Patent Application
20070187006
The present application claims priority to and the benefit of Korean Patent Application No. 10-2006-0010896, filed Feb. 4, 2006, the disclosure of which is incorporated by reference in its entirety.
1. Field
The present invention relates to an aluminum alloy, and more particularly, to a sintered aluminum base alloy.
2. Discussion of Related Technology
Recently, Al-base alloys have been actively replacing various ferrous components in automobiles to reduce the weight and improve the performance. (See The International Journal of Powder Metallurgy, 36, 2002, p 41 by F. V. Beaumont, and p 45 by C. Lall et al.) Most of aluminum alloys can be easily processed to final shapes via casting, forging, forming or machining, and also heat-treated to improve desired final properties. Many automotive components, such as space frames, an engine blocks, wheel frames, housings, etc, are currently utilizing aluminum castings or forgings. For ferrous or copper-base components in automobiles, powder metallurgy products are widely used because of their easy net-shaping capability, cost competitiveness and acceptable properties. Aluminum base powder products, however, have found very limited applications in automobiles despite many potential merits. (See The International Journal of Powder Metallurgy, 36, p 51, 2002 by W. H. Hunt) One of the main obstacles limiting the use of aluminum sintered products is the poor sintering behavior of aluminum-base powders which cause relatively low sintered properties. (See Nature, 181, p 833, 1958 by R. F. Smart et al., and Journal of Metals, 37, p 27, 1985 by Y. Kim et al.)
In general, aluminum-base powder mixtures for sintered products are prepared by mixing air atomized aluminum powder with secondary alloying powders such as copper, magnesium, silicon, zinc or others. The air atomized aluminum powder, though least expensive, contains a relatively thick aluminum oxide layer around each particle surface due to the oxidation during the atomization. Since the aluminum oxide is very stable and the sintering temperature for aluminum is relatively low, it is very difficult to reduce the oxide layer during sintering at around 600° C. The oxide layers between particles in a compact block the inter-diffusion between aluminum particles, thus severely limiting the sintering process. Compaction of a powder mixture at a relatively high pressure in order to break the surface oxide layers is known to improve the sintering to some extent.
However, forming a liquid phase during the sintering is considered as the most effective way of improving the sintering of elementally mixed aluminum powder compacts. (See “Properties and design guidelines for aluminum parts” in Proceeding 2000 International Conference on P/M Aluminum & Light Alloys for Automotive Applications, pp 51-58 by Antonio Romero, Acta Materialia, 35, p 589, 1996 by R. N. Lumley et al., and Material Chemistry and Physics, 67, p 85, 2001 by G. B. Schaffer et al.) Such liquid phases can be formed easily during the heating period of sintering by eutectic reaction between the aluminum powder and additive elemental powders such as copper, zinc, magnesium, silicon, etc. When a powder compact is heated to sintering temperature, localized alloying occurs at the contacts between aluminum and the additive powders to form a liquid phase during the heating cycle or sintering. The liquid phase, if persistently present during the sintering, can spread through particle boundaries, pulling particles and filling pores, so that higher sintered density and better bonding between particles can be obtained. On the other hand, if the liquid phase is transient, that is, formed at a relatively low temperature but disappeared by solutionizing in the matrix during heating or in the early stage of sintering, the dissolved liquid phase leaves pores behind and little persistent liquid phase in the matrix to assist particle bonding during the sintering. Therefore, controlling the characteristics and the amount of the liquid phase are most critical to improve the sintering of mixed elemental aluminum powders.
