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Development of high-workability and high heat-resistance titanium alloy sheets for automotive exhaust systems


1 Introduction

The developed automobiles exhaust system uses a titanium alloy with Cu added and a titanium alloy with Cu and Nb added. It was successfully used in a two-wheeled vehicle in 2006 and was used in a four-wheeled vehicle in 2009. The product name is Super-TIX ?10CU, Super-TIX? 10CUNB, which is currently widely used in automotive exhaust systems. In the research and development of this alloy, we chose Cu without using the precedent as an alloy addition element, and successfully achieved high processability and high heat resistance.

This article describes the reasons for choosing Cu as an additive element for alloys, and Nb is chosen to improve oxidation resistance. The design concept and characteristics of the alloy are also introduced. In particular, the research results and recent use of high temperature mechanical properties and high temperature creep properties used as exhaust gas materials have been studied. The materials for the exhaust device do not need to be high-temperature strength, high-temperature fatigue characteristics, and high-temperature oxidation resistance, and the shape, structure, and use environment of the components require good high-temperature creep resistance. Research on creep has been studied in terms of pure titanium, but it is limited to 600 ° C or less, and there is substantially no creep data on the sheet for silencer. In general, the common temperature of the two-wheel silencer is 650 ° C. In order to evalsuate the creep properties of Ti-1Cu and pure titanium (JIS 2), the mechanism is investigated.

2 required performance and alloy design of automotive exhaust system

Regardless of whether it is a two-wheeled vehicle or a four-wheeled vehicle, a titanium alloy sheet excellent in high-temperature strength and oxidation resistance which can be used in the vicinity of the engine and in the high temperature portion of the near-exhaust gas purification catalyst device is required, and several titanium alloys have been developed. sheet. On the one hand, the exhaust system components are complicated in shape due to the full play of their functions, and the finished car is subject to processability, and must be processed against complexity. That is, the material for exhaust gas needs not only excellent high-temperature strength and high-temperature oxidation resistance, but also sufficient elongation, excellent bendability, tensile formability, and deep-drawing forming at room temperature.

The required characteristics of titanium alloys for automotive exhaust must meet the following four points:

(1) There is sufficient strength at a high temperature of 600 ° C or higher, and the strength is lowered after long-term use.

(2) The reduction in wall thickness caused by oxidation at a high temperature of 700 ° C or higher is very small.

(3) Formability (bending property, bulging property, deep drawability, etc.) equivalent to JIS Class 2 and ASTM Gr. 2 pure titanium (hereinafter abbreviated as Gr. 2) at room temperature.

(4) At a high temperature of 600 ° C or higher, a certain load of less than 0.2% proof stress at a temperature is difficult to deform for a long time, that is, excellent creep resistance.

The realization of (1), that is, the improvement of high-temperature strength, representative examples are Ti-3Al-2.5V, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, and the solid-solution strengthening energy of the α phase at high temperature is a large displacement element Al, There are many additions of Sn and Si. On the other hand, the realization of (2) means that the oxidation resistance at a high temperature is improved, and a large amount of Si is often added. In this way, when designing a titanium alloy that can be used at a high temperature, Al, Sn, and Si are generally added. However, these elements have problems in increasing the room temperature strength, that is, detrimental to the excellent cold workability of pure titanium. In particular, Al can increase the lamination defect energy, suppress the twin crystal deformation of the pure titanium high-processability source at room temperature, and it is feared that the cold workability is lowered.

Under this circumstance, we have expanded the selection of alloying elements, including the solid solution of the α phase containing the quasi-stable state, the element which exhibits the solid solution strengthening energy in the high temperature region, and the element which does not inhibit the occurrence of room temperature twin crystal, and Cu is selected.

The specific alloy design concepts are as follows:

1 The maximum 2.1 mass% α phase in the equilibrium state is solid solution, and is easily over-saturated (Ti2Cu in the equilibrium phase is difficult to precipitate). The substitution element Cu is added in an amount of 1 mass% or less.

2 In order to maintain high workability of CP titanium at room temperature, it is not a normal JIS class 2 or Gr. 2, but a low oxygen amount equivalent to JIS class 1 and Gr.

(3) In order to avoid a decrease in the amount of solid solution of the α phase in the Cu concentration in the β phase, the amount of Fe of the impurity element in the strong β stabilizing element is suppressed as much as possible.

Based on this design concept, Ti-1Cu (Super-TIX? 10CU) was developed.

The use of the exhaust system in this alloy is set at 700 ° C, and the solid solution strengthening of Cu is equivalent to the low oxygen content of Gr. 1 , and high-temperature strength and room temperature high workability are achieved.

On the other hand, the high temperature oxidation property of Ti-1Cu at 500 to 800 ° C will be described in detail later. It has substantially the same properties as or better than pure titanium, especially at a high temperature of about 800 ° C. If 0.5 mass % Nb is added, the oxidation resistance of the alloy is improved. The role of Nb is to suppress the diffusion of oxygen in the oxide shell of titanium to improve oxidation resistance. At the same time, the α phase is dissolved in 1.5 to 4.5 mass% (500 to 800 ° C), the high temperature strength is lowered, and the β phase in which the solid solution amount of Cu in the α phase is reduced is not formed. In addition, Nb has no effect on room temperature processability. In this case, Ti-1Cu-0.5Nb (Super-TIX? 10CUNB) was developed, and the use temperature was designed around 800 °C.

  4 Conclusion

This paper introduces the design concept of Ti-1Cu and Ti-1Cu-0.5Nb for automotive exhaust systems and its excellent properties (room temperature formability, high temperature strength, oxidation resistance, creep resistance). These alloys have achieved low oxidation by the addition of Cu, and the necessary Nb addition. New concepts not found in the original alloy design ideas have been born. For example, Super-TIX?10CU and Super-TIX?10CUNB have been widely used as automotive exhaust systems. Application. For example, the Super-TIX® 10CU has been used in the Suzuki GSX series of mufflers for heat resistance requirements since 2007. It was used in the Nissan GT-Rspec-V in 2009 and has been used in the GT-R EGOIST silencer since 2010 (Fig. 13(a)), which is about 30% lighter than stainless steel. Super-TIX? 10CUNB is the largest exhaust component manufacturer in Europe. Akrapovic is not only used on motorcycles, but also used in car silencers (Fig. 13(b)). The high processability and high temperature strength, high oxidation resistance and creep resistance are suitable for the design and manufacturing conditions of Jiangsu tinplate alloy.

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Kunshan Minglida Stainless Steel Co., Ltd.

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