95.1%, La: 0.6%, S: 0.3%, Al: 4.0%, and unavoidable impurities; and a mass ratio of La (RE) to S is 2:1.

Furthermore, in another specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 95.5%, La: 0.5%, S: 0.4%, Al: 1.5%, V: 2.0%, Fe: 0.15%, and unavoidable impurities; and a mass ratio of La (RE) to S is 1.25:1.

Furthermore, in another specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 95.1%, La—Ce composite rare earth: 0.9%, S: 0.35%, Al: 3.2%, V: 0.5%, and unavoidable impurities; and a mass ratio of La+Ce (RE) to S is 2.6:1.

Furthermore, in another specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 95.4%, La—Ce composite rare earth: 1.6%, S: 0.5%, Al: 1.0%, Cu: 1.5%, B: 0.004%, and unavoidable impurities; and a mass ratio of La+Ce (RE) to S is 3.2:1, and a mass ratio of Cu to S is 3:1.

Furthermore, in another specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 92.8%, La—Ce composite rare earth: 2.4%, S: 0.8%, Al: 3.0%, V: 1.0%, B: 0.004%, and unavoidable impurities; and a mass ratio of La+Ce (RE) to S is 3:1.

Furthermore, in another specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 96.4%, La—Ce composite rare earth: 0.5%, S: 0.15%, Zr: 2.85%, Fe: 0.15%, and unavoidable impurities; and a mass ratio of La+Ce (RE) to S is 3.3:1, and a mass ratio of Zr to Ti accounts for 0.03:1.

Furthermore, in another specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 98.9%, Ce: 0.5%, S: 0.15%, C: 0.5%, and unavoidable impurities; and a mass ratio of Ce to S is 3.3:1.

Furthermore, in another specific embodiment of the free-cutting titanium material, the following components in percentage by mass are included: Ti: 95.2%, Mg: 0.8%, O: 0.2%, Al: 1.8%, V: 2.0%, and unavoidable impurities; and a mass ratio of the Mg to O is 4.1:1.

Specifically, a process for preparing the above free-cutting titanium material, implemented based on a vacuum consumable melting process, includes the following steps:

• • S11: mixing: mixing a matrix component group of a free-cutting titanium material according to a preset ratio, putting a resulting mixture into a mixer, and fully stirring to achieve compositional homogeneity;
  • S12: electrode block preparation: first pouring a uniformly mixed material of the matrix component group into a mold barrel of an electrode extruder, then placing a free-cutting component group of the free-cutting titanium material in a middle of the mold barrel according to a preset ratio; then, using the electrode extruder to extrude the material in the mold barrel into an electrode block;
  • S13: consumable electrode preparation: placing a preset number of electrode blocks obtained in the step S12 into a vacuum plasma welding chamber, and welding all the input electrode blocks into an integral consumable electrode by plasma welding under vacuum conditions, where the integral consumable electrode serves as an anode for vacuum consumable melting;
  • S14: primary vacuum consumable melting: clamping the consumable electrode with a clamp holder, putting the consumable electrode into a water-cooled copper crucible, evacuating a furnace to a vacuum degree of less than 3 Pa, and then starting energization, where after energization, an arc starts at an end of the electrode, the electrode starts to melt under the action of high temperature, a melt drips into the crucible, and a molten pool gradually rises to form an ingot; setting a melting voltage to 20-30 V, and setting melting current to 2-6 KA, to obtain a primarily melted consumable ingot;
  • S15: surface turning of the primarily melted consumable ingot: performing turning processing on a surface of the melted ingot to remove a defective surface layer, and obtaining a melted titanium ingot with surface defects removed after treatment; and
  • S16: multiple melting: repeating the steps S14 and S15 for the primarily melted ingot, and refining the ingot at least twice more to obtain a refined free-cutting titanium alloy ingot.

Specifically, another process for preparing the above free-cutting titanium material, implemented based on a vacuum suspension melting process, includes the following steps:

• • S21: mixing: mixing a raw material of a matrix component group and a raw material of a free-cutting component group of a free-cutting titanium material according to a preset ratio, and putting a resulting mixture into a mixer;
  • S22: feeding: using a feeder to feed the uniformly mixed raw material of the matrix component group and the raw material of the free-cutting component group into a crucible of a vacuum suspension melting furnace, and placing the raw material of the free-cutting component group in a middle of the crucible, such that the raw material of the free-cutting component group is wrapped in a middle by the raw material of the matrix component group;
  • S23: gas purging after vacuum treatment: evacuating the vacuum suspension melting furnace to below 1×10-3 Pa, and maintaining a preset vacuum degree;
  • S24: primarily melting: adjusting a voltage and current respectively according to a size of a titanium material to be prepared to raise a temperature in the furnace to 1,700-2,300° C., and maintaining the temperature for 10 min after an alloy is completely melted, to obtain a primarily melted free-cutting titanium ingot; and
  • S25: multiple melting: turning the primarily melted free-cutting titanium ingot upside down, repeating furnace charging for melting, and performing melting for three or more times to ensure a uniform structure and composition of the titanium ingot.

