JP5288717B2 - Wear-resistant magnesium alloy - Google Patents

Description

  The present invention relates to a low-friction magnesium alloy in which intermetallic compound crystals are dispersed in a magnesium matrix.

  Today, global environmental problems are becoming increasingly serious. Reviewing mass production and mass consumption in the 20th century, and building a “sustainable society” that minimizes environmental impact throughout the life cycle is an urgent issue in the 21st century. According to the Kyoto Protocol, which came into effect on February 16, 2005, global warming effect gas reduction targets including carbon dioxide are stipulated for each country. In Japan, a reduction rate of -6% is required between 2008 and 2012 based on the 1990 standard. In Japan, 20% of the total carbon dioxide emissions are due to automobile exhaust, and a significant improvement in fuel efficiency due to weight reduction of automobiles has become a required issue.

Magnesium, on the other hand, is the lightest material among practical metals and has a specific gravity of about 1/4 of iron and about 2/3 of aluminum, and is therefore an attractive material for weight reduction. Magnesium has many features such as excellent specific strength, specific rigidity, and specific strength compared to iron and aluminum, and good vibration absorption, dent resistance, and machinability. Furthermore, since the amount of recycled energy is limited to about 4 to 5% of the production energy, the amount used has increased rapidly in recent years, and it is expected to contribute greatly to energy saving in the automobile industry.
Currently, magnesium alloys are used as automotive parts in the structural material field such as steering parts, engine head covers and seat frames. In order to further improve fuel consumption, application to a poor environment such as an engine member such as a cylinder or a piston is desired. In order to apply to a sliding part, it is a required subject to improve wear resistance and heat resistance.

  In past research, Matsuoka et al. Reported that wear resistance was improved by increasing the crystal grain size of ZK60 alloy (Non-patent Document 1). Yoshioka et al. Reported the results of examining the effects of solution treatment and artificial aging on wear resistance using AZ31, AZ61 and AZ91 alloys. As a result, by increasing the aluminum concentration, low friction is exhibited, and it has been reported that the wear resistance of the solution-treated material hardly depends on the aluminum concentration (Non-patent Document 2).

  Murata et al. Have reported that low friction and wear resistance are improved by optimizing the precipitation form of the AZ91 alloy according to the aging temperature (Non-patent Document 3). However, the wear resistance of these existing magnesium alloys is insufficient in practice, and further improvement of the wear resistance is necessary. Considering that it is a commercial base, it can be said that it is indispensable to develop a magnesium alloy having wear resistance.

Furthermore, in order to apply to a sliding part, it is also required to have heat resistance. Currently, due to the problem of creep resistance at high temperatures, the application of magnesium alloys to parts exposed to high temperatures such as around engines is limited.
In recent years, in the development of heat-resistant magnesium alloys, studies have been made to improve heat resistance by adding copper, silicon and the like from the viewpoint of cost performance and die-casting properties (Non-Patent Documents 4 and 5). In the structure of the magnesium alloy formed by the addition of these elements, an intermetallic compound (Non-patent Document 6) having a higher melting point than the melting point of Mg ₁₇ Al ₁₂ which is a precipitation phase of the AZ91 alloy is formed in a dendrite shape. (Non-Patent Documents 7 to 8) are known.

