Development of heat resistant titanium alloy for exhaust valves applicable for motorcycles.
Amidst of the recent concerns on depletion of natural resources, a new heat resistant titanium alloy has been developed using the minimum amount of rare metals. Using Ti-811 as a basis and modifying the alloy composition to Ti-7Al-2Mo-0.2Si-0.15C-0.2Nb, the mechanical property, the creep resistance and the oxidation resistance at high temperatures are improved. At the same time, with the [beta] transformation point shifted to a higher temperature, the hot formability is also improved. The newly developed alloy has made it possible to expand the application of titanium material to exhaust valves in reciprocating engines.
The key to engine performance improvement is reduction of weight of component parts. The titanium alloy having a high specific strength has been used since long ago in the parts of reciprocating system and valve system in internal combustion engines. Among the parts in the valve system, it is important to reduce the weight of engine valves which have the highest share of the equivalent system weight. Accordingly, titanium alloys have been used positively in intake valves, which are used in a lower temperature environment than that of exhaust valves. In an attempt to increase applications of titanium in intake valves in the past, low-cost titanium valves for motorcycles were developed using recycled scrap materials and/or the off-grade spongy titanium material . Meanwhile the application of titanium to exhaust valves, which are subjected to high temperatures, was limited. One of the reasons is a loss of strength resulting from accelerated oxidation of surfaces when the titanium is subjected to a high temperature. A heat resistant titanium alloy contains various kinds of expensive rare metals (such as molybdenum, zirconium, vanadium) to maintain strength at high temperatures. Accordingly, the near-[alpha] structure heat resistant titanium alloy such as Ti-6242S (Ti-6Al-2Sn-4Zr-2Mo-0.1Si) has been employed for exhaust valves.
Furthermore, an application of titanium valves made of titaniumaluminum intermetallic compound is reported as an example of material exceling in the heat resistance and the specific rigidity [2,3]. However, as such a material is very expensive, applications to mass-production vehicles are limited. To extend applications to mass-production vehicles, a low-cost alloy using as less rare metal as possible is called for. Drawing attention as a general trend is how to make use of titanium "ubiquitous" using inexpensive, abundant elements instead of rare metals . Some examples of alloy development using large Clarke number elements such as Si (Silicon), Al (Aluminum), Fe (Ferrous) or interstitial type elements such as O (Oxygen), H (Hydrogen), N (Nitrogen), C (Carbon) have been introduced . There are causes of high material manufacturing cost other than the addition of rare metal elements. An issue is that scratch and/or cracks tend to occur at the time of hot forging because the strength of near-[alpha] structure heat resistant titanium alloy is high at high temperatures.
Developed in this study is the alloy having a hot formability in addition to a high strength, high creep resistance and oxidation resistance at high temperatures necessary for exhaust valves with a minimum use of expensive additives. The low-cost heat resistant titanium alloy having characteristics comparative to Ti-6242S, which is widely applied to racing machines, has thus been developed and applied to mass production. Reported hereafter is the overview of development of the new heat resistant titanium alloy for exhaust valves.
PRINCIPLES OF ALLOY DESIGN
While the working temperature of intake valve is approx 400[degrees]C, that of exhaust valves is approx 700[degrees]C or higher. As oxidation of titanium generally accelerates when the temperature exceeds 600[degrees]C, it is important to improve oxidation resistance in the temperature zone above 600[degrees]C.
Meanwhile in titanium alloys, a hot forming such as the blooming forging, the rolling and the bar rolling is conducted at a temperature below the [beta]-transus temperature when the material is hot to obtain necessary mechanical property. However, as the near-[alpha] structure alloy has high strength with superior deformation resistance at high temperatures, scratch and/or cracks tend to occur. Accordingly, the addition of grinding to remove scratch and/or cracks causes loss of material yield rate, resulting in an increase of cost.
This study has started to develop a heat resistant titanium alloy that lasts at a working temperature of 700[degrees]C and excels in the hot formability. The following are the goals:
1. Shall have the equivalent mechanical properties and oxidation resistance as Ti-6242S, which is a heat resistant titanium alloy, from the room temperature up to 700[degrees]C.
2. The alloy should be designed to maintain hot formability and compatible with the mass production line.
Regarding (1), it was decided to determine the direction of material development using the Ti-8Al-1Mo-1V (hereafter as Ti-811) containing less rare metal among many heat resistant titanium alloys and producible at a low cost as a base alloy. Note that Ti-811 is a heat resistant titanium alloy developed sometime around 1960, and widely used in the compressor blades of aircraft engines, etc. .