Specifically, another process for preparing the above free-cutting titanium material, implemented based on a powder metallurgy process, includes the following steps:

• • S31: mixing: mixing a raw material of a matrix component group and a raw material of a free-cutting component group of a powdery free-cutting titanium material according to a ratio, putting a resulting mixture into a mixer, until a particle size of the raw material of the free-cutting component group approaches a particle size of the raw material of the matrix component group, and fully stirring to achieve compositional homogeneity;
  • S32: ball milling treatment: performing ball milling and mixing on mixed powder under the protection of argon, where a ball-to-material ratio is set to 3:1-14:1, and ball milling time is set to 1-6 h;
  • S33: compacting: performing compacting on a ball-milled raw material under a pressure of 200-800 MPa;
  • S34: sintering: performing vacuum sintering on the compacted raw material, with a vacuum degree controlled at less than 0.1 Pa; heating from room temperature to 1,000-1,400° C. at a heating rate of 4-10° C./min; then heating to 1,150-1,650° C. at a heating rate of 2-5° C./min and maintaining the temperature for 2-3 h; and
  • S35: cooling: turning off power after maintaining the temperature, cooling the raw materials in a furnace, introducing inert gas into the furnace for air cooling when the furnace is cooled to 750° C., and obtaining a sintered titanium compact after air cooling to below 50° C.

In summary, due to low specific gravity, high specific strength, wear resistance, high temperature resistance, and corrosion resistance, titanium is regarded as an important strategic material, and is widely used in aerospace, marine engineering, automotive industry, and other fields. However, poor mechanical machinability of titanium has long been a major bottleneck restricting its engineering application. The present disclosure provides a free-cutting titanium material and a preparation process thereof. By adding one or more free-cutting component groups to a matrix component group of the titanium material, the present disclosure significantly enhances the machinability of the titanium material. The free-cutting titanium material has the advantages of simple composition design, low cost, simple process, strong operability, and suitability for the preparation of various titanium materials. Compared with that of traditional titanium materials, cutting efficiency of the free-cutting titanium material disclosed in the present disclosure may usually be improved by more than 30%; and therefore, the titanium material not only has the advantage of good machinability but also exhibits good hot workability and cold deformation performance, thereby having good applicability. Therefore, the free-cutting titanium material and the preparation method thereof provided by the present disclosure solve the technical problem of how to further enhance the machinability of the titanium material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a metallographic photograph of a free-cutting titanium material disclosed in Example 1 of the present disclosure.

FIG. 2 is a metallographic photograph of Comparative Example 1 compared with Example 1.

FIG. 3 is a metallographic photograph of a free-cutting titanium material disclosed in Example 2 of the present disclosure.

FIG. 4 is a metallographic photograph of Comparative Example 2 compared with Example 2.

FIG. 5 is a metallographic photograph of a free-cutting titanium material disclosed in Example 3 of the present disclosure.

FIG. 6 is a metallographic photograph of Comparative Example 3 compared with Example 3.

FIG. 7 is a metallographic photograph of a free-cutting titanium material disclosed in Example 4 of the present disclosure.

FIG. 8 is a metallographic photograph of Comparative Example 4 compared with Example 4.

FIG. 9 is a metallographic photograph of a free-cutting titanium material disclosed in Example 5 of the present disclosure.

FIG. 10 is a metallographic photograph of Comparative Example 5 compared with Example 5.

FIG. 11 is a metallographic photograph of a free-cutting titanium material disclosed in Example 6 of the present disclosure.

FIG. 12 is a metallographic photograph of Comparative Example 6 compared with Example 6.

FIG. 13 is a metallographic photograph of a free-cutting titanium material disclosed in Example 7 of the present disclosure.

FIG. 14 is a metallographic photograph of Comparative Example 7 compared with Example 7.

FIG. 15 is a metallographic photograph of a free-cutting titanium material disclosed in Example 8 of the present disclosure.

