EP2631312A1

EP2631312A1 - Magnesium-alloy member, compressor for use in air conditioner, and method for manufacturing magnesium-alloy member

Info

Publication number: EP2631312A1
Application number: EP11836462.9A
Authority: EP
Inventors: Sueji Hirawatari; Hidenori Hosoi; Tsuyoshi Fukui; Makoto Fukushima; Shigeharu Kamado; Tomoyuki Honma
Original assignee: National University Corp Nagaoka University; Sanden Corp
Current assignee: National University Corp Nagaoka University; Sanden Corp
Priority date: 2010-10-29
Filing date: 2011-10-28
Publication date: 2013-08-28
Also published as: KR20130101100A; JP2012097309A; US20130213528A1; CN103180472A; EP2631312A4; WO2012057329A1

Abstract

The present invention provides a magnesium alloy member and a method for manufacturing a magnesium alloy member capable of achieving a mechanical strength and a high-temperature fatigue strength sufficient to apply them to mechanical parts of a compressor for automotive air conditioners. The invention also provides a compressor for air conditioners using, as a mechanical part thereof, a magnesium alloy equipped with a necessary mechanical strength and high-temperature fatigue strength. More specifically, a magnesium alloy member is formed by subjecting a cast material of a magnesium alloy containing, on the basis of mass%, from 0.3% to 10% calcium (Ca), from 0.2% to 15% aluminum (Al), and from 0.05% to 1.5% manganese (Mn), and containing calcium (Ca) and aluminum (Al) at a calcium/aluminum mass ratio of from 0.6 to 1.7, with the balance being magnesium (Mg) and inevitable impurities to plastic working (extrusion processing) at from 250°C to 500°C. This makes it possible to obtain a magnesium alloy member having a room-temperature 0.2% proof stress of 300 MPa or more and a 150°C fatigue strength of 100 MPa or greater. The magnesium alloy member used for the formation of mechanical parts of a compressor for automotive air conditioners contributes to a reduction in the weight of the compressor.

Description

TECHNICAL FIELD

The present invention relates to a magnesium alloy member containing aluminum, calcium, and manganese; a compressor for air conditioners using the magnesium alloy member as or for a mechanical part of the compressor, and a method for manufacturing the magnesium alloy member.

BACKGROUND ART

Magnesium alloys having a low specific gravity may be used for automotive parts in order to reduce the weight thereof. Magnesium alloys have conventionally been used mainly for parts such as casings and covers which need neither high strength nor heat resistance. However, there have recently been developed magnesium alloys having enhanced strength or heat resistance.
For example, Patent Documents 1 to 3 disclose a magnesium alloy having enhanced castability and heat resistance, whereas Patent Document 4 discloses a magnesium alloy having enhanced high-temperature strength and forgeability.

CITATION LIST

PATENT DOCUMENTS

Patent Document 1: Japanese Laid-Open Patent Application Publication No. 2004-232060
Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2007-197796
Patent Document 3: Japanese Laid-Open Patent Application Publication No. 2004-162090
Patent Document 4: Japanese Laid-Open Patent Application Publication No. 2000-104137
Patent Document 5: Japanese Laid-Open Patent Application Publication No. 2000-109963

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

Compressors for automotive air conditioners are installed in the vicinity of engines so that they are exposed to a temperature of from about 100°C to 150°C. Materials for the parts of the compressors therefore have preferably heat resistance. Moreover, mechanical parts engaged in compression work of the compressor have preferably a high fatigue strength at high temperatures.
Magnesium alloys disclosed in Patent Documents 1 to 3 are however used for casting and have insufficient mechanical strength. They are not suited for use for parts of compressors or the like having preferably high strength at high temperatures.
Although the magnesium alloys disclosed in Patent Documents 4 and 5 are excellent in strength and forgeability, it is not certain that the can be used for mechanical parts of compressors because a high-temperature fatigue strength thereof has not yet been verified.
Moreover, when an expensive rare metal is added to a magnesium alloy, the resulting alloy has increased strength, but it raises the manufacturing cost so that it is not suited as a material for mechanical parts of compressors.
Objects of the invention are therefore to provide a magnesium alloy member and a method for manufacturing a magnesium alloy member, each capable of realizing mechanical strength and high-temperature fatigue strength facilitating application thereof to mechanical parts of a compressor for automotive air conditioners; and to provide a compressor for air conditioners equipped with mechanical parts made of a magnesium alloy having necessary mechanical strength and high-temperature fatigue strength.

