KR101346808B1 - The Titanium alloy improved mechanical properties and the manufacturing method thereof - Google Patents

Abstract

The present invention relates to a titanium alloy with improved mechanical properties and a method for manufacturing the same, to prepare an alloy ingot by adding and dissolving Al 3 ~ 7% by weight, Fe 0.5 ~ 1.5% by weight, oxidation resistance enhancing metal and the balance Ti. (Step 1); And hot working the ingot (step 2). The titanium alloy produced by the above method contains one metal selected from the group consisting of Cr, Ni, Pd, and Mo, which improves oxidation resistance to Ti-Al-Fe-based alloys. Therefore, the titanium alloy not only has improved oxidation resistance as compared to the conventional titanium alloy, it is excellent in mechanical strength and good elongation has an excellent formability effect.

Description

Titanium alloy improved mechanical properties and the manufacturing method

The present invention relates to a titanium alloy with improved mechanical properties and a method of manufacturing the same.

Titanium alloys have been applied to various industries such as aerospace, marine, sports, medical, etc. due to their high specific strength, excellent corrosion resistance and biocompatibility. Pure titanium transforms the α phase having a dense hexagonal structure at room temperature into an β phase having a body-centered cubic structure due to isomorphism at 882 ° C. When alloying elements are added to pure titanium, there exists a region where α and β phases coexist in a specific temperature section, and in some cases, the region extends to room temperature. As such, titanium alloys are generally classified into α alloys, β alloys, and α + β alloys at room temperature. The most widely used alloy is α + β alloy, which has a basic composition based on Ti-Al, which stabilizes the α phase and has a solid solution strengthening effect, and includes molybdenum (Mo), iron (Fe), and vanadium (V). And β-phase stabilizing elements such as chromium (Cr) and neutral elements such as zirconium (Zr) and tin (Sn). The most representative alloy of α + β alloy is Ti-6Al-4V, which accounts for more than 60% of the total titanium alloy usage because of excellent hot workability, weldability, and various mechanical properties obtained by heat treatment.

However, since the development of titanium alloys has been mainly made for aerospace purposes, alloy elements have been selected based on physical properties rather than prices, and have been made of alloy systems that are expensive and not easy to supply. In particular, vanadium (V), which is used for Ti-6Al-4V, is an expensive element and has harmful toxicity to humans. Therefore, there are many constraints on the special use of this alloy. Therefore, efforts have been made to develop low-cost new alloys by replacing vanadium (V), which is a representative β-phase stabilizing element, with other β-phase stabilizing elements which are inexpensive and relatively harmless to the human body. As a result, Ti-5Al-2.5 Although alloys such as Fe and Ti-6Al-0.1Si have been developed, various problems such as the tendency of the mechanical properties at room temperature and high temperature tend to be somewhat insufficient as the content of iron (Fe), which is a β-stabilizing element, are not optimized. have.

Republic of Korea Patent Publication No. 10-2004-0101267 relates to a heat-resistant titanium alloy material excellent in high temperature corrosion resistance, oxidation resistance and a method of manufacturing the same. In detail, the surface layer having the multilayer structure of the outer layer which consists of three phases of (beta) phase, (gamma) phase, and Laves phase of the Ti-Al-Cr type alloy state diagram, and the outer layer which consists of Al-Ti-Cr type alloy surfaces of a heat resistant Ti alloy base material It is formed in the alloy, and the Al concentration of the outer layer contains 50 atomic% or more, and has an effect of having heat resistance excellent in high temperature corrosion resistance and oxidation resistance.

Republic of Korea Patent Publication No. 10-2004-0044840 relates to a method for producing a (α + β) titanium alloy with improved mechanical properties and dynamic fracture characteristics, in detail the (α + β) titanium alloy at 400-600 ℃ Aging treatment to precipitate the α ₂ (Ti ₃ Al) phase inside the α phase and to quench the (α + β) titanium alloy obtained in the step (α + β) provides a method for producing a titanium alloy. The present invention is based on Ti-6Al-4V-based alloys, and has a problem of containing expensive and harmful vanadium.