FIG. 16 is a metallographic photograph of Comparative Example 8 compared with Example 8.

FIG. 17 is a metallographic photograph of a free-cutting titanium material disclosed in Example 9 of the present disclosure.

FIG. 18 is a metallographic photograph of Comparative Example 9 compared with Example 9.

FIG. 19 is a schematic diagram of a test system for testing machinability of a free-cutting titanium material of the present disclosure.

FIG. 20 is a test result diagram of testing machinability of Example 1 through a machinability test system.

DESCRIPTION OF EMBODIMENTS

In order to enable the objectives, features, and advantages mentioned above of the present disclosure to be more apparent and easily understood, specific embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Numerous specific details are set forth in the following description to facilitate a thorough understanding of the present disclosure. However, the present disclosure may be implemented in many other ways than those described herein, and those skilled in the art may make similar improvements without departing from the connotation of the present disclosure. Therefore, the present disclosure is not limited by the specific embodiments disclosed below.

Furthermore, the terms “first” and “second” are merely for the purpose of description, and cannot be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with “first” and “second” may explicitly or implicitly include at least one of the features. In the description of the present disclosure, “a plurality of” means at least two, such as two and three, unless expressly specified otherwise.

In the present disclosure, unless otherwise expressly specified and limited, the terms “mounted”, “connected”, “connecting”, “fixed” and other terms should be understood in a broad sense. For example, it may be a fixed connection or a detachable connection or integration; it may be a mechanical connection or an electrical connection; it may be a direct connection or an indirect connection via an intermediate medium; and it may be internal communication or interaction between two elements, unless otherwise expressly limited. The specific meanings of the above terms in the present disclosure may be understood on a case-by-case basis for those of ordinary skill in the art.

In the present disclosure, unless otherwise expressly stated and defined, a first feature being “above” or “below” a second feature may refer to that the first feature and the second feature are in direct contact, or the first feature and the second feature are in indirect contact via an intermedium. In addition, the first feature being “over”, “above” and “on the top of” the second feature may refer to that the first feature is over or above the second feature, or simply means that the level of the first feature is higher than that of the second feature. The first feature being “under”, “below” and “at the bottom of” the second feature may refer to that the first feature is under or below the second feature, or simply means that the level of the first feature is lower than that of the second feature.

Specifically, a free-cutting titanium material of the present disclosure includes a matrix component group and a free-cutting component group, and in particular, at least one free-cutting component group is added to the matrix component group; the matrix component group includes the following in percentage by mass: iron (Fe): 0-1.0%; nitrogen (N): 0-0.08%; hydrogen (H): 0-0.02%; oxygen (O): 0-0.50%; aluminium (Al): 0-8.0%; vanadium (V): 0-15.0%, and a balance being titanium and unavoidable impurities; and the free-cutting component group respectively refers to: a rare earth (RE) element and sulfur (S) group, a copper (Cu) and S group, a zirconium (Zr) group, a carbon (C) group, a boron (B) group, or a magnesium (Mg) group.

• • in the RE and S group, RE is an element with the atomic number of 57-71 or a rare earth alloy composed of elements with the atomic number of 57-71; in percentage by mass, the content of RE in the RE and S group is 0-4.0%, the content of S therein is 0-1.0%, and a mass ratio of RE to S satisfies: n:1, n=1-4; and the above conditions ensure that RE and S form stable rare earth sulfides that serve as free-cutting phases to improve the machinability of the titanium material.

Specifically, S is a free-cutting element, and adding S to the titanium material is conducive to enhancing the chip breaking ability, thereby enhancing the machinability of the titanium material. A mechanism by which sulfur addition alone enhances the machinability of the titanium material is as follows: S combines with titanium (Ti) to form a free-cutting phase TiS2, and as the content of S in the titanium material increases, the number of the free-cutting phases TiS2 increases significantly, which enhances the hot brittleness of the titanium material and facilitates chip breaking during cutting; and additionally, TiS2 further enhances the thermal conductivity of the titanium material and improves cutting conditions of the titanium material.

More specifically, an additive medium of S includes: sublimed sulfur (sulfur powder), an ferrosulfur alloy (FeS2, FeS, or the like), MoS2, TiS2, Cu2S, and the like.

Specifically, RE is added in the form of block-like or powder-like rare earth and rare earth alloy during raw material melting, where RE may be one (added alone) or more (added in combination) of lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd); and composite addition of RE and S enables to form rare earth sulfides between RE and S.