MEANS FOR SOLVING THE PROBLEMS

In order to achieving the above-mentioned objects, the present invention is characterized in a magnesium alloy cast material containing, on the basis of mass%, from 0.3% to 10% calcium, from 0.2% to 15% aluminum, and from 0.05% to 1.5% manganese, and containing calcium and aluminum at a calcium/aluminum mass ratio of from 0.6 to 1.7, with the balance being magnesium and inevitable impurities is subjected to plastic working at from 250°C to 500°C.
When both calcium (Ca) and aluminum (Al) are added, a Mg-Ca-based compound and a Mg-Al-Ca-based compound are crystallized at the grain boundaries, resulting in enhancement in mechanical strength at room temperature and heat resistance.
The compounds thus crystallized undergo a change, depending on a Ca/Al mass ratio. In particular, when the Ca/Al mass ratio is set at from 0.6 to 1.7, Mg₂Ca which is a Mg-Ca-based compound and (Mg,Al)₂Ca which is a Mg-Al-Ca-based compound are crystallized simultaneously, so that such a mass ratio is remarkably effective for the enhancement of mechanical strength and heat resistance.
On the other hand, when the Ca/Al mass ratio exceeds 1.7, only Mg₂Ca is crystallized or in addition thereto, a slight amount of (Mg,Al)₂Ca is crystallized, and thus, an effect for enhancing the mechanical strength may not be expected. When the Ca/Al mass ratio is below 0.6, β-Mg₁₇Al₁₂ which is a Mg-Al-based compound is crystallized and it adversely affects the heat resistance.
Addition of a small amount of manganese (Mn) decreases the crystal particle size, leading to enhancement in mechanical strength. The amount of manganese (Mn) added is preferably in a range of from 0.05% to 1.5%. When the amount is outside this range, an effect for enhancing the mechanical strength may not be expected because such an amount is less effective for decreasing the crystal particle size.
It is possible to achieve high fatigue strength at high temperatures by subjecting a cast material made of a magnesium alloy having the above-mentioned composition to plastic working at from 250°C to 500°C. The magnesium alloy member after plastic working at from 250°C to 500°C has, as mechanical strength and high-temperature fatigue strength which mechanical parts of a compressor for automotive air conditioners are desired to have, a 0.2% proof stress at room temperature of 300 MPa or greater and a 150°C fatigue strength of 100 MPa or greater.
When plastic working is conducted at below 250°C, the material is not formed and cracks appear because a sufficient strain amount is not produced. When the temperature exceeds 500°C, on the other hand, high-temperature oxidation or partial melting occurs and an effect for enhancing the fatigue strength may not be expected.
The plastic working may be followed by solution heat treatment and artificial aging treatment. It is preferred to conduct, after the plastic working, solution heat treatment to retain the resulting magnesium alloy member for 0.08 hour or more at a treatment temperature of from 450°C to 510°C and then conduct artificial aging treatment to retain it for 0.3 hour or more at a treatment temperature of from 150°C to 250°C.
When the solution heating is conducted at a treatment temperature ranging from 450°C to 510°C, the grain boundaries or inside of the grains are reinforced by fine precipitates. This suppresses local deformation and widens a uniform deformation region, so that work softening at high temperatures does not occur easily, leading to enhancement in high-temperature fatigue strength.
The solution heating conducted at a treatment temperature below 450°C makes it difficult to form a solid solution, reduces an amount of precipitates at the grain boundaries and in the grains, and prevents formation of an appropriate state, so that enhancement in high-temperature fatigue strength is not expected. When the solution heating is conducted at a treatment temperature exceeding 510°C, on the other hand, burning to melt a portion of the alloy occurs, leading to the formation of pore defects.
The solution heating time below 0.08 hour may not achieve sufficient solution heat treatment, and thus, retention time preferably exceeds 0.08 hour.
Cooling employed for hardening may be conducted with warm water or with a certain additive. Various means may be employed insofar as the well-known cooling means for hardening.
When artificial aging treatment is conducted at a temperature below 150°C, it takes longer to enhance the hardness to a proper one. Treatment temperatures exceeding 250°C may deteriorate the hardness and strength, so that the artificial aging treatment is conducted preferably in a temperature range of from 150 to 250°C.
When the retention time for the artificial aging treatment is below 0.3 hour, sufficient aging hardening is not achieved, and thus, the retention time for the artificial aging treatment is preferably 0.3 hour or more.
As the plastic working, extrusion processing can be conducted. Extrusion processing at from 250°C to 500°C can enhance the fatigue strength while suppressing cracks or surface oxidation.
The above-mentioned magnesium alloy members can be used as or for mechanical parts of a compressor for air conditioners.

