US20090269237A1

US20090269237A1 - High-strength non-combustible magnesium alloy

Info

Publication number: US20090269237A1
Application number: US12/439,601
Authority: US
Inventors: Tomio Satoh; Hidetoshi Ueno; Yoji Ogawa
Original assignee: National Institute of Advanced Industrial Science and Technology AIST
Current assignee: National Institute of Advanced Industrial Science and Technology AIST
Priority date: 2006-09-01
Filing date: 2007-02-28
Publication date: 2009-10-29
Also published as: WO2008026333A1; DE112007002016T5

Abstract

The high-strength non-combustible magnesium alloy is obtained by adding at least one supplementary additive selected from among carbon (C), molybdenum (Mo), niobium (Nb), silicon (Si), tungsten (W), alumina (Al₂O₃), magnesium silicide (Mg₂Si) and silicon carbide (SiC) to small chip-like blocks of a non-combustible magnesium alloy resulting from adding 0.5 to 5.0% by mass of calcium to a magnesium alloy to produce a crushed product, and subjecting the same to forming, sintering and plastic working. The high-strength non-combustible magnesium alloy exhibits excellent joining ability, and can therefore enhance weldability when used in a filler metal.

Description

TECHNICAL FIELD

The present invention relates to a high-strength non-combustible magnesium alloy in which the mechanical strength of a non-combustible magnesium alloy is enhanced.