A mechanism by which the combination of RE and S enhances the machinability of the titanium material is as follows: rare earth and sulfur are combined to form rare earth sulfides, and the rare earth sulfides have a relatively fixed ratio, which is conducive to chip breaking during cutting and enhances the thermal conductivity of the titanium material. When the content of RE and the content of S are both low, rare earth sulfides precipitate as fine granular spherical particles at grain boundaries and in limited quantities, which weakens an effect of enhancing the machinability of the titanium material; when the content of RE and the content of S increase, rare earth sulfides precipitate as larger-size clustered spherical particles at grain boundaries and within grains, which significantly enhances the machinability of the titanium material; and when the content of RE and the content of S further increase, an effect of enhancing the machinability of the material is limited, but a negative effect on the material increases significantly.

Specifically, metallographic analysis results indicate that when the content of RE is 0-4.0%, the content of S is 0-1.0%, and a mass ratio of RE to S satisfies: n:1 (n=1-4), that is, when addition is performed based on the content of RE, the content of S, and the mass ratio of RE to S disclosed in the present disclosure, rare earth sulfides are uniformly distributed in the titanium material, where the rare earth sulfides are spherical, numerous, and small-sized, and the uniformly distributed rare earth sulfides promote the generation and segmentation of chips of the titanium material, which significantly enhances the machinability of the titanium material without significant impact on other properties of the titanium material.

Further, in the Cu and S group, Cu is added in the form of Cu powder or copper alloy; in percentage by mass, the content of Cu is 0-4.5%, the content of S is 0-1.0%, and a mass relationship between Cu and S satisfies: Cu=3S+m, m=0-1.5%; the above conditions ensure that Cu and S enable to form CuTi2 and Cu2S compounds in the titanium material, where the compounds may serve as free-cutting phases to enhance the thermal conductivity of the titanium material and enhance the hot workability and machinability of the material.

Specifically, an addition of Cu enhances the machinability of the titanium material because: Cu has high thermal conductivity, and adding Cu to the titanium material facilitates conduction of heat in a cutting zone during cutting; and when Cu and S are added in combination, a free-cutting phase conducive to cutting is formed therebetween, and a brittle phase is formed by adding Cu to the titanium material: Ti2Cu and Ti2Cu phases may serve as a chip breaking source, thereby enhancing the machinability of the titanium material.

It should be noted that when an addition amount of Cu is low, fewer CuTi2 and Cu2S compounds are formed in the titanium material, thereby having no significant impact on enhancement in the machinability of the titanium material; and as the content of Cu further increases, when the content of Cu is 0-4.5%, the content of S is 0-1.0%, and a mass relationship between Cu and S satisfies: Cu=3S+m, where m=0-1.5%, that is, when addition is performed based on the content of the Cu, the content of S, and the mass ratio of Cu to S disclosed in the present disclosure, stable and uniform CuTi2 and Cu2S compounds are formed in the titanium material, where the compounds not only enhance the thermal conductivity of the titanium material but also improves a chip segmentation rate of the titanium material during cutting, and significantly enhances the machinability of the titanium material, with little impact on other properties of the titanium material. When the content of Cu and the content of S further increase, numerous CuTi2 and Cu2S compounds precipitate and aggregate in the titanium material, which seriously deteriorates the high-temperature resistance of the titanium material.

Further, for the Zr group, in percentage by mass, the content of Zr is 0-3.0%, where Zr is added in the form of zirconium-titanium alloy, and a relationship between Zr and Ti satisfies: Zr=0.048Ti−p, where: p=0-2.5%, which ensures that Zr is effectively solid-dissolved in the titanium material and precipitated at grain boundaries; and therefore, the hot workability and machinability of the titanium material are enhanced.

Specifically, Zr may be solid-dissolved in the titanium material, a relative atomic mass of Zr is 91.2, and a relative atomic mass of Ti is 47.9; thus, to ensure that Zr is completely solid-dissolved in titanium and excess Zr atoms precipitate in the titanium material in the form of ZrTi compound, a mass relationship between Zr and Ti should satisfy: 47.9Zr×40=91.2Ti−p, where: p=0-2.5%; when an addition amount of Zr is low, Zr is completely solid-dissolved in Ti, which has no effect on enhancing the machinability of the titanium material; and when the content of Zr is high, numerous ZrTi compounds precipitate, which adversely affects the machinability of the titanium material.

Further, for the C group, in percentage by mass, the content of C is 0-1.0%; and C is added in the form of graphite or graphene, and C precipitates in the titanium material in the form of TiC as a precipitated phase. Thus, grain boundaries may be pinned and grains may be refined during high-temperature hot working, which enhances the thermal conductivity of the titanium material and further enhances the machinability of the titanium material.