ADVANTAGEOUS EFFECT OF THE INVENTION

The present invention makes it possible to provide a magnesium alloy member capable of realizing mechanical strength and high-temperature fatigue strength sufficient to use it as or for mechanical parts of a compressor for automotive air conditioners, more specifically, a 0.2% proof stress at room temperature of 300 MPa or greater and a 150°C fatigue strength of 100 MPa or greater. In addition, the invention makes it possible to provide a compressor for air conditioners using such a magnesium alloy member as or for the mechanical parts of it.
According to the invention, a magnesium alloy member having substantially equal mechanical strength and high-temperature fatigue strength to those of high-strength aluminum alloy can be realized, so that the high-strength aluminum alloy can be replaced by the magnesium alloy member having a lower specific gravity than the high-strength aluminum alloy, making it possible to realize a drastic weight reduction of a compressor for automotive air conditioners.

MODE FOR CARRYING OUT THE INVENTION

The embodiments of the invention will hereinafter be described specifically.
Table 1 includes tensile strength (MPa) at room temperature, for example, from 10°C to 35°C and a 0.2% proof stress (MPa) of each of a plurality of magnesium alloy samples varied in aluminum (Al), calcium (Ca), and manganese (Mn) contents (mass%).
With regards to "rating" in Table 1, "○" means that a 0.2% proof stress is 300 MPa or greater, that is, a value which mechanical parts of a compressor for automotive air conditioners are desired to have and "×" means that a 0.2% proof stress is less than 300 MPa.
The "300 MPa", that is, a desired value of a 0.2% proof stress is set using, as a reference value, the 0.2% proof stress of an aluminum alloy forged material subjected to T6 treatment, that is, solution heat treatment followed by artificial aging treatment. This aluminum alloy forged material is used for mechanical parts of a compressor for automotive air conditioners.
The samples listed in Table 1 were each obtained by preparing a magnesium alloy cast material having a content as described in this table and then subjecting it to plastic working, more specifically, hot indirect extrusion processing. These samples had not been subjected to heat treatment (T6 treatment).
Described specifically, alloy melting was conducted in the atmosphere by using an electric resistance furnace and a mixed gas of SF₆ and CO₂ was used for preventing oxidation of the molten metal. After stirring, bubbling was conducted by supplying an Ar gas in order to remove an oxide formed at the time of Ca addition. The resulting mixture was then cast in a billet mold and thus, the cast material was prepared.
A hydraulic press was used for direct extrusion processing. The material sample to be extruded was charged in a mold heated to 350°C. After retention for 10 minutes, extrusion processing was started while setting an extrusion ratio at 20. It is to be noted that the term "extrusion ratio" means a (cross-sectional area before plastic working)/(cross-sectional area after plastic working) ratio.
In a tensile test for evaluating tensile properties of an extruded material, a universal tester was employed. A test specimen was collected while keeping the extruding direction and a load applying direction parallel to each other and a JIS14A test specimen having a diameter, at a test portion, of 4 mm and a gauge length of 20 mm was prepared. Moreover, an initial strain rate: 1×10^-3 s^-1 was used as a test rate.
In the bottom line of Table 1, the tensile strength (MPa) and 0.2% proof stress (MPa) of an Al alloy forged material (A4032-T6) which is a material specified by JIS are described for reference. The "rating" in this table indicates whether the Al alloy forged material (A4032-T6) has a 0.2% proof stress of 300 MPa or greater or not.
In Table 1, the samples of Examples 1 to 11 were obtained by subjecting a cast material of a magnesium alloy containing from 0.3% to 10% calcium (Ca), from 0.2% to 15% aluminum (Al), and from 0.05% to 1.5% manganese (Mn), having a calcium (Ca)/aluminum (Al) mass ratio of from 0.6 to 1.7, with the balance being magnesium Mg and inevitable impurities to plastic working (extrusion processing) at 350°C.