The relationship between heat resistance and friction / wear characteristics is also pointed out by Kato et al. (Non-patent Document 9) and Matsuoka et al. (Non-patent Document 1). In general, a temperature rise between wear surfaces is known as an important factor governing wear behavior. In these reported examples, as the wear surface temperature increases, the strength of the base material on the wear surface also begins to decrease. Accordingly, it has been reported that the wear form shifts from abrasive wear to adhesive wear. From the above report, it is expected that the use of a heat-resistant magnesium alloy having sufficient strength even at high temperatures will contribute to the improvement of low friction and wear resistance. Furthermore, improvement of wear resistance in the magnesium alloy is expected by controlling the dispersion form of the refractory intermetallic compound.
However, there has not been reported yet what has practically sufficient wear resistance.
Matsuoka Takashi, Sakaguchi Kazuhiko, Mukai Toshiji, Matsuyama Masakazu, Yoshioka Ryo, Materials, 51, 10, 1154-1159, (2002). Ryo Yoshioka, Takashi Matsuoka, Kazuhiko Sakaguchi, Toshiji Mukai, Akihiro Murata, Materials, 52, 6, 702-708 (2003). Akihiro Murata, Yohei Tanigawa, Takashi Matsuoka, Kazuhiko Sakaguchi, Hiroyuki Watanabe, Toshiji Mukai, Materials, 54, 1, 90-96, (2005). Mikihiro Kanno, Automotive Technology, 56, 10, 5, (2002). Tatsuo Sato, Metal, 71, 6, 42-50, (2001). Edited by Tsuneo Takahashi, “New Edition Non-ferrous Metal Material Selection Points” (Japan Standards Association, Tokyo, 1992), 221. M. Abulsain, A. Berkani, F. A. Bonilla, Y. Liu, M. A. Arenas, P. Skeldon, G. E. Thompson, P. Bailey, T. C. Q. Noakes, K. Shimizu, H. habasaki, Electrochimica Acta, 49, 899-904 (2004). Y. Pan, X. Liu, H. Yang, Materials Characterization, 55, 241-247, (2005). Kato, Tatsumi Serizawa, Zengo Takayama, Light Metal, 40, 12, 891-895, (1990). J. Halling, Masahisa Matsunaga, “Tribology”, (Modern Sciences, Tokyo, 1984), 92.

  In view of such circumstances, an object of the present invention is to provide a low-friction magnesium alloy that can sufficiently maintain wear resistance even at high temperatures.

The low friction magnesium alloy of the invention 1 is made of Mg—Cu, Mg—Zn—Cu, Mg—Si, or Mg—Zn—Si, and the intermetallic compound crystal has an acute angle like a sphere or an ellipse. And the average diameter of the intermetallic compound crystal is 1 to 20 μm, the yield strength of the parent phase at 473 K is 85 MPa or more, and a needle-like β phase is precipitated in the parent phase. And is an aging material or an overaging material.

Invention 2 is characterized in that in the low friction magnesium alloy of Invention 1, the yield strength at 473 K of the parent phase is 90 MPa or more .

According to the present invention , surface destruction during friction can be suppressed, the wear rate can be reduced as compared with the prior art, and the wear resistance that has not been expected so far has been achieved.
This is thought to be because the shape of the intermetallic compound crystal is such that the concentrated stress at the interface with the matrix phase is suppressed, and damage to the mating material due to sharp corners is suppressed. .

In addition, the stress dispersion action as described above can be exerted on the entire matrix phase, and the fracture can be reduced as a whole.

In addition, by providing the matrix with a high yield strength even at high temperatures , the wear resistance can be exhibited even at high temperatures, and the low friction properties at high temperatures required for practical use can be achieved.

Furthermore, the matrix was strengthened systematically to improve wear resistance and to ensure friction resistance.

Solution treatment In order to solidify the intermetallic compound present in the alloy into a super-saturated solution in the base material and to homogenize the compound, a solution treatment was performed. The processing temperature and holding time for each material are as shown in the examples. In order to prevent oxidation and combustion, solution treatment was performed in a carbon dioxide atmosphere.

Artificial aging treatment When the solid solubility decreases with decreasing temperature, aging precipitation occurs to eliminate the supersaturated solid solution state obtained by solution treatment. The aging precipitation process and the precipitation structure vary depending on the alloy system, the aging temperature, the alloy composition, the molding method, and the like. Therefore, it is necessary to examine aging treatment conditions for each test piece subjected to solution treatment. The treatment temperature was set to three conditions of 150 ° C., 175 ° C., and 200 ° C., and the holding temperature was set to six conditions of 10 minutes, 1 hour, 2 hours, 6 hours, 24 hours, and 48 hours. However, when the maximum value of the aging curve was observed, the aging time was set to that. After performing an aging treatment using an air circulation furnace, it was air-cooled.

