JP2006144063A - Titanium alloy-made engine valve manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a titanium alloy-made engine valve manufacturing method by which a valve having a preferable structure in each part, in other words, an acicular structure in a valve head part and an equi-axed structure in a shaft part is realized simultaneously with the forging without any special heating after the forging, using a extrusion-die forging method. <P>SOLUTION: A pair of upper and lower forging dies having a valve shape are used in a heated condition. A slag having the diameter larger than that of a shaft part is chamfered at an end face thereof, heated at the temperature (α+β) range, and inserted from a valve head side. A lower portion of the slag is extruded forward to extrusion-forge the shaft part, and an upper portion of the slag is die forged to develop the material in the transverse direction and mold a valve head part. By utilizing the heat generated by the composition deformation of the valve head part, the temperature of a periphery of the valve head part is set to be the value exceeding the β transus temperature of the titanium alloy, and the structure of the portion is the acicular (α+β) structure. In a Ti-6Al-4V alloy (the β transus temperature: 980°C), the heating temperature of the slag is ≥ 800°C, and, preferably, ≥ 900°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a method of manufacturing an engine valve, particularly a valve of an automobile engine, using a titanium alloy, typically a Ti-6Al-4V alloy as a material.

Engine valves such as those for automobile engines are preferably made of a material having a low specific gravity such as a titanium alloy because it is desired to be lightweight in order to achieve a good response. In addition to being light weight, the engine valve needs to have high strength and excellent heat resistance, so a material suitable for it must be selected. The Ti-6Al-4V alloy described above is suitable as a material that satisfies these requirements. However, a titanium alloy having excellent mechanical properties generally has a weakness of low formability. In this respect, a Ti-6Al-4V alloy is no exception.

Up to now, the electric upset-molding forging method has been known and used as a method for manufacturing engine valves with a titanium alloy. In this manufacturing method, first, a titanium alloy wire having the same diameter as that of the valve shaft portion is prepared, and one end thereof is energized and heated to form a large-diameter portion that becomes an umbrella portion. It consists of two steps of inserting and forging the umbrella part. This method is disadvantageous because of the process of preparing a relatively thin titanium alloy wire, because of the low formability of the titanium alloy, and the electric upset process takes time (1-2 minutes). ) As a whole, there was a problem that productivity was low. Needless to say, the low productivity, combined with the price of materials, has pushed up the cost of valves that should be manufactured at low cost.

As for the structure of the valve, the umbrella is preferably a needle-like structure having high-temperature fatigue strength, that is, needle-like α or needle-like (α + β), while the shaft is an equiaxed structure having excellent toughness. It is preferable. From this point of view, as a titanium alloy wire suitable for valve production, the microstructure of (α + β) type titanium alloy wire is an equiaxed α crystal structure, acicular α crystal structure, or equiaxed α having a particle size of 6 to 25 μm. There has been proposed a mixed structure of a crystal structure and an acicular α crystal structure (Patent Document 1).

An example of an engine valve manufacturing method by the above-described electric upset-die-insertion forging method is that β transus A manufacturing method has been developed which comprises forming an umbrella portion of needle-like α tissue at one end of a shaft portion by immediately forging hot after forming a “diameter enlarged portion” processed at a temperature exceeding the temperature. (Patent Document 2).
JP-A-6-81059 JP 2001-234313 A

As a measure to improve the productivity of a method of manufacturing an engine valve using a titanium alloy, the inventors prepared a cylindrical slag whose diameter is thicker than the shaft and chamfered the end face, and prepared the valve shape. It was put into a forging die, and was conceived to obtain a valve-shaped forging at once by stamping forging. When this method is used, the lower portion of the slag is pushed into the shaft portion of the forging die, and the slag is squeezed out by forward extrusion, while the upper portion of the slag is formed into an umbrella portion by stamping forging.

The object of the present invention is to produce a titanium alloy engine valve by the above-described squeeze-die-forging method, in which each part of the valve has a preferred structure, that is, the umbrella part has a needle-like structure and the shaft part has an equiaxed structure. An object of the present invention is to provide a manufacturing method that can be realized simultaneously with forging without performing special heat treatment after forging.

The titanium alloy engine valve manufacturing method of the present invention that achieves the above-mentioned object is a method of manufacturing an engine valve by forging that simultaneously squeezes and bevels from slag that is made of a titanium alloy and chamfered at the end face. Then, using a pair of upper and lower forging dies having a valve shape, a slag having a diameter larger than the diameter of the shaft portion is heated to a temperature in the (α + β) region. Inserting from the side, forming the shaft part by extruding the lower part of the slag forward, and then forming the umbrella part by stamping the upper part of the slag and expanding the material laterally, In principle, using the heat generated by the plastic deformation of the umbrella part, the temperature of the peripheral part of the umbrella part is set to a temperature exceeding the β transus temperature of the titanium alloy, so that the structure of the part is needle-like ( α + β) Tissue It is a manufacturing method characterized by these.

If an engine valve made of a titanium alloy is manufactured according to the manufacturing method of the present invention, it is possible to use a larger diameter cylindrical slag as a forging material without using a titanium alloy wire matched to the diameter of the shaft portion. It is easy to manufacture the material and does not require the electric upset process that was performed by the conventional method, and the valve can be obtained by one stamping and forging. Is shortened and productivity is remarkably improved. Therefore, there is an advantage that the engine valve can be provided at a lower price. In addition, the obtained valve has a needle-shaped structure with a high temperature fatigue strength and a shaft part with an equiaxed structure with high toughness and ductility, each having desirable characteristics.