BACKGROUND ART

Magnesium alloys are very lightweight, for which reason they have received attention as substitutes for aluminum or aluminum alloys. Magnesium alloys belong to the lightest metals in practical use, and has, for instance, fairly high specific strength and specific elasticity modulus values, which result from dividing strength and elastic modulus by density. As a result, the demand for such magnesium alloys is expected to grow in the future in industrial fields where lightweightness is required. Although titanium and aluminum alloys are strong enough, they have the disadvantages of, for instance, being less lightweight and having poorer shock-absorbing characteristics than magnesium alloys.
Although ordinary magnesium alloys exhibit comparatively high specific strength, their absolute strength is lower than that of titanium or aluminum alloys. Another known shortcoming of magnesium alloys is their low ignition point, which renders them readily combustible. Therefore, non-combustible magnesium alloys have been disclosed in which calcium is added to a magnesium alloy, to raise the ignition point of the latter and yield a non-combustible magnesium alloy that does not ignite readily, the non-combustible magnesium alloy being subjected to plastic working such as extrusion, rolling or the like (Patent document 1).
There have been proposed numerous improved magnesium alloys aiming at achieving a strength corresponding to that of titanium or aluminum alloys. For instance, there have been disclosed magnesium alloys, and methods for manufacturing the same, wherein the magnesium alloys have high strength, high specific strength and small grain size after plastic deformation, the non-combustible magnesium alloys being obtained by adding a predetermined amount of Ca, Zn and X to Mg (wherein X denotes at least one rare earth element selected from the group consisting of Y, Ce, La, Nd, Pr, Sm and Mm), and wherein the magnesium alloy exhibits a structure in which the foregoing compounds are finely dispersed (for instance, Patent document 2). The fine structure is achieved specifically by adding a predetermined amount of a rare earth element, and through rapid-solidification atomization.
There has also been disclosed a magnesium alloy that comprises 0.03 to 0.54 at % of one solute atom belonging to group 2 or 3 or to the lanthanoid series of the periodic table, and having an atomic radius larger than that of magnesium, and the balance magnesium, wherein the magnesium alloy exhibits simultaneously high strength and high ductility (for instance, Patent document 3). This magnesium alloy has a fine grain structure in which the average grain size is no greater than 1.5 μm and the solute atoms in the vicinity of the grain boundary are locally more abundant at a concentration of 1.5 to 10 times that of the solute atoms in the crystal grains.
Other disclosed magnesium alloys exhibiting both high strength and high ductility include an alloy in which magnesium comprises 1.0 to 4.0 at % of Zn and 1.0 to 4.5 at % of Y at a predetermined composition ratio of Zn and Y, the structure of the alloy comprising simultaneously an intermetallic compound Mg₃Y₂Zn and Mg₁₂YZn having a long-period structure (for instance, Patent document 4). The proof stress, tensile strength and elongation of the alloy are enhanced thanks to the simultaneous presence of the intermetallic compound Mg₃Y₂Zn and long-period phase Mg₁₂YZn.
As described above, non-combustible magnesium alloys resulting from adding calcium to a magnesium alloy exhibit a high ignition point, high mechanical strength and good handleability. Therefore, there have been disclosed technologies for exploiting these advantageous features by making these non-combustible magnesium alloys into articles such as helmets (for instance, Patent document 5) and spectacle frames (for instance, Patent document 6). Also, magnesium alloys can potentially be used in structural members in a wide variety of applications, for instance in mobile structures of automobiles, two-wheel vehicles, rail vehicles, aircraft, robots and the like, as well as in assistive devices, devices for the elderly and the like.
Such structural members inevitably require joining, in particular welding, between members. Various magnesium alloy welding technologies are being developed, for instance laser welding, TIG welding, MIG welding and the like. A magnesium welding line has been disclosed in which, for instance, a base material such as an extruded product of a magnesium base alloy, although not a non-combustible magnesium alloy according to the present invention, is drawn, followed by surface shaving, to yield a magnesium welding wire having excellent surface cleanability (for instance, Patent document 7). Although not a non-combustible magnesium alloy according to the present invention, known wires of a magnesium-base alloy having excellent strength and ductility include, for instance, magnesium base alloys comprising components such as Al, Mn, Zn, Zr and rare earth elements (for instance, Patent document 8).
As described above, non-combustible magnesium alloys resulting from adding calcium to a magnesium alloy exhibit a high ignition point, high mechanical strength and good handleability. Specific examples in which these advantages are exploited include the above-described Patent documents 5 and 6, which disclose joining where separate members are integrally joined, with the members butting each other, by melt welding, for instance by laser welding, TIG welding or MIG welding.
Patent document 1: JP 2000-109963 A
Patent document 2: JP 9-41065 A
Patent document 3: JP 2006-16658 A
Patent document 4: JP 2006-97037 A
Patent document 5: JP 2005-350808 A
Patent document 6: JP 2005-196094 A
Patent document 7: JP 2006-263744 A
Patent document 8: Japanese Patent No. 3592310
As described above, various technologies have been proposed for improving the mechanical characteristics of magnesium alloys. However, magnesium alloys continue to be beset by numerous problems, being still unsatisfactory and insufficient for being used in actual articles. The applicants had already claimed the technology of Patent document 1, in which a high-strength non-combustible magnesium alloy is an alloy comprising 0.1 to 15% by mass of Ca and has Al and Zn partially added thereto. The present invention, which further develops the above technology, is a yet stronger alloy. In Patent document 2, expensive rare earth elements must be added, and thus the alloy obtained as a result is inevitably a high-cost alloy. Also, the technology of Patent document 2 uses rapid-solidification atomization and must resort to high-technology means. Although the alloy afforded by Patent document 2 exhibits high strength, with a proof stress of 510 to 635 MPa, the fracture elongation of the alloy is very small, of 1.0 to 4.0%, characteristic of a highly brittle material.
Patent document 3 features an alloy having improved yield stress and elongation. Except for the specified solute atom Ca, however, all other elements are rare earth elements, which makes hence for a high-cost alloy, as is the case above. The magnesium alloy set forth in Patent document 4 achieves a tensile strength of 390 to 520 MPa and a fracture elongation of 4.5 to 10.3% only under the simultaneous presence of the intermetallic compound Mg₃Y₂Zn and the long-period phase Mg₁₂YZn. This indicates that a combination of high strength and high ductility cannot be achieved in the presence of either the intermetallic compound or the long-period phase alone.
The technology of Patent document 7 relates to a magnesium welding line, the purpose of the technology being to enhance the surface cleanability of the welding line. The technology does not relate to the composition of a so-called filler metal, such as a welding line or a welding rod. Also, the welding line is not a non-combustible magnesium alloy. Although the technology of Patent document 8 relates to a magnesium-base alloy wire, the wire comprises components such as Al, Mn, Zn, Zr and rare earth elements. The composition of the magnesium-base alloy wire differs thus from that of the high-strength non-combustible magnesium alloy of the present invention. The purpose of Patent document 8 is to provide a spring using the wire. Although Patent document 8 mentions the possibility of using the wire as a welding line, no specific example of such use is disclosed at all. Moreover, the welding line does not relate to a non-combustible magnesium alloy.
Patent document 5 discloses the possibility of using a non-combustible magnesium alloy in a helmet, and the feature of welding the non-combustible magnesium alloy, as the case may require, by melt welding such as laser welding, TIG welding, MIG welding or the like. However, Patent document 5 discloses no specific example of such a feature. In the subject matter set forth in Patent document 5, moreover, the non-combustible magnesium alloy is the material to be welded on the welding side, and not a filler metal for welding.
Thus, the advantages of above-described magnesium alloys of Patent documents 2, 3 and 4 are offset by their shortcomings as regards characteristics required in engineering materials. In all cases, expensive rare earth materials are added to the alloys. This is problematic in that the magnesium alloys ultimately manufactured are likewise expensive. Although Patent documents 5 and 6 disclose joining technologies, the non-combustible magnesium alloys involved are not the high-strength non-combustible magnesium alloy according to the present invention. The technologies set forth in Patent documents 7 and 8 involve improving the surface characteristics of a welding line, or using expensive rare earth elements, and relate to improving the mechanical characteristics of the wire itself. In both Patent documents 7 and 8, the characteristics of the wire, which is not the non-combustible magnesium alloy according to the present invention, are insufficient.