Form of crystallized material In order to observe the structure of the as-cast material (As-cast material) and each magnesium alloy subjected to the solution treatment, the test piece was buffed.
Thereafter, the structure was observed with an optical microscope (Nikon Corp .; ECLIPSE LV150).

Verification of age hardening behavior FIG. 1 shows age hardening curves at various temperatures when the artificial aging treatment is performed after the solution treatment.

Hardness test The hardness of the worn surface has a significant effect on the friction and wear characteristics.
Therefore, the hardness of the specimen wear surface before the friction / wear test was examined using a Vickers micro hardness tester (Akashi Co., Ltd. (currently Mitutoyo Co., Ltd .; MVK-H2)). A diamond regular square pyramid having a vertex angle of 136 ° was used as the indenter, and it was pressed at 100 gf for 15 seconds in a room temperature environment. The hardness was Vickers hardness, and each test piece was measured by randomly selecting 10 points within a certain distance from the bottom, and adopting an average value of 8 points from which the maximum and minimum values were removed.

High temperature compression test In order to examine the high temperature strength of the test piece used in the present invention, a high temperature compression test was conducted. In order to make it close to the load acting environment of the Pin-on-desk type friction / wear test, the compression test was used.
Moreover, since the wear surface temperature reached about 150 to 200 ° C., a high temperature compression test was performed at 150 ° C. and 200 ° C.
For comparison, a compression test in a room temperature (25 ° C.) environment was also performed. The compression test was conducted with an Instron material tester 5567 type for the high temperature compression test and the 5569 type for the room temperature compression test. The shape of the test piece was a cylindrical test piece having a diameter of 6 mm and a height of 9 mm, and the test was performed under a strain rate of 10 ⁻³ sec ⁻¹ .

Friction / Abrasion Test Specimen Dimensions and Surface Roughness To conduct the friction / abrasion test, the test piece was processed into a cylindrical pin shape having a diameter of 4 mm and a length of 12 mm. The surface of the contact surface during the friction / abrasion test is polished by using a polishing machine (Buhler; Ecomet polishing machine type 3) with water resistant abrasive paper of # 400-600, and a surface measuring device. (Mitutoyo; SURFTEST SV-400) was used to adjust the arithmetic average roughness to Ra = 0.40-0.60 μm. In order to always obtain a constant contact area from immediately after the start of the friction / wear test to the end, the edge portion was not chamfered.

Selection of mating material Considering generality, the disc mating material for friction and wear tests is one of the general-purpose bearing steels, and a high-carbon chromium bearing steel that can provide good sliding characteristics (hereinafter referred to as SUJ2 or mating material). Selected.
SUJ2 was subjected to a non-oxidizing quenching and tempering treatment. The dimensional shape was a disk type having a diameter of 60 mm and a thickness of 14 mm, and a Vickers hardness of HV = 720 was used. The contact surface with the test piece at the time of the friction / wear test was polished by using a polishing machine (Buhler; Ecomet polishing machine type 3) attached with water resistant abrasive paper of # 240-400, and a surface measuring device (Mitutoyo Co., Ltd.). Using SURFTEST SV-400), the arithmetic average roughness was Ra = 0.08 to 0.12 μm.

Friction / Abrasion Tester In the present invention, a Pin-on-Disk type frictional wear tester (manufactured by Murayama Seisakusho Co., Ltd.) was used as the tester. This testing machine is mainly composed of an air cylinder, a rotating mechanism, and a measuring device.
Compressed air is supplied to the air cylinder (manufactured by CKD Co., Ltd.) as needed from a loading air supply reducing compressor (manufactured by Anest Iwata Co., Ltd.). With this air cylinder, the pin test piece side moves up and down through a sliding mechanism. By controlling the pressure of the compressed air existing in the air cylinder, the load acting between the test piece and the counterpart material can be adjusted. The rotation mechanism is composed of a three-phase dielectric motor (manufactured by Mitsubishi Electric Corporation) and a turntable.
Since the electric motor is connected to the turntable, the number of rotations of the disk fixed to the turntable can be adjusted. Since the rotational speed of the disk can be controlled to 1500 rpm at 1 rpm intervals, a wide range of friction and wear tests from low speed to high speed are possible. Torque cell LT-8NS and load cell CLZ-5KNS (both manufactured by Tokyo Sokki Kenkyujo Co., Ltd.) were used as measuring instruments. Thereby, since the torque and vertical load which fluctuate during a test can be measured at arbitrary time intervals, it is possible to calculate an accurate friction coefficient.

