JP2015206080A - Aluminum sintered body - Google Patents
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本発明はアルミニウム焼結体に関し、詳細には引張強度等の機械的強度を高めたアルミニウム焼結体に関する。 The present invention relates to an aluminum sintered body, and more particularly, to an aluminum sintered body with increased mechanical strength such as tensile strength.
粉末冶金は、直接最終製品またはそれに近い形状を得ることができ、棒材や板材を研削加工する方法に比べて材料歩留まりに優れることから利用が拡大している。この粉末冶金では、原料粉末を成形して焼結し、熱処理や仕上げ加工を施すのが一般的であるが、焼結工程には条件によって幾つかの方法がある。一般的な焼結工程としては、大気圧で焼結する常圧焼結、一軸加圧で焼結するホットプレス、三次元的圧力で焼結するHIP等が挙げられるが、これらは高温で長時間保持する必要があるため、時間的、エネルギー的に効率的とは言えない。純アルミニウムまたはアルミニウム合金は、軽量であることから種々の装置や部品に多用されているが、純アルミニウム粉末またはアルミニウム合金粉末を原料粉末に用いる場合には500〜650℃程度で、1〜6時間程度の保持が必要となる。 Powder metallurgy can be directly used to obtain a final product or a shape close thereto, and its use is expanding because it is superior in material yield as compared with a method of grinding a bar or plate. In this powder metallurgy, raw material powder is generally molded and sintered, and heat treatment and finishing are performed, but there are several methods in the sintering process depending on conditions. Common sintering processes include atmospheric pressure sintering at atmospheric pressure, hot press sintering at uniaxial pressure, HIP sintering at three-dimensional pressure, etc., but these are long at high temperatures. Since it is necessary to hold time, it is not efficient in terms of time and energy. Pure aluminum or aluminum alloy is widely used for various devices and parts because of its light weight, but when pure aluminum powder or aluminum alloy powder is used as a raw material powder, it is about 500 to 650 ° C. for 1 to 6 hours. A degree of retention is required.
しかし、アルミニウムは鉄や銅等に比べて酸化しやすく、空気中に放置しただけでも粉末表面にはnmオーダーの安定な酸化物が形成されているため、粉末冶金を行った際に、粉末間での原子の拡散が阻害されて十分な結合を得るのが難しい。また、粉末表面には酸化物だけでなく水分等も吸着しており、これらを十分に除去せずに固化するとガス成分が残留し、加熱時の膨れや機械的特性が低下する要因となる。そのため、上記したような一般的な焼結方法では、真空中または非酸化性雰囲気中で表面に吸着している水分等を除去し、長時間かけて昇温し、高温で保持する必要がある。 However, aluminum is easier to oxidize than iron, copper, etc., and even if left in the air, a stable oxide of nm order is formed on the powder surface. It is difficult to obtain a sufficient bond because the diffusion of atoms is hindered. Further, not only oxides but also moisture and the like are adsorbed on the powder surface, and if they are solidified without sufficiently removing them, gas components remain, which causes a reduction in swelling and mechanical characteristics during heating. Therefore, in the general sintering method as described above, it is necessary to remove moisture adsorbed on the surface in a vacuum or in a non-oxidizing atmosphere, raise the temperature over a long time, and maintain at a high temperature. .
また、焼結体の機械的特性は、焼結粒子が微細粒であるほど優れるため、短時間で焼結して粒成長を抑制することが望ましいが、高温に長時間曝されると焼結粒子が著しく粗大化して機械的特性に劣るようになる。そのため、焼結体の機械的特性を低下させないためには、できるだけ短時間で焼結する必要があるが、ホットプレスやHIPでは上記したように短時間で焼結を完了させることは困難である。 In addition, since the mechanical properties of the sintered body are more excellent as the sintered particles are finer, it is desirable to sinter in a short time to suppress grain growth. The particles become extremely coarse and have poor mechanical properties. Therefore, in order not to deteriorate the mechanical properties of the sintered body, it is necessary to sinter in as short a time as possible, but it is difficult to complete the sintering in a short time as described above with hot press and HIP. .