DISCLOSURE OF THE INVENTION

In the light of the above issues of conventional art, the present invention achieves the following goals.
An object of the present invention is to provide a non-combustible magnesium alloy imparted with high strength characteristics, including high tensile strength and high proof stress, through the addition of versatile elements and/or compounds, without using alloying elements limited to rare earth elements.
Another object of the present invention is to provide a non-combustible magnesium alloy, having improved stable weldability at low cost, that can be used as a filler metal.
The present invention achieves the above objects on the basis of the following means.
The high-strength non-combustible magnesium alloy of Invention 1 is a high-strength non-combustible magnesium alloy, obtained by adding at least one supplementary additive selected from among carbon (C), molybdenum (Mo), niobium (Nb), silicon (Si), tungsten (W), alumina (Al₂O₃), magnesium silicide (Mg₂Si) and silicon carbide (SiC) to a non-combustible magnesium alloy resulting from adding 0.5 to 5.0% by mass of calcium to a magnesium alloy.
The high-strength non-combustible magnesium alloy of Invention 2 is Invention 1, wherein the amount of carbon (C) of the supplementary additive is 0.1 to 0.3% by mass.
The high-strength non-combustible magnesium alloy of Invention 3 is Invention 1, wherein the amount of molybdenum (Mo) of the supplementary additive is 1.0 to 12.0% by mass.
The high-strength non-combustible magnesium alloy of Invention 4 is Invention 1, wherein the amount of niobium (Nb) of the supplementary additive is 0.5 to 5.0% by mass.
The high-strength non-combustible magnesium alloy of Invention 5 is Invention 1, wherein the amount of silicon (Si) of the supplementary additive is 0.5 to 6.0% by mass.
The high-strength non-combustible magnesium alloy of Invention 6 is Invention 1, wherein the amount of tungsten (W) of the supplementary additive is 5.0 to 40.0% by mass.
The high-strength non-combustible magnesium alloy of Invention 7 is Invention 1, wherein the amount of alumina (Al₂O₃) of the supplementary additive is 1.0 to 5.0% by mass.
The high-strength non-combustible magnesium alloy of Invention 8 is Invention 1, wherein the amount of magnesium silicide (Mg₂Si) of the supplementary additive is 2.0 to 6.0% by mass.
The high-strength non-combustible magnesium alloy of Invention 9 is Invention 1, wherein the amount of silicon carbide (SiC) of the supplementary additive is 0.7 to 20.0% by mass.
The high-strength non-combustible magnesium alloy of Invention 10 is Invention 1, wherein the magnesium alloy is one magnesium alloy comprising 0 to 12.0% by mass of aluminum, 0 to 5.0% by mass of zinc and not more than 0.5% by mass of manganese.
The high-strength non-combustible magnesium alloy of Invention 11 is Invention 1, wherein the magnesium alloy is a magnesium alloy selected from among AZ31 alloy, AZ61 alloy, AZ80 alloy, AZ91 alloy, AZ92 alloy, AM50 alloy, AM60 alloy and AM100 alloy according to the American Society for Testing and Materials (ASTM).
The high-strength non-combustible magnesium alloy of Invention 12 is Invention 1, wherein the non-combustible magnesium alloy comprises a crushed product obtained from a base material of the non-combustible magnesium alloy.
The high-strength non-combustible magnesium alloy of Invention 13 is Invention 1, wherein the high-strength non-combustible magnesium alloy is an alloy manufactured by adding the supplementary additive, followed by plastic working in which permanent deformation is imparted through application of an external force.
The high-strength non-combustible magnesium alloy of Invention 14 is Invention 12, wherein the crushed product is cutting chips obtained by cutting, or a powder thereof.
The high-strength non-combustible magnesium alloy of Invention 15 is Invention 13, wherein the plastic working is one type of working from among extrusion, drawing, roll forging and rolling, or a combination of two or more of the foregoing.
The high-strength non-combustible magnesium alloy of Invention 16 is any of Inventions 1 to 15, wherein an alloy formed by adding the supplementary additive is an alloy that makes up a filler metal.
The high-strength non-combustible magnesium alloy of Invention 17 is Invention 16, wherein the filler metal is a wire-shaped or rod-shaped welding material.
As explained above, the high-strength non-combustible magnesium alloy of the present invention is a low-cost non-combustible magnesium alloy, to which high tensile strength and high proof stress is imparted by way of supplementary additives in the form of versatile elements and/or compounds, without using alloying elements limited to expensive rare earth elements, and by forming, sintering and plastic-working a crushed product. Thanks to having the ignition point thereof raised through addition of Ca, the non-combustible magnesium alloy of the present invention can be joined, as a filler metal, under ordinary conditions, giving rise to little fumes during welding (such fumes being substances that are vaporized by heat during welding or shearing and which cool into solid microparticles). Furthermore, the joining ability of the high-strength non-combustible magnesium alloy is enhanced, at low cost, by using effectively a crushed product of cutting chips or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a data diagram of tensile strength test results of a high-strength non-combustible magnesium alloy in which C is added to a crushed product of a non-combustible magnesium alloy “AM60B+2Ca alloy”;

FIG. 2 is a data diagram of tensile strength test results of a high-strength non-combustible magnesium alloy in which Mo is added to a crushed product of a non-combustible magnesium alloy “AM60B+2Ca alloy”;