Friction / wear test conditions During the test, the torque and load generated between the pin and the disk were measured at 1 second intervals. The test conditions were a sliding speed of 80 mm / sec and a load of 50N. The rotation radius of the pin, which is the distance from the center of the disk to the center of the pin, is 15 mm, and the sliding speed is 51 rpm when converted to the rotation speed. Since the test time was 1 hour, the sliding distance was calculated to be 288 m. The test environment was a room temperature of 23 ± 2 ° C., and a sliding friction / abrasion test was performed under non-lubrication at room temperature.

Evaluation Method of Friction / Wear Characteristics For the evaluation of wear resistance, the wear rate w, which is the “wear volume per unit slip distance” shown below, was calculated in consideration of comparison with past studies. Here, m is the amount of wear, p is the density, and L is the slip distance. (See Non-Patent Document 10)
(Formula 1)
During the experiment, the friction coefficient μ shown below was calculated using the torque T generated between the pin and the disk and the vertical load W. Here, r is the rotation radius of the pin.
(Formula 2)

Evaluation of wear powder It is considered that the wear powder had a great influence on the friction and wear characteristics.
Therefore, in order to examine the influence of the wear powder on the tribological characteristics, the wear powder dropped off by each test piece was observed by SEM.
In order to examine the wear powder in more detail, a component analysis of the wear powder was performed using an energy dispersive X-ray analyzer (manufactured by EDAX; Genesis, hereinafter referred to as EDAX) attached to the SEM.

1: Mg—Cu alloy copper and pure magnesium (purity: 99.95%) were completely dissolved in a carbon dioxide atmosphere, cast into an iron mold, and Mg-4.5 wt. % Cu alloy was produced (hereinafter, this sample is referred to as an as-cast material). The obtained cast alloy was kept in a furnace at a temperature of 440 ° C. for 24 hours, and then cooled with water to give a solution treatment. Thereafter, heat treatment was performed at 150, 175, 200 ° C., 10 minutes, 1, 2, 6, 24, and 48 hours to prepare an aging material and an overaged material (hereinafter, this sample was a peak-aged material, Over-aged material). FIG. 2 shows the results of observation of the optical microscope structure of the over-aged material. The formation of an intermetallic compound having a Mg—Cu composition can be confirmed. These samples were machined to produce cylindrical test pieces having a diameter of 4 mm and a length of 12 mm, and the wear characteristics were investigated using a Pin-on-disk type wear tester. The results are shown in Table 1 (the lower the wear rate value, the better the characteristics). Table 1 shows, for comparison, Mg-9 wt., Which is the most commonly used magnesium alloy. % Al-1 wt. The value of% Zn (AZ91) alloy is also shown. It can be confirmed that the wear rate of the sample used in the present invention is superior to that of the conventional material. FIG. 3 shows the results of observation of the sample after the abrasion test with a scanning electron microscope. Scratches parallel to the slip direction are observed, but adhesion of wear powder can also be confirmed, and the formation of adhesive wear is predicted.