On the other hand, samples of Comparative Examples 1 to 7 were obtained by subjecting a cast material of a magnesium alloy which did not satisfy at least one of the above-mentioned ranges of the calcium (Ca) content, the aluminum (Al) content, the manganese (Mn) content, and the calcium (Ca)/aluminum (Al) mass ratio to plastic working (extrusion processing) at 350°C.
It is to be noted that "Ca+Al" in Table 1 means a total mass% of calcium (Ca) and aluminum (Al).
As shown in Table 1, the samples of Examples 1 to 7 which satisfy the calcium (Ca) content of from 0.3% to 10%, the aluminum (Al) content of from 0.2% to 15%, the manganese (Mn) content of from 0.05% to 1.5%, and the calcium (Ca)/aluminum (Al) mass ratio of from 0.6 to 1.7, each have a 0.2% proof stress of 300 MPa or greater, that is, the desired value, thus satisfying the mechanical strength which mechanical parts of a compressor for automotive air conditioners are desired to have. Thus, it is possible for them to be used as a mechanical part of a compressor.
On the other hand, the samples of Comparative Examples 1 and 4 having a calcium (Ca) content outside the range of from 0.3% to 10% and the samples of Comparative Examples 2 and 3 having an aluminum (Al) content outside the range of from 0.2% to 15% have a 0.2% proof stress below 300 MPa, that is, the desired value. Thus, it is impossible that they are not suited for use as a mechanical part of a compressor.
Even if the calcium (Ca) content and the aluminum (Al) content are within the range of from 0.2% to 15%, when the calcium (Ca)/aluminum (Al) mass ratio is outside the range of from 0.6 to 1.7 as in Comparative Example 5 or Comparative Example 6, a 0.2% proof stress does not reach the desired value, that is, 300 MPa, and such samples are not suited for use as mechanical parts of a compressor.
Moreover, even if the calcium (Ca) content and the aluminum (Al) content are within the range of from 0.3% to 10% and the calcium (Ca)/aluminum (Al) mass ratio is within the range of from 0.6 to 1.7, the sample of Comparative Example 7 not containing manganese is not suited for use as a mechanical part of a compressor, because the 0.2% proof stress is below 300 MPa and does not satisfy the desired value.
It has been found from the results of the tensile test that a magnesium alloy preferably satisfies the following conditions, that is, a calcium (Ca) content of from 0.3% to 10%, an aluminum (Al) content of from 0.2% to 15%, a manganese (Mn) content of from 0.05% to 1.5%, and a calcium (Ca)/aluminum (Al) mass ratio of from 0.6 to 1.7 in order to achieve the mechanical strength necessary for mechanical parts of a compressor for automotive air conditioners, more specifically, to achieve a 0.2% proof stress of 300 MPa or greater.
By the addition of both calcium (Ca) and aluminum (Al), a Mg-Ca-based compound and a Mg-Al-Ca-based compound are crystallized at the grain boundaries, leading to enhancement in mechanical strength at room temperature and heat resistance. When the calcium (Ca)/aluminum (Al) mass ratio is adjusted to from 0.6 to 1.7 as in Examples 1 to 11, Mg₂Ca which is a Mg-Ca-based compound and (Mg,Al)₂Ca which is an Mg-Al-Ca-based compound are crystallized simultaneously, which is presumed to lead to enhancement in mechanical strength and heat resistance.
When the calcium (Ca)/aluminum (Al) mass ratio exceeds 1.7 as in Comparative Example 6, only Mg₂Ca is crystallized or in addition thereto, a slight amount of (Mg,Al)₂Ca is crystallized, which does not lead to sufficient enhancement in mechanical strength. When the calcium (Ca)/aluminum (Al) mass ratio falls below 0.6 as in Comparative Example 5, β-Mg₁₇Al₁₂ which is a Mg-Al-based compound is crystallized, which is presumed to adversely affect the heat resistance.
Even when the calcium (Ca)/aluminum (Al) mass ratio falls within the range of from 0.6 to 1.7, in a case in which magnesium (Mn) is not added as in Comparative Example 7, sufficient mechanical strength may not be achieved. On the other hand, by the addition of a small amount of manganese (Mn) as in Examples 1 to 11, a 0.2% proof stress of 300 MPa or greater can be achieved, which is presumed to occur because the crystal particle size becomes finer and mechanical strength is enhanced by the addition of a small amount of manganese (Mn). The amount of manganese (Mn) is appropriately in a range of from 0.05% to 1.5%. Amounts outside this range are less effective for reducing the particle size of crystals and an effect of enhancing the mechanical strength may not be expected.

