JPH01259152A - Manufacture of tough metallic material for structural use - Google Patents

Abstract

PURPOSE:To obtain a metallic material excellent in metal fatigue resistance and having high hardness and high strength by repeatedly carrying out cold forging from lateral side while inhibiting deformation in the direction of crystal growth and changing the direction of pressurization at the time of applying plastic working to a unidirectionally solidified ingot of single-crystal metal, single-solid solution metal, etc. CONSTITUTION:A unidirectionally solidified ingot consisting of single crystals or a unidirectionally solidified ingot of single-solid solution alloy cast by a heated mold-type continuous casting method where heating is applied to the internal surface of a mold up to a temp. of the solidification temp. of a metal to be cast or above is subjected to a repetition of cold forging from lateral side while inhibiting deformation in a forging direction and changing the direction of pressurization so as to be hardened and provided with required strength, by which a hard and strong metallic material excellent in fatigue resistance can be manufactured. This material is suitable for use as a metallic material for structural machines influencing the safety of lives, such as airplanes and automobiles.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a strong structural metal material with excellent fatigue resistance. More specifically, crystals are produced in one direction by a heated mold continuous casting method for ingots, which is characterized by using a mold in which the wall surface of the mold in contact with the molten metal is kept at a temperature higher than the solidification temperature of the cast metal. A metal ingot consisting of an ingot that has grown only in grains or a single crystal without solidification grain boundaries is cast, and deformation in the direction of crystal growth is prevented, and the sides of the ingot are repeatedly deformed by changing the direction of pressure. The present invention relates to a method for manufacturing a strong structural metal material that is work hardened and resistant to fatigue fracture.

Generally, pure metals are too soft to harden even when subjected to plastic deformation and cannot be used as structural materials, so they have been used in the form of alloys by adding alloying elements to increase their strength. If the alloy is too soft by simply adding alloying elements, heat treatment such as quenching or age hardening treatment is used to harden and strengthen the alloy and use it as a structural material.

For example, materials for airplanes need to be light and strong, and for this reason, Aρ-based alloys with low specific gravity have been used for bulk materials. Pure Aβ is too soft and strong enough to be used as a structural material for airplanes, so it is used by adding Cu, Zn, Mg, etc. to create a precipitation-hardening alloy, and applying age-hardening treatment to give it strength. Ta.

It is a well-known fact that airplanes made using such alloys often crash, and the cause of the crashes has often been concluded to be due to fatigue failure of the metal material. Materials such as aircraft structural materials that are subjected to repeated stress such as vibration for long periods of time are prone to fatigue failure, so strict inspections must always be conducted to detect fatigue-induced cracks at an early stage. Ta. Even if cracks that occur on the surface of a material can be detected, cracks that occur inside the material cannot be discovered until they propagate to the surface. When cracks occur inside such materials,
It is not known whether it will propagate to the surface of the material and lead to surface destruction of the material. Therefore, passengers on airplanes have always been made to feel anxious about the possibility of such an accident occurring. There has been a strong desire for the rapid development of structural materials for airplanes that are resistant to metal fatigue.

The risk of accidents due to destruction of structural materials due to metal fatigue is
This is not limited to just airplanes, but is always associated with mechanical parts that are subjected to repeated stress such as long-term vibration, and we aim to eliminate the factors that cause fatigue failure and find a method for producing highly reliable structural materials that can be used with peace of mind. is extremely important.

When the ingot is made of polycrystalline material, the method that has been used for casting the grain boundaries between the crystals formed during solidification, that is, the solidification grain boundaries, is to pour molten metal into a cooling mold.
The ingot was obtained by solidifying the metal by removing heat from the mold. In such an ingot, columnar crystal zones were formed in which crystals grew almost perpendicular to the surface, and equiaxed crystal zones were often formed inside the ingot.

When such an ingot is cold worked, cracks tend to form at the grain boundaries of the columnar crystals on the surface, so before cold working, the ingot is first heated to soften it and then worked. It was necessary to crush the columnar crystals in the surface layer to prevent cracks from propagating from the surface to the inside during subsequent cold working.

Gases and impurities tend to segregate in these grain boundaries formed during solidification, and fine pores are particularly likely to form at the so-called triple points of grain boundaries, where three or more grain boundaries intersect. It is known. These solidification grain boundaries are the weakest places in the structure of the ingot, and when such solidification grain boundaries exist on the surface of machine parts made by processing the ingot, Once a solidified grain boundary is formed, it remains in the final product as a history of the solidified grain boundary even if plastic working or heat treatment is performed. It is a well-known fact that solidification grain boundaries, where such impurities are segregated and where intercrystalline bonds are incomplete, are vulnerable sites for fatigue failure in structural materials.

If we consider why such materials without solidification grain boundaries, which are susceptible to fatigue fracture, such as single crystals and unidirectionally solidified ingots in which crystals grow only in the casting direction of the ingot, have not been used as structural materials in the past. There seem to be two main reasons for this.

The first reason is that in the past, in order to make single crystals or unidirectionally solidified ingots, the solidification rate had to be extremely slow, such as several mm per hour, resulting in low productivity and special functions. Although it could be used as a material, it was thought to be impossible to manufacture it cheaply in large quantities as a structural material.