2: Mg—Cu—Zn alloy copper, zinc and pure magnesium (purity: 99.95%) were completely dissolved in a carbon dioxide atmosphere, cast into an iron mold, and Mg-4.5 wt. % Cu-6.0 wt. % Alloy was produced (as-cast material). Thereafter, a peak-aged material and an over-aged material were prepared and processed under the same heat treatment conditions as in Example 1, and an abrasion test was performed. From Table 1, it can be confirmed that the wear rate of the sample used in the present invention is superior to that of the conventional material. In addition, cylindrical test pieces having a parallel part diameter of 4 mm × length of 8 mm were prepared by machining the as-cast, peak-aged, and over-aged materials, and subjected to a compression test at room temperature, 150 ° C., and 200 ° C. The obtained stress-strain curve and the results at that time are shown in FIG. It can be seen that at 150 ° C., it is equal to or higher than the compressive strength at room temperature and exhibits excellent heat resistance. From Table 2, Mg-9 wt. % Al-1 wt. It shows a value higher than the compressive strength of% Zn, indicating that the heat resistance is excellent.

3: Mg—Si alloy silicon and pure magnesium (purity: 99.95%) were completely dissolved in a carbon dioxide atmosphere, cast into an iron mold, and Mg-2.0 wt. % Si alloy was produced (as-cast material). The obtained cast alloy was held in a furnace at a temperature of 590 ° C. for 24 hours, and then cooled with water to give a solution treatment. Thereafter, a peak-aged material and an over-aged material were prepared and processed under the same heat treatment conditions as in Example 1, and an abrasion test was performed. From Table 1, it can be confirmed that the wear rate of the sample used in the present invention is superior to that of the conventional material.

4: Mg—Si—Zn alloy silicon, zinc and pure magnesium (purity: 99.95%) were completely dissolved in a carbon dioxide atmosphere, cast into an iron mold, and Mg-2.0 wt. % Si-6.0 wt. % Alloy was produced (as-cast material). Thereafter, a peak-aged material and an over-aged material were prepared and processed under the same heat treatment conditions as in Example 1, and an abrasion test and a compression test were performed. FIG. 5 shows the result of the structure observation using a typical transmission electron microscope of the over-aged material. The oval intermetallic compound is the Mg ₂ Si phase, and the intermetallic compound precipitated in a needle shape with respect to the magnesium matrix is the b phase. Fig. 6 shows the stress-strain curve obtained by the compression test. 6 and Tables 1 and 2, it can be seen that the sample used in the present invention is excellent in wear resistance and heat resistance. FIG. 7 shows the result of observation of the sample after the abrasion test with a scanning electron microscope. Although scratches parallel to the slip direction are observed, it is considered that the wear resistance is further improved because the form of adhesive wear as shown in FIG. 3 is not observed.

Table 1 shows the results of the wear test, and Table 2 shows the results of the compression test.

Aging curve at each temperature (a) Mg-Cu alloy, (b) Mg-Si alloy Optical microstructure of Mg-4.5wt.% Cu alloy over-aged material Wear test piece surface of Mg-4.5wt.% Cu alloy over-aged material by scanning electron microscope Stress-strain curve in compression test of Mg-4.5wt.% Cu alloy over-aged material Mg-2.0 wt. % Si-6.0 wt. % Electron over-aged transmission electron microscopic structure (the elliptical intermetallic compound in the figure is Mg2Si, and the intermetallic compound precipitated in a needle shape in the parent phase is the β phase) Mg-2.0 wt. % Si-6.0 wt. Stress-strain curve in compression test of% alloy over-aged material Mg-2.0 wt. % Si-6.0 wt. Wear test piece surface of% alloy over-aged material

Claims (2)

A low-friction magnesium alloy in which an intermetallic compound crystal is dispersed in a magnesium matrix, which is made of Mg-Cu, Mg-Zn-Cu, Mg-Si, or Mg-Zn-Si , The crystal is a fine particle having no acute angle such as a sphere or an ellipse, the average diameter of the intermetallic compound crystal is 1 to 20 μm, and the yield strength at 473 K of the matrix is 85 MPa or more, A low-friction magnesium alloy characterized in that a needle-like β phase is precipitated in the matrix and is an aging material or an overaging material.   The low-friction magnesium alloy according to claim 1, wherein the yield strength at 473K of the parent phase is 90 MPa or more.