Regarding (2), attempts were made to ensure producibility by increasing the temperature at the time of hot forming. When the temperature exceeds [beta]-transus temperature in a hot forming, as the crystal grains grow very large, deterioration of mechanical property tends to occur. Therefore, forming should be performed below the [beta]-transus temperature. We have therefore aimed at improvement of hot formability by raising the [beta]-transus temperature using carbon, which is an interstitial type element originally treated as an impurity element. The relationship between the [beta]-transus temperature and the carbon content calculated by thermodynamics analysis is shown in Figure 1. In this calculation, formulation of TiC is not assumed.
Along with an increase of carbon content, the [beta]-transus temperature goes up. However, when the carbon content exceeds 0.2%, the ductility may lower because TiC educes . Therefore, it was decided to first add carbon by 0.15% to the Ti-811 and analyze oxidation resistance in the working temperatures of exhaust valves. This titanium alloy is hereafter referred to as Ti-811C.
Oxidation Resistance of Ti-811C
To assess oxidation resistance, changes of tensile properties before and after the high temperature exposure were measured. The high temperature exposure conditions were set at 700[degrees]C x 20 h for both Ti-811C and Ti-6242S. The measured data are compared in Figure 2. In Ti-6242S, although the elongation and the reduction of area decline after the high temperature exposure, no decrease of tensile strength or lowering of proof stress is obtained. Meanwhile in Ti-811C, lowering is confirmed in tensile strength, elongation and reduction of area. With respect to the proof stress, it was impossible to take measurement after the high temperature exposure because the test piece showed fracture from brittleness. That phenomenon can be considered due to the composition in that the oxygen diffusion advances easier than in Ti-6242S. Accordingly, as another method to assess oxidation resistance, changes of surface hardness relative to the exposure time period were compared. It is because the hardness increases from the material surface to the inside when the oxygen diffusion advances in the base material. We consider that the less changes of surface hardness occur when the oxidation resistance is high and the diffusion of oxygen in the material surface is restricted. Measured changes of surface hardness at each exposure time at 700[degrees]C are shown in Figure 3. The surface hardness increases proportionally to the exposure time period. The rate of change of surface hardness of Ti-811C is approximately twice more than those of Ti-6242S, which is an indication of deterioration of oxidation resistance. As the Ti-811C as it was did not show adequate oxidation resistance at 700[degrees]C, it was decided to deal with it by changing the alloy composition.
Selection of Additional Elements
Influence of Al Equivalent on Thermal Stability of Titanium Alloy
Aluminum, which is an [alpha]-stabilizing element, tin and zirconium which are a neutral element to improve solubility, are generally added in a heat resistant titanium alloy. As these elements form [Ti.sub.3]Al in high temperatures when large amounts are added, and make the alloy brittle, the additions are limited to the degree indicated by the following equation from experiences .
[Al]eq = [Al]+1/3[Sn]+1/6[Zr]+10[O] [??]9 mass%
As oxygen is contained in a titanium alloy at a rate of approximately 0.1%, the Al equivalent of Ti-811C is approximately 9%. The brittleness of Ti-811C after the high temperature exposure in Figure 2 may be attributable to the formation of [Ti.sub.3]Al. Therefore, the amount of aluminum was restricted at 7%.
Effects of Additional Element on Oxidation Resistance
With respect to the lowering of ductility from the high temperature exposure, while the earlier-mentioned formation of [Ti.sub.3]Al is a concern, the deterioration of material from oxidation can be considered another major cause. For example, compared to Ti-6Al-4V, the increase of weight after the high temperature exposure is less in Ti-5Al-2.5Sn, Ti-7Al-4Mo . The degree of deterioration after the high temperature exposure in Ti-8Al-1Mo-1V-C and Ti-6Al-2Sn-4Zr-2Mo-0.1Si are compared in Figure 2. While the degree of deterioration is higher in Ti-811C, it is supposed that V (vanadium) lowers oxidation resistance because the aluminum content of 5%-8% does not affect significantly to the oxidation resistance taking into account the above-mentioned difference of oxidation between the alloys. Accordingly considering that Mo (molybdenum) was more effective for improvement of oxidation resistance than V, the 1% V in Ti-811C was substituted by Mo to increase Mo, and formulated the basic composition as Ti-7Al-2Mo-C.