Looking at automotive engine valves, titanium alloy valves can only be used for racing cars and high-end cars, but they can also be used for intermediate and lower cars by reducing costs and achieving high performance. In addition to improving engine performance, such as improving response, it also contributes to reducing environmental impact through improvements in fuel consumption.

The titanium alloy engine valve manufacturing method of the present invention based on the above principle will be described in more detail. First, the chamfering of the end surface of the slag prevents the temperature drop of the slag due to the sharp edge, and the temperature is reduced. The effect of keeping it uniform is given. Chamfering also serves to prevent galling and cracking during forging.

In order to reliably obtain the effects of the present invention, the ultimate temperature determined by the relationship between the heating temperature of the slag to be forged and the amount of equivalent plastic strain generated in the peripheral portion of the umbrella portion by die forging according to the titanium alloy to be used. However, it is important to select a heating temperature that exceeds the β transus temperature of the titanium alloy. The inventors calculated by computer simulation what the distribution of equivalent plastic strain at each position of the umbrella part would be when the umbrella part was formed by stamping forging. The result is as shown in FIG. According to this, the amount of equivalent plastic strain generated in the peripheral part of the umbrella part is 1.8 to 2.0.

On the other hand, when a titanium alloy heated to a certain temperature in advance is strained by forging, the heat generated thereby raises the temperature of the material, and as a result, how much temperature is reached is determined as Ti-6Al- A 4V alloy (β transus temperature: 980 ° C.) was examined, and the results shown in FIG. 2 were obtained.

1 and 2 can be said collectively if the slag to be forged is heated to at least 800 ° C. or more, the equivalent plastic strain is 1.8 to 2.0 at the peripheral edge of the umbrella. As a result, the temperature of the part easily exceeds the β transus temperature. In order to reliably exceed the β transus temperature, it is preferable to set the heating temperature to 900 ° C. or higher and about 950 ° C. In short, although it does not reach the β transus temperature, it is preferably a temperature close to it.

As shown in FIG. 2, the main body portion of the umbrella portion generates a higher equivalent plastic strain (2 to 3), so that the β transus temperature is more easily exceeded. In many cases, the base portion connected to the shaft of the umbrella portion does not exceed the β transus temperature because the equivalent plastic strain is low (0.7 to 1.0). In this way, an engine valve having a needle-like (α + β) structure in the peripheral part and the main body of the umbrella part and an equiaxed (α + β) structure from the base part to the shaft part of the umbrella part is obtained.

As the forging die, a die made of conventional hot working die steel can be used. Prior to forging, it is desirable to heat the mold to an acceptable limit in order to ensure that the temperature of the material reaches the β transus temperature or higher. Heat to at least 100 ° C. or higher. Preferably, it is set to 300 ° C. or higher. However, the limit of the temperature at which a metal mold manufactured with conventional hot working die steel can be heated is 500 ° C., and beyond that, the mold is easily deformed and is not suitable. Practically, it is better to stop at about 450 ° C.

In practicing the present invention, it is recommended to use techniques known in the field of die forging. For example, a graphite film is applied to the slag to be forged, or a lubricant is applied to the mold. As the lubricant, mineral oil is suitable.

Using the Ti-6Al-4V alloy as a material, an engine valve having the shape shown in FIG. 3 was manufactured. A bar with a diameter of 16 mm was manufactured from this alloy and cut into a length of 40 mm to obtain a forging slag. Forging was performed by heating the slag to 950 ° C. and the mold to 200 ° C. The obtained valve product was cut vertically to examine the tissue. About 90% of the peripheral part and the main body of the umbrella part had a needle-like structure and the rest had an equiaxed structure, and almost 100% of the part from the base part to the shaft part of the umbrella part had an equiaxed structure.

Sectional drawing which shows the quantity of the equivalent plastic strain which arises in each position of a valve umbrella part, when an engine valve is manufactured by stamping forging according to the manufacturing method of this invention. The graph which showed the relationship between the amount of the equivalent plastic strain which a forging brings, and the temperature which a forging material reaches | attains with the temperature rise resulting from it at various heating temperature. The side view which shows the shape and dimension of the engine valve manufactured in the Example of this invention.

Claims (5)

It is a method of manufacturing an engine valve by forging which is made of a titanium alloy and which is subjected to crushing and umbrella machining simultaneously from a slag whose end face is chamfered, using a pair of upper and lower forging molds having a valve shape, A slag with a diameter larger than the diameter of the shaft is heated to a temperature in the (α + β) region, inserted into the heated mold from the umbrella side, and the lower part of the slag is extruded forward to squeeze the shaft. And then forming the umbrella part by stamping the upper part of the slag and deploying the material in the lateral direction. Using the heat generated by the plastic deformation of the umbrella part, A method for producing an engine valve made of titanium alloy, characterized in that the temperature of the titanium alloy exceeds the β transus temperature of the titanium alloy so that the structure of the portion has a needle-like (α + β) structure. Depending on the titanium alloy used, the ultimate temperature determined by the relationship between the heating temperature of the slag to be forged and the amount of equivalent plastic strain generated at the periphery of the umbrella by die-casting forging exceeds the β transus temperature of the titanium alloy. The method for manufacturing a titanium alloy engine valve according to claim 1, wherein the heating is performed by selecting a proper heating temperature. The titanium alloy engine valve manufacturing method according to claim 1 or 2, wherein a Ti-6Al-4V alloy (β transus temperature: 980 ° C) is used as the titanium alloy, and the heating temperature of the slag is 930 to 1030 ° C. The titanium alloy engine valve manufacturing method according to any one of claims 1 to 3, wherein the slag is coated with a graphite film and mineral oil is used as a mold lubricant. The method for producing an engine valve made of titanium alloy according to any one of claims 1 to 3, wherein the forging die is heated to a temperature of 100 to 500 ° C.

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