Accordingly, the inventors of the present invention prepared an alloy by adding inexpensive iron (Fe) and an oxidation-resistant metal, and confirmed that the alloy produced had excellent oxidation resistance, mechanical properties, and moldability, and completed the present invention.

An object of the present invention is to provide a titanium alloy having improved oxidation resistance and excellent mechanical properties.

Another object of the present invention to provide a method for producing a titanium alloy excellent in the mechanical properties.

In order to achieve the above object, the present invention provides a titanium alloy including 3 to 7% by weight of aluminum (Al), 0.5 to 1.5% by weight of iron (Fe), oxidation resistance enhancing metal and balance Ti.

In addition, the present invention comprises the steps of preparing an alloy ingot by adding and dissolving 3-7% by weight of aluminum (Al), 0.5-1.5% by weight of iron (Fe), an oxidation resistance-promoting metal and the balance Ti (step 1); And hot working the ingot (step 2). The method provides a method of manufacturing a titanium alloy having improved mechanical properties.

Titanium alloy according to the present invention has the effect of reducing the production cost by replacing vanadium (V) with low-cost iron (Fe) from the conventional Ti-6Al-4V alloy. In addition, conventional Ti-6Al-4V is added to a Ti-Al-Fe-based alloy without adding toxic vanadium by adding one selected from the group containing Cr, Ni, Pd and Mo, which enhances oxidation resistance. Compared with titanium alloy, it has excellent oxidation resistance even at a high temperature of 600 to 800 ° C., has high strength at room temperature and high temperature, and has excellent elongation, and has good formability.

1 is a graph showing a hot working process in the alloy manufacturing method of the present invention,
Figure 2 is a graph showing the results of the high temperature oxidation test by temperature,
3 is a SEM photograph of the surface oxide layer after performing a high temperature oxidation test of the titanium alloy,
4 and 5 are photographs and spectra showing the results of EPMA analysis after the high temperature oxidation test of the titanium alloy by temperature,
FIG. 6 is a SEM photograph of an oxide scale peeled off after an oxidation test of a titanium alloy. FIG.

Hereinafter, the present invention will be described in detail.

The present invention

It provides a titanium alloy with improved mechanical properties, including Al 3-7% by weight, Fe 0.5-1.5% by weight, oxidation resistance enhancing metal and balance Ti.

Titanium alloy with improved mechanical properties according to the present invention has the effect of reducing the production cost by replacing vanadium with low-cost iron from the conventional Ti-6Al-4V alloy. In addition, by adding a metal that enhances oxidation resistance to Ti-Al-Fe-based alloys without using toxic vanadium, superior oxidation resistance is achieved even at a high temperature of 600 to 800 ° C. compared to conventional Ti-6Al-4V titanium alloys. It also has an excellent effect at room temperature and high temperature. In addition, the elongation is remarkably excellent and the moldability is good.

The titanium alloy includes 3 to 7 wt% Al, 0.5 to 1.5 wt% Fe, an oxidation resistance enhancing metal and the balance Ti, and the metals forming the alloy are as follows.

1) Aluminum (Al)

Aluminum is an element that solidifies and strengthens the α phase of the alloy. As the aluminum content is increased, the strength is increased by the solid solution of aluminum to the titanium (Ti) base. In addition, aluminum is lighter than titanium and serves to reduce the density of the alloy to achieve high specific strength. When the content of aluminum is less than 3% by weight, there is a problem in that the effect of reducing the density is not large and the strength is lowered. When the content of aluminum is more than 7% by weight, Ti ₃ Al is formed to rapidly decrease the ductility of titanium.

2) Fe

Iron plays a role in stabilizing the β phase. If the iron content is less than 0.5% by weight, there is a problem that the β phase cannot be sufficiently stabilized at room temperature. If the iron content is more than 1.5% by weight, the strength is increased while the creep strength is lowered.