When the content of C is low, C is solid-dissolved in Ti, and no TiC precipitated phase is formed, which fails to enhance the machinability of the titanium material. When the content of C is high, numerous TiC inclusions are formed in the titanium material and aggregated and distributed in the titanium material, which seriously affects the plasticity of the titanium material and embrittles the titanium material; and additionally, cutting tool wear aggravates during machining. When an addition amount of C accounts for 0-1.0%, TiC precipitates in the titanium material relatively uniformly, and numerous fine TiC precipitated phases not only enhance the thermal conductivity of the titanium material but also have the effect of pinning grain boundaries and refining grains, which is conducive to enhancing the machinability of the titanium material.

Further, for the B group, in percentage by mass, the content of B is 0-0.5%. B is added in the form of pure boron or ferroboron alloy to ensure that B atoms are solid-dissolved in the titanium material, which inhibits the grain growth behavior of the titanium material during high-temperature hot working, and plays a role of refining grains and homogenizing the structure, thereby enhancing the machinability of the titanium material.

Specifically, B is solid-dissolved in the titanium material, and therefore, when an addition amount of B is low, B is completely solid-dissolved in the titanium material, and the effect of enhancing the machinability is not ideal. When an addition amount of B is too high, hard borides TiB and TiB2 precipitate in large quantities, which further deteriorates the machinability of the titanium material and aggravates cutting tool wear. When an addition amount of B accounts for 0-0.5%, on the one hand, it is ensured that B atoms are solid-dissolved in the titanium material, and on the other hand, borides disperse and precipitate in the titanium material, which not only has the effect of pinning grain boundaries, refining and homogenizing the structure, but also enhances the machinability of the titanium material.

Further, for the Mg group, in percentage by mass, the content of Mg is 0-1.0%, where Mg is added in the form of magnesium-iron alloy or magnesium-aluminum alloy, and an addition amount of Mg satisfies: Mg=4O+q, where q=0-0.05%; the above conditions ensure that MgO is formed during a melting process of the titanium material, and MgO serves as a dispersedly distributed precipitated phase, which enhances the chip fragmentability of the material, and increases thermal conductivity, thereby further enhancing the machinability of the titanium material.

Specifically, when an addition amount of Mg is too high, numerous large-sized and aggregated MgO particles are formed in the titanium material, which weakens the chip fragmentability of the titanium material, and reduces the cleanliness of the titanium material, thereby seriously deteriorating the mechanical properties of the titanium material. When an addition amount of Mg is low, Mg is completely solid-dissolved in the titanium material and cannot be used as a free-cutting phase for enhancing the machinability of the titanium material. Therefore, when an addition amount of Mg is 0-1.0%, and the addition amount of Mg satisfies: Mg=4O+q, where q=0-0.05%, Mg combines with O to form MgO. In this case, MgO is dispersed in the titanium material, which, as a free-cutting phase, significantly enhances the chip fragmentability of the titanium material, and enhances the thermal conductivity of the titanium material, thereby enhancing the machinability of the titanium material.

Further, the above free-cutting titanium material may be prepared by any one of three methods: vacuum consumable melting, vacuum suspension melting, and powder metallurgy.

Specifically, when the vacuum consumable melting method is used to prepare the above free-cutting titanium material, a preparation process includes the following steps:

• • S11: mixing: mixing a matrix component group of a free-cutting titanium material according to a preset ratio, putting a resulting mixture into a mixer, and fully stirring to achieve compositional homogeneity;
  • S12: electrode block preparation: first pouring a uniformly mixed material of the matrix component group into a mold barrel of an electrode extruder, then placing a free-cutting component group of the free-cutting titanium material in a middle of the mold barrel according to a preset ratio; then, using the electrode extruder to extrude the material in the mold barrel into an electrode block;
  • S13: consumable electrode preparation: placing a preset number of electrode blocks obtained in the step S12 into a vacuum plasma welding chamber, and welding all the input electrode blocks into an integral consumable electrode by plasma welding under vacuum conditions, where the integral consumable electrode serves as an anode for vacuum consumable melting;
  • S14: primary vacuum consumable melting: clamping the consumable electrode with a clamp holder, putting the consumable electrode into a water-cooled copper crucible, evacuating a furnace to a vacuum degree of less than 3 Pa, and then starting energization, where after energization, an arc starts at an end of the electrode, the electrode starts to melt under the action of high temperature, a melt drips into the crucible, and a molten pool gradually rises to form an ingot; setting a melting voltage to 20-30 V, and setting melting current to 2-6 KA, to obtain a primarily melted consumable ingot;
  • S15: surface turning of the primarily melted consumable ingot: performing turning processing on a surface of the melted ingot to remove a defective surface layer, and obtaining a melted titanium ingot with surface defects removed after treatment; and
  • S16: multiple melting: repeating the steps S14 and S15 for the primarily melted ingot, and refining the ingot at least twice more to obtain a refined free-cutting titanium alloy ingot. Specifically, multiple melting refers to further refining based on primary melting, which is intended to improve the uniformity of alloy elements in a titanium alloy ingot, remove surface impurity elements, and ensure material consistency.