FIG. 3 is a data diagram of tensile strength test results of a high-strength non-combustible magnesium alloy in which Nb is added to a crushed product of a non-combustible magnesium alloy “AM60B+2Ca alloy”;

FIG. 4 is a data diagram of tensile strength test results of a high-strength non-combustible magnesium alloy in which Si is added to a crushed product of a non-combustible magnesium alloy “AM60B+2Ca alloy”;

FIG. 5 is a data diagram of tensile strength test results of a high-strength non-combustible magnesium alloy in which W is added to a crushed product of a non-combustible magnesium alloy “AM60B+2Ca alloy”;

FIG. 6 is a data diagram of tensile strength test results of a high-strength non-combustible magnesium alloy in which Al₂O₃is added to a crushed product of a non-combustible magnesium alloy “AM60B+2Ca alloy”;

FIG. 7 is a data diagram of tensile strength test results of a high-strength non-combustible magnesium alloy in which Mg₂Si is added to a crushed product of a non-combustible magnesium alloy “AM60B+2Ca alloy”;

FIG. 8 is a data diagram of tensile strength test results of a high-strength non-combustible magnesium alloy in which SiC is added to a crushed product of a non-combustible magnesium alloy “AM60B+2Ca alloy”;

FIG. 9 is a data diagram illustrating tensile strength test results, including a comparative example, of a plate welded using a high-strength non-combustible magnesium alloy, containing supplementarily added elements, as a filler metal; and

FIG. 10 is a data diagram illustrating tensile strength test results, including a comparative example, of a plate welded using a high-strength non-combustible magnesium alloy, containing supplementarily added compounds, as a filler metal.

BEST MODE FOR CARRYING OUT THE INVENTION

High-Strength Non-Combustible Magnesium Alloy

Embodiments of the non-combustible magnesium alloy of the present invention are explained in detail next. To make the present invention easier to grasp, the magnesium alloy will be explained first. The magnesium alloy is standardized, for instance, according to the American Society for Testing and Materials (hereinafter “ASTM”) or Japanese Industrial Standard (hereinafter “JIS”). Magnesium alloys can be roughly divided into cast magnesium alloys and wrought magnesium alloys. The ranges from minimum to maximum values for mechanical characteristics, as prescribed in ASTM and JIS for both kinds of alloy, are given below. The chemical component compositions of these alloys are standardized and known in the art, and thus a detailed explanation thereof will be omitted.
The mechanical characteristics of cast magnesium alloys are as follows. Tensile strength: 140 MPa (AM100A-F material) to 270 MPa (ZK61A-T5, T6 treated material). Proof stress: 70 MPa (AM100A-F material) to 180 MPa (ZK61A-T5, T6 treated material). Elongation: about 0% (AM100A-F material) to 10% (AM50A-F material).
The mechanical characteristics of wrought magnesium alloys are as follows. Tensile strength: 190 MPa (AZ31C-O material) to 310 MPa (ZK60A-T5 treated material). Proof stress: 90 MPa (AZ31C-O material) to 230 MPa (ZK60A-T5 treated material). Elongation: 4% (AZ31C-H14 treated material) to 13% (AZ31C-O material).
Plastic working and thermomechanical treatment cause mechanical properties of metals, such as strength and ductility, to improve considerably more in wrought alloys than in cast alloys. Although mechanical properties improve also in magnesium alloys, as described above, strength and ductility do not improve as much as they do in other metals. Further technical developments have resulted in the technologies featured in the above-described patent documents.
In the present embodiment, a non-combustible magnesium alloy, having been imparted non-combustibility through the addition of Ca, is further added supplementary additives, in the form of low-cost elements or compounds, to enhance the mechanical strength of the non-combustible magnesium alloy. The present embodiment proposes a high-strength non-combustible magnesium alloy having mechanical characteristics that include a tensile strength not smaller than 419 MPa at room temperature and a proof stress not smaller than 380 MPa, obtained by using a crushed product of a non-combustible magnesium alloy that is then formed, sintered and plastic-worked.
This alloy is explained next. The magnesium alloy illustrated in the embodiments of the present invention is a cast magnesium alloy designated as “AM60B” in ASTM. The alloys for which the embodiment of the present invention can be used are not necessarily limited to this cast magnesium alloy “AM60B”, and may be other magnesium alloys. To this alloy there is added 0.5 to 5.0% by mass of Ca. In the present embodiment there is added 2% by mass of Ca.
AM60B is a die-cast high-purity magnesium alloy that contains little Fe, Ni and Cu impurities with a view to enhancing corrosion resistance. The basic chemical composition of the alloy is Al 5.5 to 6.5% by mass, Mn 0.24 to 0.60% by mass, and balance magnesium. Calcium is added to this alloy to yield a non-combustible magnesium alloy. As described above, the amount of calcium added ranges preferably from 0.5 to 5.0% by mass.
Magnesium has a close-packed hexagonal crystal structure. Therefore, the plastic workability of magnesium at room temperature is very poor, and so magnesium cannot be cold-worked. Although plastic workability increases with hot working, magnesium remains harder to work into fine shapes than other metals. The manufacture of magnesium alloys involves thus mainly casting methods. The castings obtained by casting, as well as forged and wrought materials obtained by plastic working, are often finished to their final shape by cutting. However, there are limits to the treatment of cutting chips generated during cutting, on account of, for instance, the high cost of the treatment, while re-using untreated cutting chips, as recycled material, is fraught with numerous problems.
Although effective use of cutting chips has been the object of ongoing research, no conclusive instances have been disclosed in which such research has been translated into practice. In the present example cutting chips of non-combustible magnesium alloy are used as a base. This non-combustible magnesium alloy can be cut at high speed thanks to its good cutting properties. Cutting chips can be formed therefore in substantial amounts. However, the present invention is not limited to cutting chips, and may encompass powders or small-piece blocks so long as these are chip-like.