このような背景から、近年では、放電プラズマ焼結法、プラズマ活性化焼結法及び放電焼結法等の通電焼結法も知られるようになってきている。通電焼結法は、ホットプレスやHIP等に比べて低い温度域での焼結が可能であり、更には短時間で焼結を行うことができるという利点を有する(例えば、特許文献1参照)。 Against this background, in recent years, electric current sintering methods such as a discharge plasma sintering method, a plasma activated sintering method, and a discharge sintering method have come to be known. The electric current sintering method has an advantage that it can be sintered in a lower temperature range than hot press, HIP, and the like, and further, can be sintered in a short time (for example, see Patent Document 1). .
しかしながら、更なる強度の向上の要求が強く、本発明ではより高強度のアルミニウム焼結体を提供することを目的とする。 However, there is a strong demand for further improvement in strength, and the present invention aims to provide a higher-strength aluminum sintered body.
上記課題を解決するために本発明は、平均粒径が10μm以下の純アルミニウム粉末またはアルミニウム合金粉末を、20MPa以上の加圧力で、パルス電流を印加して通電焼結してなり、かつ、酸化物が分散していることを特徴とするアルミニウム焼結体を提供する。 In order to solve the above-described problems, the present invention is a method in which pure aluminum powder or aluminum alloy powder having an average particle size of 10 μm or less is subjected to current sintering by applying a pulse current at a pressure of 20 MPa or more, and is oxidized. Provided is an aluminum sintered body characterized in that a product is dispersed.
本発明のアルミニウム焼結体では、平均粒径が10μm以下の微細な純アルミニウム粉末またはアルミニウム合金粉末を用い、パルス電流を印加して通電焼結したため、焼結粒子が微細であることに加えて、焼結前に粉末表面に生成していた酸化物が微細粒子となって分散しており、高強度になっている。 In the aluminum sintered body of the present invention, since a fine pure aluminum powder or aluminum alloy powder having an average particle size of 10 μm or less was applied and subjected to current sintering by applying a pulse current, the sintered particles were fine. The oxide produced on the powder surface before sintering is dispersed as fine particles, and the strength is high.
以下、本発明に関して図面を参照して詳細に説明する。 Hereinafter, the present invention will be described in detail with reference to the drawings.
本発明のアルミニウム焼結体は、平均粒径が10μm以下の微細な純アルミニウム粉末またはアルミニウム合金粉末(以下「アルミニウム粉末」という)を、パルス電流を印加して通電焼結したものである。アルミニウム粉末の種類には制限はなく、純アルミニウム系の1000系、溶接性に優れる5000系、高強度な7000系等、必要機能や入手性、コスト等に応じて適宜選択する。 The aluminum sintered body of the present invention is obtained by subjecting a fine pure aluminum powder or aluminum alloy powder (hereinafter referred to as “aluminum powder”) having an average particle diameter of 10 μm or less to current sintering by applying a pulse current. There is no restriction | limiting in the kind of aluminum powder, It selects suitably according to a required function, availability, cost, etc., such as pure aluminum type 1000 series, 5000 series excellent in weldability, and high strength 7000 series.