Further, when the vacuum suspension melting method is used to prepare the above free-cutting titanium material, a preparation process includes the following steps:

• • S21: mixing: mixing a matrix component group of a free-cutting titanium material according to a preset ratio, and putting a resulting mixture into a mixer;
  • S22: feeding: using a feeder to feed the uniformly mixed raw material of the matrix component group and the raw material of the free-cutting component group of the free-cutting titanium material (with a preset ratio) into a crucible of a vacuum suspension melting furnace, and placing the raw material of the free-cutting component group in a middle of the crucible, such that the raw material of the free-cutting component group is wrapped in a middle by the raw material of the matrix component group;
  • S23: gas purging after vacuum treatment: evacuating the vacuum suspension melting furnace to below 1×10-3 Pa, and maintaining a preset vacuum degree;
  • S24: primarily melting: adjusting a voltage and current respectively according to a size of a titanium material to be prepared to raise a temperature in the furnace to 1,700-2,300° C., and maintaining the temperature for 10 min after an alloy is completely melted, to obtain a primarily melted free-cutting titanium ingot; and
  • S25: multiple melting: turning the primarily melted free-cutting titanium ingot upside down, repeating furnace charging for melting, and performing melting for three or more times to ensure a uniform structure and composition of the titanium ingot.

Further, when the powder metallurgy method is used to prepare the above free-cutting titanium material, a preparation process includes the following steps:

• • S31: mixing: mixing a raw material of a matrix component group and a raw material of a free-cutting component group of a powdery free-cutting titanium material according to a ratio, putting a resulting mixture into a mixer, until a particle size of the raw material of the free-cutting component group maximally approaches a particle size of the raw material of the matrix component group, and fully stirring to achieve compositional homogeneity;
  • S32: ball milling treatment: performing ball milling and mixing on mixed powder under the protection of argon, where a ball-to-material ratio is set to 3:1-14:1, and ball milling time is set to 1-6 h;
  • S33: compacting: performing compacting on a ball-milled raw material under a pressure of 200-800 MPa;
  • S34: sintering: performing vacuum sintering on the compacted raw material, with a vacuum degree controlled at less than 0.1 Pa; heating from room temperature to 1,000-1,400° C. at a heating rate of 4-10° C./min; then heating to 1,150-1,650° C. at a heating rate of 2-5° C./min and maintaining the temperature for 2-3 h; and
  • S35: cooling: turning off power after maintaining the temperature, cooling the raw materials in a furnace, introducing inert gas into the furnace for air cooling when the furnace is cooled to 750° C., and obtaining a sintered titanium compact after air cooling to below 50° C.

Specifically, RE, S, Cu, Zr, C, B, and Mgs disclosed in a free-cutting titanium material and a preparation process thereof the present disclosure are all free-cutting elements, and adding these elements to the titanium material is conducive to enhancing the machinability of the titanium material.

In particular, in an embodiment of a free-cutting titanium material of the present disclosure, a matrix component group thereof further preferably includes the following in percentage by mass: Fe: 0-0.5%; N: 0-0.05%; H: 0-0.015%; O: 0-0.4%; Al: 0-4.0%; V: 0-3.0%; and a balance being titanium and unavoidable impurities. In addition to the above components, one or more free-cutting component groups should be further contained.

In particular, in another embodiment of a free-cutting titanium material of the present disclosure, the free-cutting titanium material containing RE and S elements further preferably includes the following RE and S in percentage by mass: content of RE: 0.5-2.4%; content of S: 0.1-1.0%; and a mass ratio of RE to S should satisfy: RE:S=n1:1, where n1=1.5-3.0.