As added Nb (niobium) dissolves in Ti[O.sub.2] as an element having a larger valence (more than 5) than Ti, it is known that the addition of Nb reduces oxygen vacancy, restricts oxygen diffusion, and improves oxidation resistance. It is also reported that addition of a small amount (0.2%) of Nb to the Ti-Al-Si alloy lowers the surface hardness after the high temperature exposure. That is considered due to the formation of dense oxidized scale which restricts penetration of oxygen . Accordingly, Nb was added to Ti-Al-Mo-C alloy and the tensile properties at high temperature were investigated while varying the content of Nb. The effects of added Nb to the tensile properties at 760[degrees]C are shown in Figure 4. It has been clarified that the small changes of Nb content do not affect significantly to the strength or elongation at high temperatures.
The surface hardness with (0.2%) and without Nb after exposure at 700[degrees]C is compared in Figure 5. With Nb added, the effect of lowering Vickers hardness by approximately 100-200 points is verified across the entire retaining time period of 1 h-100 h. It can be considered from the said findings that even a small amount of Nb is effective to restrict oxygen diffusion on the base material surface.
It was determined from these findings to add Nb by 0.2% to the previously mentioned, modified basic composition as the minimum necessary amount of additive.
In addition to that, Si was selected to improve creep resistance of the basic composition of Ti-7Al-2Mo-C-0.2Nb. In a heat resistant titanium alloy, it is considered that Si lowers stacking fault energy, prevents cross slip and restricts dislocation motions. A small amount of Si is often added to improve creep resistance . Accordingly, the effects of Si to the high temperature tensile properties were analyzed. The properties at a high temperature were analyzed at various Si contents. Effects of Si content on tensile properties at 760[degrees]C are shown in Figure 6.
Although the effects of Si to the high temperature strengths are small, an effect on the elongation is obtained. The peak elongation was 0.20%. It is known that although Si is classified as a [beta] stabilizing element, its solid solubility is very low, and forms an intermetallic compound of Ti5Si3 [12,13]. The effect of Si additive to the heat resistant titanium alloy is also analyzed, and the Si content that ensures favorable tensile strength and ductility is reported . In this material, the highest tensile strength and ductility were achieved at 0.20%. It has thus been decided to add Si also by 0.2%.
Optimum Carbon Content
From the above-mentioned standpoint, Ti-7Al-2Mo-0.2Nb-0.2Si-C and the composition were reviewed to prevent deterioration of Ti-811C after the high temperature exposure. Lastly how the carbon content determined from the [beta]-transus temperature affected the high temperature tensile properties were analyzed. The tensile properties at 760[degrees]C in various carbon contents are shown in Figure 7.
Virtually no effect of carbon to the high temperature strengths were obtained. With respect to the elongation, the maximum appeared at 0.15%. Accordingly, the carbon content was determined at 0.15%.
Taking the above-mentioned into account, the nominal composition was set at Ti-7Al-2Mo-0.2Si-0.15C-0.2Nb (hereafter as Ti-72SiCN). The composition of Ti-72SiCN is shown in Table 1.
ASSESSMENT OF PROPERTY OF NEWLY DEVELOPED MATERIAL
The high temperature property and the oxidation resistance of the newly developed titanium alloy reported in "principles of alloy design" were validated.
Material Production Conditions
Production conditions of Ti-72SiCN and Ti-6242S for comparison are shown. For Ti-72SiCN, a 20 kg ingot was melted by CCIM (Cold Crucible Induction Melt), rolled, and forged to bars. After that, heat-treated at 1140[degrees]C x 1 h / AC + 760[degrees]C x 2 h / AC, and provided for tests. Regarding Ti-6242S, the 5-ton ingot was melt by the arc melting, rolled and forged to bars. After that, heat treated at 1050[degrees]C x 1 h / AC + 510[degrees]C x 2 h / AC, and provided for tests. Note that such heat treatment was conducted to obtain acicular structure which excels in creep resistance at high temperatures.
Assessment of High Temperature Property
The high temperature properties were assessed by the tensile tests, fatigue tests, and creep tests.