アルミニウム粉末は微細粒であるほど、得られる焼結体の機械的強度が高まる。また、アルミニウム粉末が大径であるほど、得られる焼結体における単位質量当りの酸化物量が少なくなり、分散強化が機能しなくなり、強度向上が望めなくなる。金属の塑性変形は、格子欠陥である転位が移動することにより生じ、転位が動きやすい材料は塑性変形が容易で、強度が低い材料である。そのため、転位を動きにくくすることで強度を上げることが可能であり、その一つに分散強化がある。分散強化とは、材料中にマトリックスよりも硬く、塑性変形しにくい粒子(分散粒子)を分散させ、転位を動きにくくする強化現象のことである。そして、分散粒子が微細で、単位質量当りの分散粒子が占める割合が大きくなるほど、転位が動きにくくなり、強度も向上する。表面酸化物の厚さはアルミニウム粉末の粒子径によって大きく変化するものではないため、粒子径が大きくなると単位質量当りの表面積が少なくなって表面酸化物量も少なくなり、焼結体における単位質量当りの酸化物が占める割合も小さくなる。このような理由から、本発明では、アルミニウム粉末の平均粒径を10μm以下とする。 The finer the aluminum powder, the higher the mechanical strength of the resulting sintered body. In addition, the larger the aluminum powder diameter, the smaller the amount of oxide per unit mass in the resulting sintered body, the dispersion strengthening does not function, and the improvement in strength cannot be expected. Metallic plastic deformation is caused by the movement of dislocations, which are lattice defects. A material in which dislocations easily move is a material that is easy to plastically deform and has low strength. Therefore, it is possible to increase the strength by making dislocations difficult to move, one of which is dispersion strengthening. Dispersion strengthening is a strengthening phenomenon that disperses particles (dispersed particles) that are harder than a matrix and difficult to plastically deform in a material, and that makes dislocations difficult to move. As the dispersed particles are finer and the proportion of the dispersed particles per unit mass increases, the dislocations are less likely to move and the strength is improved. Since the thickness of the surface oxide does not change greatly depending on the particle size of the aluminum powder, as the particle size increases, the surface area per unit mass decreases and the amount of surface oxide also decreases. The proportion of oxide is also small. For these reasons, in the present invention, the average particle size of the aluminum powder is 10 μm or less.
尚、アルミニウム粉末の平均粒径の下限は制限されるものではなく、取り扱いの容易さや、入手性等に応じて選択する。 In addition, the minimum of the average particle diameter of aluminum powder is not restrict | limited, It selects according to the ease of handling, availability, etc.
また、本発明では、パルス電流を印加して通電焼結を行う。通電焼結法は、ホットプレスと同様に一軸加圧焼結であるが、カーボングラファイト等の導電性の焼結容器に粉末材料を封入し、加圧しながら直接電流を流して加熱する焼結方法である。ホットプレス等の一般的な焼結方法では外部加熱によって内部へと熱が伝わるのに対し、通電焼結法では内部から加熱するためガス成分が抜けやすく、粉末の接触点から発熱するため、粉末間の元素拡散が促進されて短時間で緻密化される。そして、パルス通電を行うことにより、粉末間がプラズマ放電状態となり、粉末表面の酸化物が破壊されて焼結体中に分散する。その結果、分散した酸化物が転位を移動しにくくして焼結体の強度を向上させる。 Further, in the present invention, current sintering is performed by applying a pulse current. The electric current sintering method is uniaxial pressure sintering as in the hot press. However, the powder material is enclosed in a conductive sintering container such as carbon graphite, and heated by applying a direct current while pressing. It is. In general sintering methods such as hot pressing, heat is transferred to the inside by external heating, while in the current sintering method, gas components are easily removed because of heating from the inside, and heat is generated from the contact point of the powder. The diffusion of the elements between them is promoted, and densification takes place in a short time. Then, by performing pulse energization, the powder is in a plasma discharge state, and the oxide on the powder surface is destroyed and dispersed in the sintered body. As a result, the dispersed oxide is less likely to move dislocations and improves the strength of the sintered body.
通電焼結を行うのに際し、その加圧力を20MPa以上とする。加圧力が20MPa未満では粒子間の密着性が低く均一な電流が流れないため、元素拡散も進み難く緻密な焼結体が得られなくなる。上限についての制限はないが、焼結体サイズやプレス容量、焼結容器やパンチの耐荷重性等に応じて選択する。 When performing the electric current sintering, the applied pressure is set to 20 MPa or more. If the applied pressure is less than 20 MPa, the adhesion between the particles is low and a uniform current does not flow, so that element diffusion is difficult to proceed and a dense sintered body cannot be obtained. Although there is no restriction | limiting about an upper limit, it selects according to the sintered compact size, press capacity | capacitance, the load resistance of a sintering container, or a punch.