In particular, in another embodiment of a free-cutting titanium material of the present disclosure, the free-cutting titanium material containing Cu and S further preferably includes the following Cu and S in percentage by mass: Cu: 0.5-2.5%; S: 0.1-0.8%, and a mass relationship between Cu and S satisfies: Cu=3S+m1, where m1=0.1-0.9%.

In particular, in another embodiment of a free-cutting titanium material of the present disclosure, the free-cutting titanium material contains Zr, a relationship between an addition amount of Zr and an addition amount of Ti satisfies: Zr=0.048Ti−p1, where p1=0-1.3%; and the above conditions ensure that Zr is effectively solid-dissolved in the titanium material and precipitated at grain boundaries.

In particular, in another embodiment of a free-cutting titanium material of the present disclosure, the free-cutting titanium material containing C further preferably includes C in percentage by mass: C: 0.4-0.65%; and the above conditions ensure that C precipitates in the titanium material in the form of TiC, which enables to pin grain boundaries and refine grains during high-temperature hot working.

In particular, in another embodiment of a free-cutting titanium material of the present disclosure, the free-cutting titanium material containing B further preferably includes the following B in percentage by mass: B: 0.002-0.030%.

In particular, in another embodiment of a free-cutting titanium material of the present disclosure, the free-cutting titanium material containing Mg further preferably includes the following Mg in percentage by mass: a mass relationship between Mg and O satisfies: Mg=4O+q1, where q1=0.01-0.03%.

Further, in percentage by mass, nine specific examples and nine corresponding comparative examples are disclosed below, as shown in Table 1:

TABLE 1
Chemical compositions of different examples and comparative examples

RE

S

Cu

Zr

B

C

Mg

Al

V

O

Fe

Ti

Example 1	0.5×	0.3	—	—	—	—	—	1.6	2.0	—	0.15	Balance
Comparative	—	—	—	—	—	—	—	1.6	2.0	—	0.15	Balance
Example 1
Example 2	0.6×	0.3	—	—	—	—	—	4.0	—	—	—	Balance
Comparative	—	—	—	—	—	—	—	4.0	—	—	—	Balance
Example 2
Example 3	0.5×	0.4		—	—		—	1.5	2.0	—	0.15	Balance
Comparative	—	—	—	—	—	—	—	1.5	2.0	—	0.15	Balance
Example 3
Example 4	0.9××	0.35	—	—	—	—	—	3.2	0.5	—	—	Balance
Comparative	—	—	—	—	—	—	—	3.2	0.5	—	—	Balance
Example 4
Example 5	1.6××	0.5	1.5	—	0.004	—	—	1.0	—	—	—	Balance
Comparative	—	—	—	—	—	—	—	1.0	—	—	—	Balance
Example 5
Example 6	2.4××	0.8	—	—	0.004	—	—	3.0	1.0	—		Balance
Comparative	—	—	—	—	—	—	—	3.0	1.0	—	—	Balance
Example 6
Example7	0.5××	0.15	—	2.85	—	—	—	—	—	—	0.15	Balance
Comparative	—	—	—	—	—	—	—	—	—	—	0.15	Balance
Example 7
Example 8	0.5×××	0.15	—	—	—	0.5	—	—	—	—	—	Balance
Comparative	—	—	—	—	—	—	—	—	—	—	—	Balance
Example 8
Example 9	—	—	—	—	—	—	0.8	1.8	2.0	0.2	—	Balance
Comparative	—	—	—	—	—	—	—	1.8	2.0	0.2	—	Balance
Example 9
It should be noted that in the above Table 1, × represents content of added La; ×× represents an addition amount of a La—Ce composite; ××× represents content of added Ce; and the vacuum consumable melting method is employed in all examples and comparative examples.

Further, the following methods are used to perform performance tests for all examples and comparative examples disclosed in Table 1 of the present disclosure, and comparison results of cutting effects in all examples and comparative examples are shown in Table 2 below.

Hardness test: An MH-5L micro Vickers hardness tester is used to perform hardness tests (10 tests in total) for all examples and comparative examples, and an average hardness value is selected as a final test result.

Machinability test: A milling cutter is a dedicated standard tool for titanium alloy cutting with a diameter of 4 mm (D0101.002027), a cutting fluid is a special microemulsion cutting fluid for titanium alloy, a spindle speed is 5,000 rev/min, a feed rate is 800 mm/min, a cutting depth is 4 mm, and a radial cutting depth is 0.4 mm. A machinability test is performed on a V-8L machining center, and a Kistler 9257B dynamometer is used to collect a cutting force during milling.