3) Chromium (Cr), Nickel (Ni), Palladium (Pd) and Molybdenum (Mo)

In titanium alloys, chromium, nickel, and molybdenum play a role of stabilizing the beta phase and improving oxidation resistance. Chromium has the effect of improving the ductility by adding a small amount. Palladium has a great effect on the improvement of corrosion resistance only by adding 0.1 to 0.2% by weight, but when added in excess of 0.2% by weight, the production cost is increased due to the high cost of palladium is not preferable. When molybdenum is added, strength and hardenability can be increased, but the weldability impairs weldability. However, the amount of molybdenum may vary depending on the use of the alloy. However, in the present invention, when the molybdenum content is less than 0.3 to 1.0% by weight, The effect of an increase etc. cannot be acquired, and when it exceeds 1.0 weight%, there exists a problem that weldability falls. Nickel, along with molybdenum, is an economical alloying element that can replace palladium. Therefore, the titanium alloy according to the present invention has excellent performance of oxidation resistance by adding one selected from the group comprising Cr, Ni, Pd and Mo, which enhances oxidation resistance.

In addition,

Preparing an alloy ingot by adding and dissolving 3 to 7 wt% Al, 0.5 to 1.5 wt% Fe, an oxidation resistance enhancing metal, and the balance Ti (step 1); And

It provides a method of producing a titanium alloy with improved mechanical properties comprising the; step of hot working the ingot (step 2).

Hereinafter, the present invention will be described in detail by steps.

3 to 7% by weight of Al, 0.5 to 1.5% by weight of Fe, an oxidation-resistant metal and the balance Ti are added and dissolved to prepare an alloy ingot. After melting by adjusting the weight ratio of titanium, aluminum and iron, it can be dissolved by adding an oxidation resistance enhancing metal. At this time, the dissolution method that can be used to dissolve the titanium alloy may be one selected from the group consisting of electrode electrode vacuum dissolution method (VAR), electron beam dissolution method, plasma arc dissolution method, non-consumer arc dissolution method, induction skull dissolution method. It is more preferable to use an induction skull dissolution method. Induction skull dissolving method is to replace the crucible with water cooling in the conventional induction melting to avoid the problem of melt contamination by the crucible. In induction melting, the electromagnetic force acts on the molten metal in the center direction, so there is little opportunity for the molten metal to come into direct contact with the water-cooled crucible. Because you can't. The molten alloy is poured into a mold made of graphite to produce an ingot.

In step 1, aluminum is preferably added in 3 to 7% by weight. When the content of aluminum is less than 3% by weight, there is a problem that the strength is lowered, and when added in excess of 7% by weight, Ti ₃ Al is formed, and thus the ductility of titanium is sharply lowered.

In the step 1, the iron content is preferably added in 0.5 to 1.5% by weight. If it is less than 0.5% by weight, there is a problem that the β phase cannot be sufficiently stabilized at room temperature. If the iron content is more than 1.5% by weight, the strength is increased, while the creep strength is decreased, and the weak ω phase and the intermetallic such as TiFe There is a problem in that a compound is formed to rapidly reduce the ductility of the alloy.

The oxidation resistance enhancing metal of step 1 is preferably one selected from the group containing Cr, Ni, Pd, and Mo. In titanium alloys, chromium, nickel and molybdenum serve to stabilize the beta phase and to improve oxidation resistance.

Chromium has the effect of improving the ductility by adding a small amount. Palladium has a great effect on the improvement of corrosion resistance only by adding 0.1 to 0.2% by weight, but when added in excess of 0.2% by weight is not preferable because the production cost is increased due to the high palladium cost. When molybdenum is added, an increase in strength and hardenability may be obtained, but the amount of addition may vary depending on the use of the alloy because it impairs weldability. In the present invention, it is preferable to add 0.3 to 1.0 wt% of the molybdenum content. If the molybdenum content is less than 0.3% by weight, the effect of improving strength and hardenability may not be obtained. If the molybdenum content is more than 1.0% by weight, there is a problem in that weldability is lowered. Nickel, on the other hand, is an economical alloying element that can replace palladium with molybdenum.