[Table 1]

	Ca [wt%]	Al [wt%]	Mn [wt%]	Ca/Al [-]	Ca+Al [wt%]	Tensile strength [MPa]	0.2% proof stress [MPa]	Rating
Example 1	0.34	0.54	0.29	0.63	0.88	328	312	○
Example 2	1.5	2.4	0.23	0.63	3.9	350	335	○
Example 3	3.3	3.7	0.33	0.89	7	349	330	○
Example 4	3	5	0.31	0.60	8	340	330	○
Example 5	5.8	8.1	0.36	0.72	13.9	339	321	○
Example 6	7.6	6.8	0.39	1.12	14.4	327	319	○
Example 7	9.4	8.2	0.43	1.15	17.6	321	310	○
Example 8	9.2	5.8	0.24	1.59	15	322	309	○
Example 9	3.8	14.5	0.37	0.26	18.3	318	304	○
Example 10	3.7	3.9	0.06	0.95	7.6	329	318	○
Example 11	0.33	0.23	0.26	1.43	0.56	321	304	○
Comp. Ex. 1	0.23	0.3	0.21	0.77	0.53	320	288	×
Comp. Ex. 2	0.31	0.19	0.18	1.63	0.5	313	285	×
Comp. Ex. 3	9.7	15.4	0.33	0.63	25.1	326	297	×
Comp. Ex. 4	11.6	14.2	0.24	0.82	25.8	263	231	×
Comp. Ex. 5	3.3	6.8	0.25	0.49	10.1	298	272	×
Comp. Ex. 6	6.1	3.4	0.21	1.79	9.5	311	289	×
Comp. Ex. 7	3.2	3.7	○	0.86	6.9	304	278	×
A4032-T6			-			350	300	-