焼結温度は550〜650℃、昇温開示から加熱停止までのトータル焼結時間は1時間以下とすることが好ましい。焼結温度を550℃以上としたのは、元素拡散には熱エネルギーが必要になり、550℃未満では元素拡散が進み難く緻密な焼結体を得るには焼結温度を550℃以上にする必要である。但し、焼結温度が650℃を超えると、焼結中の結晶粒の粗大化により焼結体の強度が低下するだけでなく、部分的にアルミニウムが溶融して焼結容器からの漏れが生じ、狙ったサイズの焼結体が得られない等の問題が生じる。また、トータルの焼結時間が1時間を越えると、焼結中の結晶粒の粗大化により焼結体の強度が低下するようになる。 The sintering temperature is preferably 550 to 650 ° C., and the total sintering time from the disclosure of the temperature rise to the stop of heating is preferably 1 hour or less. The reason why the sintering temperature is set to 550 ° C. or higher is that heat energy is required for element diffusion, and if the temperature is less than 550 ° C., element diffusion does not proceed easily, and the sintering temperature is set to 550 ° C. or higher to obtain a dense sintered body. is necessary. However, when the sintering temperature exceeds 650 ° C., not only does the strength of the sintered body decrease due to the coarsening of crystal grains during sintering, but also aluminum partially melts and leaks from the sintering vessel. There arises a problem that a sintered body having a target size cannot be obtained. On the other hand, when the total sintering time exceeds 1 hour, the strength of the sintered body decreases due to the coarsening of crystal grains during sintering.
パルス通電は、焼結開始直後の数分間印加する。このパルス電流の印加により、アルミニウム粉末表面の酸化物が破壊されて焼結体中に分散し、焼結体に増強効果をもたらす。尚、パルス電流は焼結温度や焼結時間、焼結体サイズ等により選択できるが、電流量100〜15000Aとすることができる。 Pulse energization is applied for several minutes immediately after the start of sintering. By applying this pulse current, the oxide on the surface of the aluminum powder is destroyed and dispersed in the sintered body, thereby providing an enhancing effect on the sintered body. The pulse current can be selected depending on the sintering temperature, the sintering time, the size of the sintered body, etc., but the current amount can be set to 100 to 15000A.
焼結は大気環境下、真空環境下、不活性ガス環境下で行って構わないが、上記したように本発明ではアルミニウム粉末表面の酸化物を有効利用するため、真空環境下や不活性ガス環境下で行う必要はなく、真空設備や不活性ガスの供給設備が不要となり、低コストとなる。 Sintering may be performed in an atmospheric environment, a vacuum environment, or an inert gas environment. However, as described above, in the present invention, the oxide on the surface of the aluminum powder is effectively used. There is no need to perform the operation below, and vacuum equipment and inert gas supply equipment are not required, resulting in low costs.
即ち、焼結条件としては、例えば、一軸加圧力を40MPa、最高到達温度を600℃、トータル焼結時間を0.5時間、パルス電流(電流値:1000A)を焼結開始直後2分間印加し、大気中で焼結することができるが、粉末組成や粒子径、焼結体サイズ等により最適条件が異なるため、必要に応じて適宜変更する。 That is, as sintering conditions, for example, a uniaxial pressure is 40 MPa, the maximum temperature reached is 600 ° C., a total sintering time is 0.5 hours, and a pulse current (current value: 1000 A) is applied for 2 minutes immediately after the start of sintering. Although it can be sintered in the atmosphere, the optimum conditions differ depending on the powder composition, particle size, sintered body size, etc., so that they are appropriately changed as necessary.
以下に実施例及び比較例を挙げて本発明を更に説明するが、本発明はこれにより何ら制限されるものではない。 Examples The present invention will be further described below with reference to examples and comparative examples, but the present invention is not limited thereby.