High Temperature Property Assessment Method
For the tensile tests, test pieces of 3 mm diameter and 15 mm gauge length were prepared. Starting the tests after retaining at each designated temperature for 5 min, the 0.2% proof stress, the tensile strength, elongation and the reduction of area were assessed. The tests were conducted at the room temperature, 300[degrees]C, 500[degrees]C, 700[degrees]C and 850[degrees]C.
For the fatigue tests, test pieces having 8 mm diameter in the parallel part was prepared, and Ono-type rotary bending fatigue tests were conducted at 3600 rpm. The number of repetition was limited to 1.0 x [10.sup.7]. The tests were conducted at the room temperature and 700[degrees]C. As the lowering of fatigue strength from a notch tends to be large in a high strength titanium alloy, the comparison was also made by the shape having a V cut corresponding to a stress concentration factor [K.sub.T] = 1.5 in addition to the smooth shape.
The creep tests were conducted at 760[degrees]C, and at a test stress of 3 MPa. For comparison of creep characteristics from various metal compositions, the test pieces having an equiaxed structure of Ti-6242S was added to the assessment.
Results of High Temperature Property
The tensile test results of Ti-72SiCN and Ti-6242S at various test temperatures are shown in Figure 8 and Figure 9. The 0.2% proof stress and the tensile strength of Ti-72SiCN are the same as those of Ti-6242S from the room temperature up to the high temperature. Also, the elongation and the reduction of area of Ti-72SiCN were approximately twice more than those of Ti-6242S at 700[degrees]C. It has thus been clarified that Ti-72SiCN has better mechanical property than Ti-6242S across the entire test temperature range.
The fatigue test results of Ti-72SiCN and Ti-6242S at room temperature of 23[degrees]C and 700[degrees]C are shown in Figure 10 and Figure 11. In the fatigue characteristics at the room temperature and 700[degrees]C, equivalent fatigue strength to Ti-6242S is obtained in the high repetition cycle of [10.sup.7] both in the smooth shape and the notched shape. No difference from Ti-6242S was verified in the lowering of strength due to a notch compared to the smooth shape.
The high temperature creep characteristics of Ti-72SiCN and Ti-6242S are shown in Figure 12. The creep distortion is lower in Ti-72SiCN than in Ti-6242S, which can be considered an effect of a higher Si additive content than in Ti-6242S. For reference, the creep distortion in the equiaxed structure and the acicular structure of Ti-6242 are compared. As generally considered, the creep distortion is lower in the acicular structure than in the equiaxed structure.
It has been confirmed from the above-mentioned findings that Ti-72SiCN has equivalent high temperature properties to Ti-6242S.
Assessment of Oxidation Resistance
In Ti-811C, the brittle fracture occurred in the tensile test after the high temperature exposure. Also in the changes of surface hardness, approximately twice higher rate of change than in Ti-6242S was found. Therefore, it was decided to assess oxidation resistance of Ti-72SiCN and compare to Ti-6242S. The oxidation resistance was assessed by the tensile tests, fatigue tests and surface hardness measurement after the high temperature exposure. The high temperature exposure was conducted by heating and holding in the muffle furnace in an atmospheric environment.
Oxidation Resistance Assessment Method
The previously-mentioned test piece was used for the tensile tests after the high temperature exposure. At the test temperature of 700[degrees]C, the exposure time for Ti-6242S was set at 1 h and 20 h, and that for Ti-72SiCN set at 1 h, 100 h and 200 h. After that, the tensile tests were conducted at the room temperature with the oxide scale remaining on the surface.
The previously mentioned smooth fatigue test piece was used for the fatigue tests after the high temperature exposure. Selecting only Ti-72SiCN, tests were conducted under the same conditions as mentioned earlier. Using the test piece heated and held at 700[degrees]C for 100 h and 200 h prior to the test, the fatigue tests were conducted at the test temperature of 700[degrees]C with the oxide scale remaining on the surface.
For surface hardness measurement after the high temperature exposure, the test pieces for hardness measurement were prepared, and after buffing the hardness measurement surface, heated and held at 700[degrees]C. Fixing the test temperature at 700[degrees]C, the exposure duration was set at 1 h, 2 h, 4 h, 20 h and 100 h. After buffing the measurement surface after the exposure, measurements by Vickers 100 gf were taken at 10 points in each sample, and an average of 8 points excepting the maximum and the minimum was taken.