The samples listed in Table 2 are the cast materials of a magnesium alloy of Example 3 having the contents indicated in Table 1, that is, a calcium (Ca) content of 3.3%, an aluminum (Al) content of 3.7%, and a manganese (Mn) content of 0.33%, a calcium (Ca)/aluminum (Al) mass ratio of 0.89, and a total content of calcium (Ca) and aluminum (Al) of 7%. These cast materials were subjected to extrusion processing (plastic working) at varied extrusion ratios and extrusion temperatures and the results of a 0.2% proof stress test of the samples obtained by extrusion processing are listed.
In the tests of Table 2, the extrusion ratio was set at four ratios, that is, 10, 20, 40, and 60. When the extrusion temperature at each of these extrusion ratios was in a range of from 250°C to 500°C, neither cracks nor surface oxidation occurred and the 0.2% proof stress exceeded the desired value, that is, 300 MPa.
On the other hand, when the extrusion ratio was set at 20 and the extrusion temperature was set at 230°C, which was a temperature below the range of from 250°C to 500°C, cracks occurred and desired mechanical strength was not attained. When the extrusion temperature was set at 517°C, which was a temperature exceeding the range of from 250°C to 500°C, surface oxidation occurred and the 0.2% proof stress was below the desired value, that is, 300 MPa.
It has been found that by controlling the temperature of the plastic working (extrusion processing) to fall within a range of from 250°C to 500°C, the 0.2% proof stress of 300 MPa or more can be attained. Plastic working results in failure and cracks appear at a plastic working temperature below 250°C, because a sufficient strain amount cannot be secured. At the plastic working temperature exceeding 500°C, high-temperature oxidation or partial melting occurs and therefore enhancement in fatigue strength may not be expected.

[Table 2]

Component	Extrusion ratio [-]	Temperature [°C]	0.2% Proof stress [MPa]	Rating	Remarks
Example 3	10	350	311	○
	20	350	330	○
	20	230	-	×	Cracks
	20	470	308	○
	20	517	278	×	Surface oxidation
	20	280	303	○
	50	350	334	○
	60	400	339	○
	60	300	342	○

Table 3 includes measurement results of a 150°C fatigue strength (high-temperature fatigue strength) of a sample subjected to plastic working (extrusion processing) at from 250°C to 500°C, followed by heat treatment (T6 treatment) and a sample not subjected to heat treatment (T6 treatment) after the plastic working.
It is to be noted that as a sample, used was that obtained by subjecting the cast material of a magnesium alloy of Example 3 having the contents indicated in Table 1, that is, a calcium (Ca) content of 3.3%, an aluminum (Al) content of 3.7%, a manganese (Mn) content of 0.33%, a calcium (Ca)/aluminum (Al) mass ratio of 0.89, and a total content of calcium (Ca) and aluminum (Al) of 7% to extrusion processing at an extrusion ratio of 20 and an extrusion temperature of 350°C.
In Table 3, the 150°C fatigue strength of an Al alloy forged material (A4032-T6) which is a material specified by JIS is also listed for comparison in Table 3. As describe above, this Al alloy forged material (A4032-T6) has been used for a compressor for automotive air conditioners, and thus, a material capable of achieving a 150°C fatigue strength at least equal to the 150°C fatigue strength of this A4032-T6 (100 MPa) can be used as a substitute member of A4032-T6.
The fatigue test (rotary bending test) for finding the fatigue strength of Table 3 and calculation of a fatigue strength were conducted in accordance with "The Japan Society of Mechanical Engineers, Standard method of statistical fatigue testing (revised) JSME S-002-1994" ed. by The Japan Society of Mechanical Engineers. The test was conducted at a test temperature of 150°C, a rotating speed of 3000rpm, a frequency of 50Hz, and a stress ratio R of -1. The fatigue strength in Table 3 is the result of the tests conducted 10⁷ times.
The test specimen used for the fatigue test is a rod-type test specimen. The diameter at a chuck portion was 8.5 mm and the diameter at a breaking portion was 4 mm. It was collected so that the extruding direction and the load applying direction were orthogonal to each other. In order to remove the influence of streaks due to cutting, the breaking portion was polished with waterproof abrasive paper, followed by buffing to finish.
In addition, as the T6 treatment, solution heat treatment to retain the sample in an Ar gas stream of 500°C for 30 minutes (0.5 hour) was conducted in a horizontal tubular furnace and then, artificial aging treatment was conducted in an oil bath of 180°C for 2 hours. The heat treatment time (retention time) is a period of time starting with the charging of the sample.
As listed in Table 3, the 150°C fatigue strength of A4032-T6 is 100 MPa; the 150°C fatigue strength of the magnesium alloy member subjected to plastic working at a temperature of from 250°C to 500°C, more specifically, subjected to extrusion processing at 350°C and not subjected to the heat treatment (T6 treatment) thereafter was 117 MPa; and the 150°C fatigue strength of the magnesium alloy member having the same composition, subjected to the same plastic working, and then subjected to the heat treatment (T6 treatment) was 132 MPa.
This means that the 150°C fatigue strength exceeding that of A4032-T6 can be achieved by subjecting the magnesium alloy cast material having a calcium (Ca) content of from 0.3% to 10%, an aluminum (Al) content of from 0.2% to 15%, a manganese (Mn) content of from 0.05% to 1.5%, and a calcium (Ca)/aluminum (Al) mass ratio of from 0.6 to 1.7 to plastic working at from 250 to 500°C even without subjecting it to heat treatment (T6 treatment). The material subjected to the heat treatment (T6 treatment) in addition to the plastic working has an enhanced 150°C fatigue strength compared with the material not subjected to the heat treatment (T6 treatment).
In other words, a magnesium alloy member obtained by subjecting the magnesium alloy cast material having a calcium (Ca) content of from 0.3% to 10%, an aluminum (Al) content of from 0.2% to 15%, a manganese (Mn) content of from 0.05% to 1.5%, and a calcium (Ca)/aluminum (Al) mass ratio of from 0.6% to 1.7 to plastic working at from 250°C to 500°C can achieve a room-temperature 0.2% proof stress and a high-temperature fatigue strength suited for use for mechanical parts of a compressor for automotive air conditioners, more specifically, the room-temperature 0.2% proof stress of 300 MPa or greater and 150°C fatigue strength of 100 MPa even without the heat treatment (T6 treatment). The member subjected to the heat treatment (T6 treatment) in addition to the plastic working has a further enhanced high-temperature fatigue strength.
It is therefore possible to use a magnesium alloy member instead of a conventionally used high-strength aluminum alloy for the formation of mechanical parts of a compressor for automotive air conditioners. This makes it possible to realize a remarkable reduction in the weight of the compressor.