The second reason is that single crystals and directionally solidified ingots are soft and difficult to harden even when processed, so a strong material must be a polycrystalline body consisting of fine crystals. This is thought to be due to the fact that

The present inventor first developed a heated mold continuous casting method (Patent No. 10) in which the temperature of the inner wall of the mold is heated above the solidification temperature of the cast metal.
No. 49146). And by that method, A7! It has been found that unidirectionally solidified ingots of copper, copper, nickel, etc. can be easily produced. Then, an ingot made of a single AA-based solid solution without solidification grain boundaries that grew from the ingot surface toward the inside, which was cast using a heated mold continuous casting method, was cold-cast from the side while preventing deformation in the casting direction. The present invention was completed by discovering that a hard and strong material with low fatigue resistance can be obtained by repeatedly forging the material while changing the direction of pressure.

When plastic working is applied to a polycrystalline ingot, the ingot is deformed due to movement of dislocations generated within the crystals. and,
When these dislocations move and gather at grain boundaries, the movement stops there, the crystal hardens, and eventually breaks from the grain boundaries. Therefore, it has been said that the finer the crystal grains, the less work-hardened the metal material will be. In other words, it has been said that the larger the crystal grains, the softer the metal material is and the harder it is to harden.

This means that in the case of a single crystal without grain boundaries, the dislocations generated by W-based processing move and exit the crystal, preventing the movement of the generated dislocations and hardening the material. This indicates that in order to work harden such an ingot, it is necessary to find an effective means for preventing the movement of dislocations within the crystals.

It is known that directionally solidified ingots containing single crystals deform preferentially in the casting direction. Therefore, in order to work harden a unidirectionally solidified ingot, it was first necessary to find an effective means to prevent deformation in the casting direction and further to prevent movement of dislocations within the crystal.

The inventor of the present invention did not apply simple plastic working such as rolling or drawing to a unidirectionally solidified ingot such as a single crystal, but instead applied it to the side surface of the ingot while preventing deformation in the direction in which the crystal tends to deform, that is, in the casting direction. Af Using an ingot made of a rod-shaped polycrystalline body of -4% Cu alloy and a single crystal ingot obtained by solidifying in one direction, using a curved body from the side while preventing deformation in the length direction, 1. Compression processing was performed repeatedly while changing the pressure direction. As a result, while the polycrystalline body already broke at a Pinkers hardness of 100, the single crystal ingot did not break even at a Vickers hardness of 130.
We have discovered that a soft, unidirectionally solidified single crystal ingot can be transformed into an extremely strong structural material.

In order to obtain such structural materials strengthened by cold working, pure metals are too soft, so it is necessary to use an alloy containing other elements to the extent that the second phase does not crystallize. . When the second phase, which is weak in the solid structure, crystallizes in the form of a plate due to the addition of alloying elements, this becomes the starting point for cracking during repeated processing. However, the second phase is not crystallized during solidification from the liquid phase, but is scattered within the crystal as fine precipitated particles that precipitate after aging after the ingot forms a single solid phase body that is supersaturated with solutes. This is rather favorable for the hardening of the ingot.

Such a solute-supersaturated solid solution alloy ingot can be cast by rapidly cooling and solidifying the alloy using a heated mold continuous casting method.

All conventional structural materials are alloys, and they always have surface defects, such as solidification grain boundaries and second-phase crystallized substances such as compounds, that serve as starting points for fractures due to bending stress and fatigue fractures. Ta.

The present invention provides a method for producing highly reliable structural materials that are free from such surface defects as well as solidification grain boundaries and second phase crystallization that can become starting points for cracks inside the material. This is what we provide.

The manufacturing method for the structural material of the present invention is based solely on Al or N.
It can be applied not only to materials using i and Cu, but also to all metals that can be melt-cast and work-hardened, such as Fe, Co, and Mg.

The present invention simply changes the solidification method of the ingot, uses a heated mold continuous casting method, creates a unidirectionally solidified ingot without solidified grain boundaries that are likely to cause cracks on the surface of the ingot, and changes the direction of pressure to the ingot. This method is used to manufacture structural materials for machinery and equipment that are related to human life, such as airplanes and automobiles.It is made possible by repeatedly applying cold working to harden the materials, giving them the necessary strength and making it possible to manufacture highly reliable structural materials. I think this is groundbreaking.

Claims (1)

[Scope of Claims] 1. A method for producing a strong structural metal material, which comprises repeatedly subjecting a directionally solidified ingot to hardening while preventing deformation in the direction of crystal growth. 2. The method for producing a strong structural metal material according to claim 1, wherein the unidirectionally solidified ingot is made of a single crystal. 3. The strong structural metal according to claim 1, wherein the unidirectionally solidified ingot is obtained by a heated mold continuous casting method in which the inner wall surface of the mold is heated to a temperature higher than the solidification temperature of the cast metal. Method of manufacturing materials. 4. The method for producing a strong structural metal material according to claim 1, wherein the unidirectionally solidified ingot is a single solid solution alloy.