Results of Oxidation Resistance
The tensile test results of Ti-72SiCN and Ti-6242S after maintaining at the test temperature of 700[degrees]C for each duration are shown in Figure 13. In both alloys, the longer the exposure time, the lower the elongation and reduction of area became while showing a slight increase in the proof stress. Especially noteworthy is that the rate of lowering of elongation and reduction of area in relation to the exposure time is lower in Ti-72SiCN than in Ti-6242S. It can be considered that it is because the oxygen diffusion from the high temperature exposure is more restricted in Ti-72SiCN than in Ti-6242S.
The results of fatigue test conducted after retaining Ti-72SiCN at 700[degrees]C for each exposure duration are shown in Figure 14. The test results of the test piece without heating and retaining are also shown in the picture. The 107 fatigue strength in the exposure duration of 0 h and 200 h is the same. No lowering of fatigue strength is verified from the high temperature exposure. It is considered that the oxygen diffusion was suppressed that generally occurs during the high temperature exposure test, carried out at 700[degrees]C, because the alloy was designed so as to include elements with oxidation resistance. It is understood that there was no difference observed in the fatigue strength as that consequence.
The measured surface hardness of Ti-72SiCN and Ti-6242S after heating and holding at 700[degrees]C for each duration are shown in Figure 15. The surface hardness of Ti-72SiCN shows virtually equivalent changes to Ti-6242S. We consider that this data shows the effect of Nb additive seen in Figure 5.
It has been confirmed from the above-mentioned findings that Ti-72SiCN has equivalent oxidation resistance to that of Ti-6242S, which is considered as attributable to such an effect of small amount of Nb that restricts oxygen diffusion on the base material surface.
Furthermore, productivity of round bar using an actual production line was verified by preparing a 500 kg ingot using the VAR melting for mass production. As a result, the workability that permits mass production has been confirmed.
The new alloy was developed on the basis of such a concept that uses inexpensive aluminum, etc. as the main additional element while restricting use of expensive additional element as little as possible. The composition was improved to restrict brittleness of Ti-811C after the high temperature exposure on which we first focused our attention to and reached to Ti-72SiCN. Upon the comparison of high temperature property, oxidation resistance and hot formability, it has been confirmed that Ti-72SiCN has equivalent properties to the target Ti-6242S.
APPLICATION OF NEWLY DEVELOPED MATERIAL TO MOTORCYCLE EXHAUST VALVES
Explained hereafter are the forging method and detailed specifications employed for application of the newly developed alloy Ti-72SiCN to the exhaust valves. The results of actual endurance tests are also reported.
Forging Method of Exhaust Valve
There are two kinds of forging methods for valve manufacturing; the upset forging and the extrusion forging. The relationship between the exhaust valve head diameter and the valve stem diameter are shown in Figure 16. The valves of motorcycles have a higher rate of valve head diameter relative to the valve stem diameter when compared to those of automobiles. Accordingly, the valves for motorcycles are often produced by upset forging. In the case of an upset forging, a wire rod having a diameter of 6-10 mm, which is slightly larger than the valve stem diameter, is often used. The smaller the diameter, the higher the cost of titanium material becomes because repetitive rolling and annealing processes require removal of oxide scale from the surface each time, causing a low material yield rate. Meanwhile in the case of extrusion forging, a bar having a diameter of 15-20 mm is used. Compared to the upset forging, as a large diameter titanium material can be used, the blank material can be produced in a simple manufacturing process. In this study, applications to mass production have been realized by selecting the most optimum extrusion forging conditions such as the forging temperature to prevent occurrence of wrinkles and/or cracks after forging.
The valve is composed of the valve head, the valve stem and the valve stem end. The required characteristics are different for each part of valve. As the valve head is subjected to high temperatures, it requires creep resistance in addition to the high temperature strength. The valve face also has to be wear resistant. The stem requires a high temperature strength along with wear resistance to withstand reciprocating motions. The end of stem requires wear resistance along with an anti-fretting property where comes in contact with the cotter.
Taking into account the outcomes from the previously-mentioned testing with test pieces, the double-phase structure having the acicular structure which excels in the creep resistance in the valve head and the equiaxed structure in the valve stem and stem end is employed in this study. To make the valve head of an acicular structure, the heat treatment using induction heating is applied. As the [beta]-transus temperature of the newly developed alloy is approximately 1100[degrees]C, the heat treatment was conducted at 1150[degrees]C, which is approximately 50[degrees]C higher. The microscopic structures of the valve head and the valve stem are shown in Figure 17. The valve head part is of an acicular structure in which acicular [alpha] is formed in the entire surface within the original [beta] grains having an approximate diameter of 300-400 [micro]m. The stem part shows a fine [alpha]+[beta] equiaxed structure.