[Table 3]

Component, T6 treatment	150°C Rotary bending fatigue strength [MPa]
Example 3, subjected to T6 treatment	132
Example 3, not subjected to T6 treatment	117
A4032 forged product, subjected to T6 treatment	100

It is to be noted that in the heat treatment (T6 treatment), it is preferred that the solution heat treatment after plastic working (extrusion processing) be conducted while retaining the magnesium alloy member for 0.08 hour or more at a treatment temperature of from 450°C to 510°C and the artificial aging treatment after hardening treatment is conducted while retaining the member for 0.3 hour or more at a treatment temperature of from 150°C to 250°C.
At treatment temperatures of solution heat treatment within a range of from 450°C to 510°C, the grain boundaries and the inside of the grains are reinforced with fine precipitates, local deformation is suppressed, and a uniform deformation area is widened, so that work softening at high temperatures is suppressed and the resulting alloy member has an enhanced high-temperature fatigue strength.
The solution heating conducted at a treatment temperature below 450°C makes it difficult to form a solid solution, reduces an amount of precipitates at the grain boundaries and in the grains, and prevents formation of an appropriate state, so that enhancement in high-temperature fatigue strength is not expected. When the solution heating is conducted at a treatment temperature exceeding 510°C, on the other hand, burning to melt a portion of the alloy occurs, leading to the formation of pore defects.
The solution heating time below 0.08 hour may not achieve sufficient solution heat treatment, and thus, retention time is preferably greater than 0.08 hour.
Treatment temperatures of the artificial aging treatment below 150°C may increase the treatment time in order to attain a proper hardness, whereas treatment temperatures exceeding 250°C deteriorate the hardness and strength. The temperature of the artificial aging treatment is preferably in a range of from 150°C to 250°C.
Retention time of the artificial aging treatment below 0.3 hour cannot achieve sufficient aging hardening, and thus, the retention time of the artificial aging treatment is preferably 0.3 hour or greater.
The temperature and retention time of the heat treatment (T6 treatment) which have provided the results of Table 3 satisfy the above-mentioned temperature range and time range.
As described above, the magnesium alloy member and the method for manufacturing the magnesium alloy member according to the invention can realize a room-temperature 0.2% proof stress of 300 MPa or greater and a 150°C fatigue strength of 100 MPa or greater which are necessary for mechanical parts of a compressor for automotive air conditioners and therefore the magnesium alloy member of the invention can be used instead of the conventionally used Al alloy forged material A4032.
Since the magnesium alloy member has a lower specific gravity than the Al alloy forged material A4032, when mechanical parts of a compressor for automotive air conditioners are made of the magnesium alloy, it is possible to markedly reduce the weight of the compressor, reduce the weight of the automotive, and therefore enhance the fuel efficiency.
Examples of the magnesium alloy member of the invention and mechanical parts of a compressor for automotive air conditioners to which the magnesium alloy member is applied include shoes and pistons for swash plate compressors and spiral bodies for scroll type compressors.
The magnesium alloy member and the method for manufacturing a magnesium alloy member according to the invention have been developed with a view to applying them to mechanical parts of a compressor for automotive air conditioners, but they can be applied not only to the mechanical parts of a compressor for automotive air conditioners but also to mechanical parts of a compressor for stationary air conditioners.
In addition, the plastic working is not limited to extrusion processing and it may be forging, rolling, or drawing processing.

