JPH0466265A - Method for removing residual stress in fiber-reinforced metal - Google Patents

Abstract

PURPOSE:To remove the residual tensile stress and to prevent defect of crack, etc., by leaving under the room temp. after dipping and rapidly cooling a fiber- reinforced metal having the residual tensile stress into super low temp. liquid of liquefied nitrogen, etc., and raising the temp. CONSTITUTION:The reinforcing fiber (fine filament-state carbon fiber, etc.) are embedded into the metal (Al casting, etc.) under arranging to one direction and molten metal casting (high pressure casting) is executed, and it is formed and cooled to the room temp., to manufacture the fiber-reinforced metal. As this fiber-reinforced metal has the residual tensile stress, after dipping and rapidly cooling this into the super low temp. liquid of liquefied nitrogen (about -195 deg.C), etc., this is taken out and left under the room temp. and the temp. thereof is raised. By this method, residual compressive stress is developed to cancel the residual tensile stress and the development of defect of crack, etc., to the fiber reinforced metal is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for removing residual stress in fiber-reinforced metals, and particularly for removing tensile residual stress easily and economically by using an ultra-low temperature liquid such as liquid nitrogen. The present invention relates to a method for removing residual stress that can prevent defects such as cracks in fiber-reinforced metal.

Conventional technology Conventionally, carbon fiber reinforced metal matrix composite materials, as an example of fiber reinforced metal (FRM), are made by arranging very thin filament-like carbon fibers extremely densely in one direction, and forming carbon fibers in aluminum, which is an example of a base material. Aluminum is buried in aluminum and formed by molten metal forging (high-pressure casting), but when cooling from a high temperature state to room temperature, due to the difference in the coefficient of thermal expansion of the two materials, the aluminum cannot contract, resulting in tensile residual stress. As a result, this composite material had the drawback of being susceptible to cracking during use.

That is, the coefficient of thermal expansion α of aluminum casting is 2IXIO
-6/'C, whereas the thermal expansion coefficient α of carbon fiber is
is -0, IXI O-6/ 'C to 10, 4XI
O-'/l, and its length tends to change little with changes in heat, or to shrink slightly as the temperature rises, and to stretch slightly as the temperature falls, so when aluminum is cooled from a high temperature state, it , the carbon fiber tends to shrink significantly, but the length of the carbon fiber does not change or, on the contrary, it tries to shrink slightly, so the shrinkage action of the aluminum is blocked by the carbon fiber, and as a result, the fiber reinforcement Residual tensile stress will be generated in the metal.

However, there is no conventional technology for removing this residual stress, and the drawback of this type of fiber-reinforced metals, such as their tendency to break, remains unresolved at present.

Purpose The present invention has been made in order to eliminate the drawbacks of the above-mentioned prior art, and its purpose is to align reinforcing fibers in one direction and embed them in metal to perform molten metal casting (high pressure forging). ), a fiber-reinforced metal that has been molded and cooled to room temperature and has tensile residual stress is put into an ultra-low temperature liquid such as liquid nitrogen, rapidly cooled, and then taken out of the ultra-low temperature liquid and left at room temperature to rise in temperature. The purpose is to completely eliminate residual tensile stress by allowing compressive residual stress to settle and offset with tensile residual stress, and thereby to increase the cracking strength of carbon fiber-reinforced metal matrix composite materials in particular. .

Structure In short, the present invention is a fiber-reinforced metal that is formed by molten metal casting (high-pressure forging) by aligning reinforcing fibers in one direction, embedding them in metal, and having tensile residual stress after being cooled to room temperature. It is characterized in that it is put into an ultra-low temperature liquid, rapidly cooled, and then taken out from the ultra-low temperature liquid and left to stand at room temperature to raise the temperature, thereby generating a compressive residual stress that offsets the tensile residual stress.

In general, residual strain ε, which occurs in the base material (aluminum casting, etc.) in fiber reinforced composite materials, increases the Young's modulus of the fibers E
f, the volume fraction of the fiber is Vf, the thermal expansion coefficient of the base material is α1,
If the coefficient of thermal expansion of the fiber is α1, the temperature difference is ΔT, and the Young's modulus of the composite material is EC, then 8m = E, V, (a, -ex
, ) is expressed as ΔT/Ec.

Therefore, if a fiber-reinforced metal that has been solidified by high-pressure casting, cooled to room temperature, and has tensile residual stress, is subjected to the reverse process of high-pressure casting and solidification, it will be The present invention was developed based on the idea that since compressive residual stress is generated, the compressive residual stress and the tensile residual stress cancel each other out, and the harmful tensile residual stress is removed.

Therefore, in the present invention, reinforcing fibers are aligned in one direction, embedded in metal, molded by molten metal casting (high-pressure forging), and cooled to room temperature to create a fiber-reinforced metal that has tensile residual stress. After being put into a liquid and rapidly cooled, it is taken out from the ultra-low temperature liquid and left to stand at room temperature to raise the temperature, thereby generating compressive residual stress that offsets the tensile residual stress.

Liquid nitrogen is an ultra-low temperature liquid with a temperature of approximately -195°C, so when fiber-reinforced metal at room temperature is put into it and cooled down, it is cooled very rapidly, so it takes time for the base metal to completely shrink. It is thought that there is no increase in tensile residual stress due to shrinkage at this time.

However, when a fiber-reinforced metal is removed from an ultra-low temperature liquid and left at room temperature, the base material gradually rises in temperature and attempts to elongate (if left unattended), but in this case too, the fibers block this and do not allow the base material to elongate. Therefore, a compressive residual stress is generated in the base material, and this compressive residual stress and the previous tensile residual stress cancel each other out, and both residual stresses are removed.

Function The present invention is constructed as described above, and its function will be explained below. Since the fiber-reinforced metal produced by the method of the present invention has no residual stress in its base material,
In particular, it does not crack early due to tensile loads, has sufficient strength, and successfully solves the drawbacks of conventional fiber-reinforced metals of this type.

Effects As described above, the present invention is made by arranging reinforcing fibers in one direction, embedding them in metal, molding them by molten metal casting (high-pressure forging), cooling them to room temperature, and making the fiber-reinforced metal with tensile residual stress into a liquid. After being put into an ultra-low temperature liquid such as nitrogen, rapidly cooled, and then taken out from the ultra-low temperature liquid and left at room temperature to raise the temperature, compressive residual stress is generated and offset with the tensile residual stress, so the residual tensile stress is Stress can be completely removed, and as a result, the strength against cracking of the carbon fiber-reinforced metal matrix composite material can be particularly increased.

Patent applicant: Hino Motors Co., Ltd.

Claims (1)

[Claims] The reinforcing fibers are aligned in one direction, embedded in the metal, molded by molten metal casting (high-pressure forging), cooled to room temperature, and has tensile residual stress.The fiber-reinforced metal is then placed into an ultra-low temperature liquid such as liquid nitrogen. A method for removing residual stress in a fiber-reinforced metal, the method comprising: quenching the material, then removing it from the ultra-low temperature liquid, leaving it at room temperature and raising the temperature to generate compressive residual stress to offset the tensile residual stress.