Method for Manufacturing the High-Strength Non-Combustible Magnesium Alloy

A method for manufacturing the high-strength non-combustible magnesium alloy of the present invention is explained next. In the present embodiment, the alloy used as the base magnesium alloy was a non-combustible magnesium alloy “AM60B+2Ca alloy” having added thereto 2% by mass of Ca. Although AM60B is originally a cast magnesium alloy, it can also undergo plastic working such as extrusion or the like when hot-worked. Plastic working includes, for instance, extrusion, drawing, forging, roll forging, rolling and the like. Adding 2% by mass of Ca to the AM60B allows raising to 200 to 300° C. the ignition temperature of the non-combustible magnesium alloy “AM60B+2Ca alloy” obtained through addition of 2% by mass of Ca to this AM60B.
As a result, melting operations can be safely conducted in the atmosphere. The non-combustible magnesium alloy “AM60B+2Ca alloy” must afford small chip-like blocks that are suitable for a subsequent crushing step. In the present embodiment there were suitably used cutting chips resulting from cutting the non-combustible magnesium alloy “AM60B+2Ca alloy”. Needless to say, the small chip-like blocks are not limited to cutting chips formed by cutting, and there may be used, for instance, various cutting chips and grinding dust, as waste of various mechanical working processes, press scrap resulting from shearing, punching and the like, crushed waste from crushers, or a crushed product of small chip-like blocks from castings and forgings. A ball mill or the like is used to obtain a crushed product of such small chip-like blocks.
In the case of the non-combustible magnesium alloy of the present embodiment, non-combustibility is imparted through addition of Ca, and hence it is safe to leave the crushed product to stand in the atmosphere, at normal temperatures. For instance, the lowest explosive limit of a crushed product of the non-combustible magnesium alloy “AM60B+2Ca alloy” having an average particle size of 146 μm is 100 mg/m³. This value is higher than that of aluminum powder (35 mg/m³), and of the order of iron powder (<120 mg/m³). The crushed product can thus be handled easily, with a substantially reduced danger of explosion.
A predetermined element or compound is added, as a supplementary additive characteristic of the present embodiment, when obtaining a crushed product out of small chip-like blocks. The supplementary additive is not limited to a rare earth element, and may include various elements, or predetermined compounds thereof, in proportions such as, for instance, 0.1 to 0.3% by mass of C, 1.0 to 12.0% by mass of Mo, 0.5 to 5.0% by mass of Nb, 0.5 to 6.0% by mass of Si, 5.0 to 40.0% by mass of W, 1.0 to 5.0% by mass of Al₂O₃, 2.0 to 6.0% by mass of Mg₂Si, 0.7 to 20.0% by mass of SiC.
These limitations imposed on the types of elements or compounds and the addition amounts thereof denote the ranges within which the manufactured non-combustible magnesium alloy can be imparted high strength, the strength-increasing effect being weaker outside these ranges. One type or selected plural types of the elements or compounds are supplementarily added, to carry out simultaneously crushing of the small chip-like blocks and combining of the element or compound. Specifically, the solidification structure of the non-combustible magnesium alloy, in the form of small chip-like blocks, is broken in the crushing process, and is modified into a fine homogeneous structure. Simultaneously therewith, the supplementary additive is taken up uniformly into the powder, to afford thereby the fine and homogeneous structure of the non-combustible magnesium alloy.