(試験A)
純アルミニウム粉末及び7075アルミニウム合金粉末を用い、パルス電流の印加による酸化物分散強化を確認した。焼結条件は表1に示すとおりであるが、一軸加圧力40MPa、最高到達温度600℃、トータル焼結時間0.5時間、大気中で焼結し、パルス電流有りの場合は焼結開始直後から2分間パルス電流(電流:1000A)を流した。また、焼結体サイズは、φ50mm、t=5mmの円板とした。尚、7075アルミニウム合金粉末を用いた場合は、時効硬化(T6処理)した。
(Test A)
Pure aluminum powder and 7075 aluminum alloy powder were used, and oxide dispersion strengthening by applying a pulse current was confirmed. Sintering conditions are as shown in Table 1, but
そして、得られた焼結体の引張強度を測定した。結果を表1に示すが、パルス電流を印加することにより焼結体の引張強度が高まっており、これは原料粉末表面の酸化物が焼結財中に分散したことにより、酸化物分散強化が現れた結果である。 And the tensile strength of the obtained sintered compact was measured. The results are shown in Table 1, and the tensile strength of the sintered body is increased by applying a pulse current. This is because the oxide on the surface of the raw material powder is dispersed in the sintered article, and thus the oxide dispersion strengthening is enhanced. It is the result that appeared.
(試験B)
表2に示すように、平均粒径が異なる原料粉末を用い、試験Aと同条件にてパルス電流を印加して通電焼結を行い、焼結体の引張強度を測定した。結果を表2、並びに図1、2に純アルミニウムフ粉末または7075アルミニウム合金粉末の平均粒径と、得られた焼結体の引張強度との関係を示す。
(Test B)
As shown in Table 2, using raw material powders having different average particle diameters, applying a pulse current under the same conditions as in Test A and conducting current sintering, the tensile strength of the sintered body was measured. The results are shown in Table 2, and FIGS. 1 and 2 show the relationship between the average particle diameter of pure aluminum powder or 7075 aluminum alloy powder and the tensile strength of the obtained sintered body.
表2及び図1、2から、純アルミニウム粉末及び7075アルミニウム合金粉末ともに、平均粒径が大きくなるのに伴って引張強度が低下しており、特に10μmより大きくなると急激に強度低下が起こっている。これは、大径になるほど焼結体の単位質量当りの酸化物が占める割合が減少して酸化物分散強化が機能しなくなり、特に平均粒径で10μmを超えると顕著になることを示している。 From Table 2 and FIGS. 1 and 2, both the pure aluminum powder and the 7075 aluminum alloy powder show a decrease in tensile strength as the average particle size increases, and particularly when the size exceeds 10 μm, the strength decreases rapidly. . This indicates that the larger the diameter, the smaller the proportion of oxide per unit mass of the sintered body decreases, and the oxide dispersion strengthening does not function, and the average particle diameter is particularly prominent when it exceeds 10 μm. .
(試験C)
7075アルミニウム合金粉末を用い、一軸加圧力を変えて試験Aと同条件にてパルス電流を印加して通電焼結を行い、時効硬化(6T処理)を施して焼結体とした。そして、各焼結体の引張強度を測定した。
(Test C)
A 7075 aluminum alloy powder was used, uniaxial pressing force was changed and pulsed current was applied under the same conditions as in Test A to conduct current sintering, and age hardening (6T treatment) was applied to obtain a sintered body. And the tensile strength of each sintered compact was measured.
結果を表3及び図3に示すが、一軸加圧力が20MPa以上になると引張強度が急激に高まっているのがわかる。これに対し、一軸加圧力が20MPa未満では緻密化が進まず、焼結体の断面を観察すると多数の空隙が存在していた。 The results are shown in Table 3 and FIG. 3, and it can be seen that the tensile strength is rapidly increased when the uniaxial pressure is 20 MPa or more. On the other hand, when the uniaxial pressure is less than 20 MPa, densification does not proceed, and a large number of voids exist when the cross section of the sintered body is observed.
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