Claims

A magnesium alloy member obtained by subjecting a cast material of a magnesium alloy containing, on the basis of mass%, from 0.3% to 10% calcium, from 0.2% to 15% aluminum, and from 0.05% to 1.5% manganese, and containing calcium and aluminum at a calcium/aluminum mass ratio of from 0.6 to 1.7, with the balance being magnesium and inevitable impurities to plastic working at from 250°C to 500°C.
The magnesium alloy member according to claim 1, wherein the plastic working is followed by solution heat treatment and artificial aging treatment.
The magnesium alloy member according to claim 2, wherein after the plastic working, the magnesium alloy member is subjected to solution heat treatment to retain the magnesium alloy member for at least 0.08 hour at a treatment temperature of from 450°C to 510°C, and then, the resulting member is subjected to artificial aging treatment to retain the resulting member for at least 0.3 hour at a treatment temperature of from 150°C to 250°C.
A magnesium alloy member obtained by subjecting a cast material of a magnesium alloy containing, on the basis of mass%, from 0.3% to 10% calcium, from 0.2% to 15% aluminum, and from 0.05% to 1.5% manganese, and containing calcium and aluminum at a calcium/aluminum mass ratio of from 0.6 to 1.7, with the balance being magnesium and inevitable impurities to plastic working; and having a room-temperature 0.2% proof stress of 300 MPa or greater and a 150°C fatigue strength of 100 MPa or greater.
The magnesium alloy member according to claim 1, wherein the plastic working is extrusion processing.
A compressor for air conditioners using, as a mechanical part thereof, the magnesium alloy member as claimed in claim 1.
A method for manufacturing a magnesium alloy member, comprising subjecting a cast material of a magnesium alloy containing, on the basis of mass%, from 0.3% to 10% calcium, from 0.2% to 15% aluminum, and from 0.05% to 1.5% manganese, and containing calcium and aluminum at a calcium/aluminum mass ratio of from 0.6 to 1.7, with the balance being magnesium and inevitable impurities to plastic working at from 250°C to 500°C.
The method for manufacturing a magnesium alloy member according to claim 7, wherein the plastic working is followed by solution heat treatment and artificial aging treatment.
The method for manufacturing a magnesium alloy member according to claim 8, further comprising, after the plastic working, subjecting the magnesium alloy member to solution heat treatment to retain the member for at least 0.08 hour at a treatment temperature of from 450°C to 510°C, and then, subjecting the resulting member to artificial aging treatment to retain the resulting member for at least 0.3 hour at a treatment temperature of from 150°C to 250°C.
The method for manufacturing a magnesium alloy member according to claim 7, wherein the plastic working is extrusion processing.