Step 2 according to the present invention is a step of performing a hot working, as shown in Figure 1 performs a hot working. Hot working is a processing performed at a temperature higher than the recrystallization temperature (T _β ), when the hot processing is performed has the effect of improving the mechanical properties and workability. α + β type titanium alloy may have various microstructures according to the processing heat treatment process, and thus the mechanical properties and workability of the alloy may be changed. In particular, the mechanical properties vary depending on the phase fraction formed by the processing heat treatment process, the shape and size of the α grains, the distribution and the texture of the aggregates.

The hot working of step 2 is to maintain water for 2 hours at 1050 ~ 1150 ℃ after hot working with a strain of 50 to 70%, and to perform a second hot working at 20% strain after holding for 1 hour at 850 ~ 950 ℃ desirable. The hot working process is a process for crushing the cast structure of the manufactured ingot, and gives a large deformation of about 50% to 70% or more in the β phase region of the manufactured titanium alloy at 1050 to 1150 ° C. It serves to refine the grains by crushing them. If the hot working process is less than 1050 ° C., there is a problem in that cracking occurs with a high processing pressure, and if it exceeds 1150 ° C., it may cause significant grain growth. It is easy to generate an α phase of an equiaxed crystal in the secondary processing process only by forming a fine Widmanstatten structure through water cooling after hot working at the temperature. In addition, when the amount of deformation is 50% or less, there is a problem that the cast structure is not completely fractured and remains in a coarse form, and when the amount of deformation is 70% or more, there is a high possibility of severe cracking on the material surface.

Secondary hot working is preferably carried out at 850 ~ 950 ℃. This secondaryly performs hot forging in the α + β region in order to spheroidize the α phase of the plate. When forming at 850 ℃ or lower during secondary hot processing, there is a problem that cracking may occur on the surface of the material because high molding pressure is required.When forming at 950 ℃ or higher, processing in β-phase region occurs due to processing heat. There is a problem that the + β mixed phase is not formed. In addition, the secondary hot working strain is preferably 20%.

Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are merely to illustrate the invention, but the content of the present invention is not limited by the following examples.

< Example  1> Ti -6 Al -One Fe -0.3 Mo  Manufacture of alloys

Step 1. Melt the metal forming alloy Ingot  Steps to manufacture

6% by weight of aluminum, 1% by weight of iron, 0.3% by weight of molybdenum and 92.7% by weight of titanium were added and dissolved in an induction skull melting furnace. At this time, the skull formed inside the water-cooled crucible used was made of a Ti-Al-Fe-based alloy. The molten alloy was injected into a mold made of cylindrical graphite having a diameter of 50 mm and a length of 150 mm to prepare a Ti-6Al-1Fe-0.3Mo alloy ingot.

Step 2. Hot working  Steps to Perform

The ingot prepared in step 1 was heated to 1100 ° C. at a temperature increase rate of 15 ° C./min, held for 120 minutes, and subjected to hot working with about 50% strain, followed by water cooling. Secondary forging was carried out with an amount of deformation of about 20% after holding for 1 hour at 950 ° C to air-cool to room temperature to prepare a Ti-6Al-1Fe-0.3Mo alloy having an α + β mixed structure.

< Comparative Example  1> Ti -6 Al -One Fe  Manufacture of alloys

A Ti-6Al-1Fe alloy was prepared in the same manner as in Example 1, except that ingot was manufactured using 6 wt% aluminum, 1 wt% iron, and 93 wt% titanium in Step 1 of Example 1. Prepared.

< Comparative Example  2> Ti -6 Al -4V alloy

A Ti-6Al-4V alloy was prepared in the same manner as in Example 1, except that ingot was manufactured using 6 wt% of aluminum, 4 wt% of vanadium, and 90 wt% of titanium in Step 1 of Example 1. Prepared.