To improve wear resistance, the OD (oxygen diffusion) treatment, which is an inexpensive surface treatment , is applied to the entire valve surfaces. This treatment significantly improves wear resistance of stem and stem end where the ambient temperature is low when the engine is operating. Also in the valve head where the ambient temperature is high when the engine is running, a wear resistance effect is attainable in the early stage of engine operation.
The measured hardness curve in the cross section of the valve stem is shown in Figure 18. The data of conventional intake valve material Ti-6Al-4V is also compared. The OD treatment condition is 710[degrees]C x 5 h for either example. Using Fisher Instrument's Fischer scope Hm2000, measurements were taken with the max load set at 20 mN, the length of time up to the max load at 20 sec, and the max load retaining time at 5 sec. The hardness of outermost surface shows approximately 700HV in either case, showing no difference of hardness distribution in the direction of depth from the surface. When the OD depth is defined as Vickers hardness 500HV or higher, it is approximately 12.5 [micro]m.
The microscopic structures of the surface are shown in Figure 19. In the zone having a 12.5 [micro]m depth from the surface where the Vickers hardness is 500HV or higher, a white oxygen diffusion layer is noticeable.
In addition to that, the shot peening using high-speed-steel beads is applied to the valve cotter fitting area to improve fretting fatigue resistance.
The exhaust valve manufacturing processes are shown in Figure 20, and the exhaust valve construction in Figure 21.
Results of Valve Endurance Tests in Actual Engine
The valves having the above-mentioned specifications were tested for endurance in the actual engine for quality assurance purposes. No fracture or crack was observed. The wear on the valve face was approximately several [micro]m. Remaining OD layer was also observed. Thus confirmed is the applicability of the exhaust valves made of newly developed alloy Ti-72SiCN to motorcycle engines in mass production.
To extend application of titanium exhaust valves, the heat resistant titanium alloy has been newly developed using the minimum necessary rare metal. Using Ti-811 as a basis, it has equivalent high temperature properties and the oxidation resistance to Ti-6242S. By selecting Ti-7Al-2Mo-0.2Si-0.15C-0.2Nb as the nominal composition, the mechanical property, creep resistance and oxidation resistance at high temperatures have been achieved. With the shift of [beta]-transus point to a higher temperature, the hot formability is also improved.
When applying the new alloy to the engine parts, the extrusion forging of bar material, the simplified anti-wear treatment, and the partial [beta] structuring are introduced to allow mass production at a high productivity while ensuring durability.
This study has demonstrated an actual example that meets the element strategies (ubiquitous strategies) proposed by Ministry of Education, Culture, Sports, Science and Technology in Japan, and Ministry of Economy, Trade and Industry in Japan.
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Shinji Kasatori and Yuji Marui
Honda R&D Co., Ltd.
Hideto Oyama and Kosuke Ono
Kobe Steel, Ltd.
CITATION: Kasatori, S., Marui, Y., Oyama, H., and Ono, K., "Development of Heat Resistant Titanium Alloy for Exhaust Valves Applicable for Motorcycles," SAE Int. J. Mater. Manf. 10(1):2017
Department 4 Technology Development Division #3 Honda R&D Co., Ltd. Motorcycle R&D Center 3-15-1 Senzui, Asaka-shi, Saitama, 351-8555 Japan
We would like to express our heartfelt thanks to the concerned ladies and gentlemen of Nittan Valve Co., Ltd. for their valuable advices and cooperation.
Table 1. Nominal composition of Ti-72SiCN and range Ti-72SiCN Al Mo Si C Nb O Ti Nominal 7.0 2.0 0.2 0.15 0.20 0.07 Bal composition Control range 6.5 1.5 0.16 0.08 0.20 0.09 - (wt%) ~7.5 ~2.5 ~0.24 ~0.15 ~0.30 Max
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|Author:||Kasatori, Shinji; Marui, Yuji; Oyama, Hideto; Ono, Kosuke|
|Publication:||SAE International Journal of Materials and Manufacturing|
|Date:||Jan 1, 2017|
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