Forming and Sintering

The crushed non-combustible magnesium alloy having thus a fine and homogeneous structure is then formed and sintered. Forming can be accomplished by both cold forming or hot forming, but in terms of shortening the process, forming is preferably carried out by hot forming, since the latter can take place simultaneously with sintering. Pulse electric current sintering is suitable for hot forming. Pulse electric current sintering is a known processing method in which a target sample is filled into a graphite mold and is then sintered through flow of pulse-like current while the sample is being compressed. In the present example, the sample is the above-described crushed product made of the non-combustible magnesium alloy. Such a processing method is advantageous in that the crushed product can be heated with good efficiency, and sintering can be carried out rapidly.
A billet-shaped sintered compact of the non-combustible magnesium alloy crushed product thus formed and sintered is plastic-worked next. By imparting shear deformation to the sintered compact, plastic working has the effect of strengthening the cohesion within the crushed product beyond that of the sintered compact, while making the microstructure of the sintered compact yet finer. Plastic working methods include, for instance, extrusion, rolling, drawing, forging, roll forging and the like. In the present example there is used hot extrusion carried out at a temperature at or above the recrystallization temperature of the material, since extrusion allows imparting considerable shear deformation to the work. To some extent, a high extrusion ratio causes the obtained material to exhibit increased mechanical strength. However, raising the extrusion ratio beyond necessity results in a shorter life of the extrusion mold or in mold damage, and requires larger extrusion equipment, among other drawbacks. Accordingly, the extrusion ratio is preferably of about 120 at most.
By virtue of this extrusion forming, the crushed particles in the sintered compact become strongly bonded to one another on account of the shear deformation to which they are subjected. The particles of the intermetallic compounds originally contained in the non-combustible magnesium alloy, as well as the supplementary additive, make up thereby a structure that is homogeneously dispersed in the magnesium matrix. The magnesium matrix crystal particles become yet finer thanks to the recrystallization that takes place during hot extrusion. This affords as a result enhanced mechanical characteristics and higher strength.
Other than as disclosed in the above-described embodiment, the magnesium alloy used can also be effective when comprising 0 to 12.0% by mass of aluminum, 0 to 5.0% by mass of zinc, and no more than 0.5% by mass of manganese. Effective results can also be achieved by using one magnesium alloy from among AZ31 alloy, AZ61 alloy, AZ80 alloy, AZ91 alloy, AZ92 alloy, AM50 alloy, AM60 alloy and AM100 alloy according to the American Society for Testing and Materials (ASTM).
The present invention can also be employed in a filler metal by using the high-strength non-combustible magnesium alloy having the above characteristics as a base material. Filler metals used during welding, such as welding rods or welding lines (also called “welding wires”) comprise herein the high-strength magnesium alloy according to the present invention, in which a magnesium alloy has added thereto 0.5 to 5% by mass of calcium (Ca), and supplementary, also at least one from among C, Mo, Nb, Si, W, Al₂O₃, Mg₂Si, SiC. Such an alloy has a higher ignition point and greater strength.
Non-combustibility reduces the risk of fire or the like caused by sparks during joining, which can thus proceed safely. As is known, substances that are vaporized by heat during welding cool down into fumes of solid microparticles. Occurrence of such fumes can be suppressed, however, by using the filler metal according to the present invention. The present invention can thus contribute to improving the welding environment at the actual welding site.
Filler metals are obtained, for instance, through extrusion or wire drawing using roller dies specialized for wire drawing. Thanks to the above extrusion or wire drawing, the supplementary additive contained in the filler metal of the present invention can become yet more homogeneously dispersed in the magnesium matrix. The weld structure can become as a result more homogeneous, which allows enhancing mechanical characteristics.
As a filler metal, the high-strength non-combustible magnesium alloy of the present invention can be used in all welding techniques in which magnesium or magnesium alloys are welded, but can be ideally used, in particular, in TIG welding and MIG welding. In the examples below joining is carried out by TIG welding. Needless to say, the present invention is not limited to the embodiment explained above.

Example 1

The alloy in the present example comprised a base of the non-combustible magnesium alloy “AM60B+2Ca”, having added thereto 2.0% by mass of Ca to impart non-combustibility to the AM60B alloy. To this base there were added, as supplementary additives, elements or compounds of C, Mo, Nb, Si, W, Al₂O₃, Mg₂Si and SiC so as to yield the composition given in Table 1. Cutting chips in the form of lathe-turning waste were used as the small chip-like blocks of the alloys in the present example. These cutting chips were crushed in a ball mill to yield a crushed product. The supplementary additives were added simultaneously with crushing, to homogeneously disperse and combine the additives thereby.
The crushed products of the non-combustible magnesium alloys thus prepared in the ball mill were then solid-formed in the atmosphere by pulse electric current sintering at a sintering temperature or 480° C. for 20 minutes. The sintered compacts, in the form of billets, were then hot-extruded at an extrusion ratio of 110 and an extrusion temperature of 480° C. Specimens were taken in the longitudinal direction of the obtained extruded products, and were tested for tensile strength, proof stress and fracture elongation at room temperature. The test results are summarized in Table 2. The results showed that the tensile strength was not smaller than 419 MPa and the proof stress not smaller than 380 MPa in all specimens, which was indicative of the effect of the present invention.