Specifically, with reference to FIG. 19, a machinability test system includes a test machining center 1, a standard titanium alloy milling cutter 2, a fixture 3, a dynamometer 4, a charge amplifier 5, a data acquisition system 6, and a computer 7; and the standard titanium alloy milling cutter 2, the fixture 3, and the dynamometer 4 are disposed in the test machining center 1 respectively, and the dynamometer 4 is connected below the fixture 3. The charge amplifier 5 is connected to the dynamometer 4 through a connecting cable; and the data acquisition system 6 is respectively connected to the charge amplifier 5 and the computer 7. In a machinability test experiment, the test machining center 1 drives the standard titanium alloy milling cutter 2 to mill a titanium material sample clamped on the fixture 3 according to preset processing parameters; in this case, the dynamometer 4 collects a cutting force during milling; and after processing by the charge amplifier 5 and the data acquisition system 6, data is transmitted to the computer 7 for processing. With further reference to FIG. 20, cutting force test results obtained by testing the milling performance of Example 1 through the above machinability test system are shown.

Performance test results of all examples and comparative examples are shown in Table 2; metallographic photographs of all examples and comparative examples after grinding and polishing are shown in FIGS. 1 to 18. Compared with corresponding comparative examples, in all examples disclosed in the present disclosure, free-cutting phases conducive to cutting are introduced after titanium material alloying, which significantly enhances the machinability of the titanium materials.

TABLE 2
Cutting effects of all examples and comparative examples

Cutting parameters

			Radial		Cutting
		Cutting	cutting	Cutting	force
	Hardness	depth	depth	force	Enhancement
	(HV)	(mm)	(mm)	(N)	rate (%)

Example 1	156	4	0.5	55	33.7
Comparative	165	4	0.5	83
Example 1
Example 2	181	4	0.5	63	29.2
Comparative	231	4	0.5	89
Example 2
Example 3	153	4	0.5	52	35.0
Comparative	159	4	0.5	80
Example 3
Example 4	163	4	0.5	56	30.9
Comparative	170	4	0.5	81
Example 4
Example 5	165	4	0.5	58	30.1
Comparative	158	4	0.5	83
Example 5
Example 6	207	4	0.5	89	30.5
Comparative	202	4	0.5	128
Example 6
Example 7	189	4	0.5	77	33.0
Comparative	186	4	0.5	115
Example 7
Example 8	138	4	0.5	55	38.2
Comparative	135	4	0.5	89
Example 8
Example 9	193	4	0.5	79	35.8
Comparative	199	4	0.5	123
Example 9

It can be seen from the test results in Table 2 that cutting effects of titanium materials are significantly improved after adding free-cutting components; and a method for calculating a cutting force enhancement rate disclosed in Table 2 is as follows: cutting force enhancement rate (%)=(cutting force of comparative example-cutting force of example)/cutting force of comparative example×100%.

In summary, due to low specific gravity, high specific strength, wear resistance, high temperature resistance, and corrosion resistance, titanium is regarded as an important strategic material, and is widely used in aerospace, marine engineering, automotive industry, and other fields. However, poor mechanical machinability of titanium has long been a major bottleneck restricting its engineering application. The present disclosure provides a free-cutting titanium material and a preparation process thereof. By adding one or more free-cutting component groups to a matrix component group of the titanium material, the present disclosure significantly enhances the machinability of the titanium material. The free-cutting titanium material has the advantages of simple composition design, low cost, simple process, strong operability, and suitability for the preparation of various titanium materials. Compared with that of traditional titanium materials, cutting efficiency of the free-cutting titanium material disclosed in the present disclosure may usually be improved by more than 30%; and therefore, the titanium material not only has the advantage of good machinability but also exhibits good hot workability and cold deformation performance, thereby having good applicability. Therefore, the free-cutting titanium material and the preparation method thereof provided by the present disclosure solve the technical problem of how to further enhance the machinability of the titanium material.

The technical features of the above examples can be combined arbitrarily. For the sake of brevity, all possible combinations of the technical features in the above examples are not described. However, as long as the combinations of these technical features are not contradicted, it should be regarded as the scope of the description in this specification.

The examples mentioned above are merely several embodiments of the present disclosure, and are specifically described in details, but may not be interpreted as limiting the scope of the patent for the present disclosure as a result. It should be noted that for those of ordinary skill in the art, they may also make several transformations and improvements on the premise of not deviating from the conception of the present disclosure, and these transformations and improvements shall fall within the scope of protection of the present disclosure. The scope of protection of the present patent for disclosure shall be governed by the appended claims.