< Experimental Example  1> Analysis of mechanical properties

In order to determine the yield strength, tensile strength and elongation of the alloys subjected to the hot working process, measured using an Instron universal testing machine (model name: INSTRON 5881) for Example 1 and Comparative Examples 1 and 2, the results are shown in Table 1 is shown.

alloy

Yield strength (MPa)

Tensile Strength (MPa)
Elongation (%)
Room temperature

500 ℃
600 ℃
Room temperature
500 ℃
600 ℃
Room temperature
500 ℃
600 ℃
Example 1

840
480
318
901
540
347
16.8
21.1
83.4
Comparative Example 1

864
515
352
935
588
393
15.4
22.8
52.1
Comparative Example 2

805
375
-
868
488
-
7.1
16.8
-

As shown in Table 1, it can be confirmed that Example 1 has excellent mechanical properties at both room temperature and high temperature compared to the alloy prepared in Comparative Example 2. In the case of yield strength and tensile strength, the results measured at room temperature, 500 ℃ and 600 ℃ can be seen that Comparative Example 1 is somewhat superior to Example 1. However, in the case of the elongation, it can be confirmed that Example 1 is more excellent. That is, it can be seen that the alloy prepared in Example 1 is as high as the alloy prepared in Comparative Example 1 and has excellent moldability.

< Experimental Example  2> Oxidation resistance  exam

In order to determine the oxidation resistance of the prepared alloys, the alloys prepared in Example 1 and Comparative Examples 1 and 2 were isothermally oxidized at 600, 700, and 800 ° C. for 100 hours using TGA (Thermogravimetric analyzer), respectively. The change in weight per unit area is measured, and the results are shown in FIG. 2.

As shown in FIG. 2, in the case of the alloys prepared in Example 1 and Comparative Example 1, as the oxidation proceeds at 600 ° C., the weight increase amounts to 0.14 and 0.13 mg / cm 2, respectively. It can be seen that the significantly increased to mg / ㎠. In addition, the weight increase of Comparative Example 1 at 700 ℃ is 1.39 mg / ㎠, the weight increase of Example 1 was 0.94 mg / ㎠ showed excellent oxidation resistance, while Comparative Example 2, a commercial alloy, the weight increase is 9.2 mg / It can be seen that the oxidation resistance is markedly inferior in Example 2 by about 10 times the difference from Example 1. It can be seen that even at 800 ℃ the weight change per unit area of Example 1 is the smallest. Therefore, it can be seen that the alloy prepared in Example 1 has the best oxidation resistance.

On the other hand, in all experimental temperature ranges of these alloys, these curves show a parabolic behavior at the beginning, which is rapidly oxidized due to the direct exposure of the specimen surface to high temperature, but as the oxide film is formed over time, This is because oxidation is prevented by disturbing the diffusion path of various ions. Curves that initially exhibited parabolic behavior show linear oxidation behavior over time. This suggests that the increase in the diffusion rate of material ions at high temperature affects the oxidation more than the protective ability of the oxide film which served as a barrier to the diffusion path. Rapid oxidation at 800 ℃ causes the oxide film to crack due to the increase of internal residual stress and thermal stress as the oxide film grows during the formation of the oxide over time, and the gap between the oxide layer and the base material creates easy diffusion of oxygen. It is judged that it is done.

< Experimental Example  3> after oxidation test Surface oxide layer  And Known interface  Observation of the shape

In order to observe the shape of the surface oxide layer and matrix matrix after the oxidation test of the prepared alloys, the specimens of Example 1 and Comparative Example 1, which performed the isothermal oxidation test in Experimental Example 2, were observed through SEM, and the results are illustrated. 3 is shown.

As shown in FIG. 3, when the oxidation test was performed at 600 ° C., all of the alloys prepared in Example 1 and Comparative Example 1 were composed of α phase and β phase, and both alloys had a thickness of about 2 μm on the surface. It can be seen that the oxide layer was formed, and as confirmed in Experimental Example 2, the oxidation resistance in Example 1 and Comparative Example 1 was almost similar, and did not show a difference, and a great difference in the microstructure of the surface oxide layers of the two alloys Not observed. When the oxidation test was performed at 700 ° C., it can be seen that in the alloy prepared in Example 1, a dense oxide film is formed as compared with the alloy prepared in Comparative Example 1, and the thickness of the oxide layer is also thinner. When the oxidation test was performed at 800 ° C., both oxide layers were thickly separated from the specimen, and in Example 1, the oxide layer was newly formed at the interface between the base material and the partially peeled oxide. can see. Therefore, it can be seen that the oxidation resistance of the alloy prepared in Example 1 is excellent.