TABLE 1

Test	Composition (wt %)

mate-	AM60B +
rial	2Ca	C	Mo	Nb	Si	W	Al₂O₃	Mg₂Si	SiC

1	99.9	0.1
2	99.8	0.2
3	98.9		1.1
4	94.5		5.5
5	89.5		10.5
6	99.0			1.0
7	95.4			4.6
8	99.0				1.0
9	95.0				5.0
10	94.8					5.2
11	87.9					18.1
12	63.7					36.3
13	98.9						1.1
14	95.6						4.4
15	97.8							2.2
16	94.5							5.5
17	90.1							9.9
18	98.2								1.8
19	91.3								8.7
20	83.4								16.6

	TABLE 2

	Tensile test results

	Tensile	Proof
Test	strength	stress	Fracture
material	(MPa)	(MPa)	elongation (%)

1	427	423	3.3
2	441	433	8.4
3	427	410	18.4
4	434	400	13.0
5	444	426	8.6
6	433	420	13.7
7	438	431	6.1
8	439	414	6.6
9	448	427	3.8
10	424	418	7.2
11	447	420	14.6
12	476	456	6.0
13	429	397	14.3
14	444	425	9.6
15	419	383	14.3
16	459	434	8.6
17	454	430	7.7
18	428	426	11.5
19	490	443	9.4
20	467	437	5.0

FIGS. 1 to 8 illustrate data for each element or compound in the above results. The figures are data diagrams illustrating tensile strength, proof stress and fracture elongation versus the addition amounts of the various elements or compounds, wherein FIG. 1 corresponds to addition of C, FIG. 2 to addition of Mo, FIG. 3 to addition of Nb, FIG. 4 to addition of Si, FIG. 5 to addition of W, FIG. 6 to addition of Al₂O₃, FIG. 7 to addition of Mg₂Si, and FIG. 8 to addition of SiC.
As the data diagrams clearly illustrate, these results show that in all cases where the supplementary additives were added in the present example, mechanical strength increased beyond that of conventional Ca-added non-combustible magnesium alloys disclosed in Patent document 1, which contains no supplementary additives. For instance, tensile strength was not smaller than 419 MPa in all instances where a supplementary additive was added. The above high-strength non-combustible magnesium alloys can therefore be said to be yet stronger as base materials. In the figures, the values of 0% addition amount denote the results of the comparative examples below.
The comparative examples below were carried out for comparison purposes versus the present example. The results of the present example exceeded those of all the comparative examples below.

Comparative Example 1

Comparative example 1 was performed for a conventional non-combustible magnesium alloy lacking the supplementary additives according to the present invention. Cutting chips from lathe-turning of a forging of non-combustible magnesium alloy “AM60B+2Ca alloy”, having the same chemical composition as that of the Example, were made into a crushed product in a ball mill. The crushed product was then formed and sintered by pulse electric current sintering under exactly the same conditions as in the Example. The sintered compact, in the form of a billet, was then hot-extruded at an extrusion ratio R=110 and an extrusion temperature T=480° C., under the same conditions as in the Example. The obtained extruded product was tested for tensile strength at room temperature, in the longitudinal direction. The results of the tensile strength test yielded a tensile strength of 415 MPa, a proof stress of 364 MPa and a fracture elongation of 23%. These values are depicted as values of 0% of the various additives in the left end of FIGS. 1 to 8, which illustrate the results of the Example.

Comparative Example 2

Comparative example 2 was performed for a conventional non-combustible magnesium alloy lacking the supplementary additives according to the present invention. A forging of non-combustible magnesium alloy “AM60B+2Ca alloy”, having exactly the same chemical composition as that of the Example, was extruded at an extrusion ratio R=110 and an extrusion temperature T=480° C., under the same conditions as in the Example. The obtained extruded product was tested for tensile strength at room temperature, in the longitudinal direction. The results of the tensile strength test yielded a tensile strength of 305 MPa, a proof stress of 242 MPa and a fracture elongation of 18%.

Comparative Example 3

Comparative example 3 was performed for a conventional non-combustible magnesium alloy lacking the supplementary additives according to the present invention. A forging of non-combustible magnesium alloy “AM60B+2Ca alloy”, having exactly the same chemical composition as that of the Example, was hot-extruded and then hot-drawn. The obtained drawn product was tested for tensile strength at room temperature, in the longitudinal direction. The results of the tensile strength test yielded a tensile strength of 286 MPa, a proof stress of 198 MPa and a fracture elongation of 16%.

Example 2

In the present example there was assessed the joining effect when using the high-strength non-combustible magnesium alloys illustrated in FIGS. 1 to 8 as a welding wire, which is a filler metal in magnesium alloy welding. As the member to be welded there was used a plate (plate thickness 2 mm) extruded from a non-combustible magnesium alloy “AM60B+2Ca alloy” obtained by adding 2% by mass of Ca to impart non-combustibility to an AM60B alloy. Welding was carried out by TIG. The main welding conditions were as follows.
There was used a pure tungsten electrode having a diameter of 2.4 mm, the distance between electrode and base metal was 2 mm, the welding speed was 200 mm/min, with AC current of 100 A, and argon gas was used as an inert gas, at a flow rate of 12 L/min. After welding, the weld overlay was removed to shape a specimen that was then tested for tensile strength, to assess joint strength. The tensile strength test results are given in Table 3 and are illustrated in FIGS. 9 and 10. FIG. 9 illustrates results for each supplementarily added element, while FIG. 10 illustrates results for each supplementarily added compound. In FIGS. 9 and 10, the horizontal axis represents the type of supplementary additive and the composition thereof. Except for the supplementary additives 5Si and 9Mg₂Si, the results of the example exceeded those of the comparative examples, and confirmed the effect of the present invention. The results in the cases of the supplementary additives 5Si and 9Mg₂Si arose from deficient welding caused by coarse welding defects. These strength test results are thus abnormal.

	TABLE 3

	Filler metal	Tensile strength
	composition	(MPa)

	0.1C	224
	0.2C	228
	1Mo	222
	6Mo	231
	11Mo	222
	1Nb	241
	5Nb	231
	1Si	193
	5Si	59
	5W	191
	18W	196
	36W	176
	1Al₂O₃	226
	4Al₂O₃	188
	2Mg₂Si	230
	5Mg₂Si	199
	9Mg₂Si	75
	2SiC	181
	9SiC	192
	17SiC	178
	Comp. ex.	173

Comparative Example 4

In the present example there was assessed the joining effect when using as a filler metal a conventional non-combustible magnesium alloy lacking the supplementary additive according to the present invention. As the filler metal, i.e. as a welding wire in the present comparative example, there was used a drawn product manufactured through hot extrusion, followed by hot drawing, of a forging of non-combustible magnesium alloy “AM60B+2Ca alloy”. TIG welding was carried out using the same material to be welded and the same welding conditions as in the Example. After welding, the weld overlay was removed to shape a specimen that was then tested for tensile strength to assess joint strength, as in the Example. The results are given in Table 3 as “Comparative example” and are illustrated in FIGS. 9 and 10 as “Comparative example”. The joint tensile strength of the welded plate in the Comparative example was 173 MPa, lower than in all the examples.

Claims

1. A high-strength non-combustible magnesium alloy, obtained by adding at least one supplementary additive selected from among carbon (C), molybdenum (Mo), niobium (Nb), silicon (Si), tungsten (W), alumina (Al₂O₃), magnesium silicide (Mg₂Si) and silicon carbide (SiC) to a non-combustible magnesium alloy resulting from adding 0.5 to 5.0% by mass of calcium to a magnesium alloy.

2. The high-strength non-combustible magnesium alloy according to claim 1, wherein the amount of carbon (C) of the supplementary additive is 0.1 to 0.3% by mass.

3. The high-strength non-combustible magnesium alloy according to claim 1, wherein the amount of molybdenum (Mo) of the supplementary additive is 1.0 to 12.0% by mass.

4. The high-strength non-combustible magnesium alloy according to claim 1, wherein the amount of niobium (Nb) of the supplementary additive is 0.5 to 5.0% by mass.

5. The high-strength non-combustible magnesium alloy according to claim 1, wherein the amount of silicon (Si) of the supplementary additive is 0.5 to 6.0% by mass.

6. The high-strength non-combustible magnesium alloy according to claim 1, wherein the amount of tungsten (W) of the supplementary additive is 5.0 to 40.0% by mass.

7. The high-strength non-combustible magnesium alloy according to claim 1, wherein the amount of alumina (Al₂O₃) of the supplementary additive is 1.0 to 5.0% by mass.

8. The high-strength non-combustible magnesium alloy according to claim 1, wherein the amount of magnesium silicide (Mg₂Si) of the supplementary additive is 2.0 to 6.0% by mass.

9. The high-strength non-combustible magnesium alloy according to claim 1, wherein the amount of silicon carbide (SiC) of the supplementary additive is 0.7 to 20.0% by mass.

10. The high-strength non-combustible magnesium alloy according to claim 1, wherein the magnesium alloy is a magnesium alloy comprising 0 to 12.0% by mass of aluminum, 0 to 5.0% by mass of zinc, and not more than 0.5% by mass of manganese.

11. The high-strength non-combustible magnesium alloy according to claim 1, wherein the magnesium alloy is one magnesium alloy selected from among AZ31 alloy, AZ61 alloy, AZ80 alloy, AZ91 alloy, AZ92 alloy, AM50 alloy, AM60 alloy and AM100 alloy according to the American Society for Testing and Materials (ASTM).

12. The high-strength non-combustible magnesium alloy according to claim 1, wherein the non-combustible magnesium alloy comprises a crushed product obtained from a base material of the non-combustible magnesium alloy.

13. The high-strength non-combustible magnesium alloy according to claim 1, wherein the high-strength non-combustible magnesium alloy is an alloy manufactured by adding the supplementary additive, followed by plastic working in which permanent deformation is imparted through application of an external force.

14. The high-strength non-combustible magnesium alloy according to claim 12, wherein the crushed product is cutting chips obtained by cutting, or a powder thereof.

15. The high-strength non-combustible magnesium alloy according to claim 13, wherein the plastic working is one type of working from among extrusion, drawing, roll forging and rolling, or a combination of two or more of the foregoing.

16. The high-strength non-combustible magnesium alloy according to claim 1, wherein an alloy formed by adding the supplementary additive is an alloy that makes up a filler metal.

17. The high-strength non-combustible magnesium alloy according to claim 16, wherein the filler metal is a wire-shaped or rod-shaped welding material.