< Experimental Example  4> EPMA  analysis

In order to observe the oxide layer and the internal microstructure of the alloy after the oxidation test of the alloys prepared using the EPMA (Electron Probe Micro Analyzer) for the specimens of Example 1 and Comparative Example 1 subjected to the isothermal oxidation test in Experimental Example 2 The analysis was performed, and the results are shown in FIGS. 4 and 5.

4 and 5, it can be seen that the surface oxide layer of the alloy prepared in Example 1 and Comparative Example 1 contains a large amount of oxygen, and also contains some nitrogen. In the matrix, aluminum (Al), which is a stabilizing element of the α phase, is concentrated on the α phase, and iron (Fe) and molybdenum (Mo), which are the stabilizing elements of the β phase, are concentrated on the β phase. In particular, the specimens subjected to oxidation test at 700 ℃ increased the concentration of aluminum and iron in the β phase, which seems to be due to the process of decomposition of the β phase into α phase and FeTi phase by long-term maintenance at high temperature. It can be seen that oxygen forms titanium oxide and nitrogen diffuses to the interface under the oxide. In the oxide layer formed by high temperature oxidation, iron oxide was not observed, and an oxide, which is assumed to be aluminum oxide (Al ₂ O ₃ ), was produced. The addition of molybdenum (Mo) in the TiAl-based alloy reduced oxygen solubility and produced a dense aluminum oxide layer. Since it is known to form, it can be judged that the aluminum oxide layer serves as a barrier to prevent the diffusion of various metal ions to enhance oxidation resistance.

< Experimental Example  5> High temperature oxidation  after Peeled  Oxidation scale observation

In order to observe the oxidized scale peeled off after the high-temperature oxidation of the prepared alloy, the specimens of Example 1 and Comparative Example 1 which were subjected to isothermal oxidation test at 800 ° C. in Experimental Example 2 were observed through SEM, and the results are shown in Table 2 below. And FIG. 6.

alloy

Peeled Scale (μm)
Example 1

70.581
Comparative Example 1

68.618

As shown in Table 2 and Figure 6, it can be seen that the oxide scale of the alloy prepared in Example 1 to which molybdenum is added is thicker. This is because the addition of molybdenum inhibits the formation of nitride, which makes the layer vulnerable at the interface of the base material, and thus the oxidation layer is further grown because the oxidation proceeds without being separated from the base material. The thickness of the oxide scale formed from the alloy prepared in Example 1 to which molybdenum was added was found to be thicker than the oxide scale formed in Comparative Example 1. However, considering the part removed and peeled off, the thickness of the entire oxide layer was found to be thinner for the molybdenum-added alloy. In addition, looking at the structure of the peeled oxide scale of Example 1, it can be seen that the oxide scale is formed as a projection. This makes it possible to know that there is a discontinuous aluminum oxide layer between the titanium oxide layers.

Claims (7)

Titanium alloy with improved mechanical properties and oxidation resistance including 3-7 wt% Al, 0.5-1.5 wt% Fe, 0.3-1.0 wt% Mo and balance Ti.
delete delete delete Preparing an alloy ingot by adding and dissolving Al 3-7 wt%, 0.5-1.5 wt% Fe, 0.3-1.0 wt% Mo, and the balance Ti (step 1); And
Hot working the ingot (step 2); Method of producing a titanium alloy with improved mechanical properties and oxidation resistance according to claim 1 comprising a.
delete The method of claim 5, wherein the hot working of step 2 is water-cooled after hot working to 50-70% strain after holding for 2 hours at 1050 ~ 1150 ℃, and secondary at 20% strain after holding for 1 hour at 850 ~ 950 ℃ Method for producing a titanium alloy with improved mechanical properties and oxidation resistance characterized in that the hot working is performed.

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