Most of commercially available mixed elemental aluminum powder blends, however, have the compositions based on those of wrought aluminum-base alloy systems, i.e., 201AB of Alcoa Inc. and AMB2712 of Ampal Inc. from AA2014, 601AB of Alcoa Inc. and AMB6712 of Ampal Inc. from AA6061, and AMB 7775 of Ampal Inc. from AA7075, rather than being optimized for sintering. The sintered properties obtainable from these powder mixtures are relatively low when compared to those of the wrought counterparts. For example, sintered Alcoa 201AB at 95% theoretical density was reported to have the tensile strength of ˜330 MPa after T6 condition, which is about only 70% of that of AA2014, despite the only 5% of porosity level. (See above “Properties and design guidelines for aluminum parts” in Proceeding 2000 International Conference on P/M Aluminum & Light Alloys for Automotive Applications, pp 51-58 by Antonio Romero) AMB7775 was reported to have the highest tensile strength over 400 MPa, but exhibits a relatively poor wear resistance. AMB7775 contains 6-8 wt % Zn, 3-5 wt % Mg and 1-3 wt % Cu with a small amount of Si and Pb, and also forms a liquid phase during the sintering. The liquid phase, however, is mainly transient, that is, disappeared by dissolving in the aluminum matrix during the sintering because of relatively large solubility of these elements in aluminum at or below the sintering temperature. The blend thus can be well strengthened by solid solution hardening and also by precipitation hardening, but dimensional control could be rather difficult because of the pores left by the solutionized liquid phase.
U.S. Pat. No.5,902,943 describes about Al—Zn-Mg—Cu base mixed elemental aluminum alloy powder blends and sintered aluminum alloys. The blends described in the patent are basically 7xxx type alloys and contain a relatively large amount of Zn as the principal alloying addition and other elements to enhance the sintering. Depending on the relative quantities of the alloying additions and heat treatment conditions, the blends can exhibit tensile strengths over 400 MPa but require a precise control of processing variables. The patent does not describe any results on the wear properties of the blends. Wear is also a big concern for most of potential aluminum powder metallurgy (PM) parts which will replace steel parts. Aluminum alloys generally have a relatively poor wear resistance than steels and thus require methods to improve the wear resistance. Hypereutectic Al—Si alloys which contain over 20% Si possess excellent wear resistance when they are produced in a pre-alloyed powder form and consolidated to full density at elevated temperatures. The pre-alloyed powders, however, are not well suited for the press and sinter processing, and thus are not very economical. (See above Material Chemistry and Physics, 67, p 85, 2001 by G. B. Schaffer et al.) Aluminum alloy composites reinforced with hard ceramic particles are alternatives but they are generally not well sinter-able and thus possess very poor strength and ductility in as-sintered condition.
Therefore, new mixed elemental aluminum base powder blends which can provide a better combination of strength and wear resistance are needed for wider range of applications than has been possible with existing blends.
The discussion in this section is to provide general background information, and does not constitute an admission of prior art.
One aspect of the invention provides an aluminum alloy comprising Al, Cu and Zn, wherein a portion of the alloy comprises: Cu in an amount over 5.6 wt % and less than about 9 wt % with reference to the weight of the portion; and Zn in an amount from about 1 wt % to about 5 wt % with reference to the weight of the portion.
In the foregoing alloy, the portion may comprise Al-containing grains and an intergrain material disposed between and interconnecting neighboring grains, wherein a substantial amount of Cu may be present in the intergrain material, and wherein a substantial amount of Zn is present in central areas of the Al-containing grains. In one embodiment, the substantial amount of Cu present in the intergrain material is about 2.5% to about 95% of the total amount of Cu. In certain embodiments, Cu present in the intergrain material may be about 2.5, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 50, 65, 80 or 95% of its total amount in the alloy portion. In some embodiments, Cu present in the intergrain material may be within a range defined by two of the foregoing amounts. In one embodiment, the substantial amount of Zn present in the Al-containing grain is about 5% to 100% of the total amount of Zn. In certain embodiments, Zn present in the Al-containing grain may be about 5, 20, 35, 50, 60, 70, 80, 85, 90, 95, 97, 98, 99, 99.5 or 100% of its total amount in the alloy portion. In some embodiments, Zn present in the Al-containing grain may be within a range defined by two of the foregoing amounts. Substantially the entire amount of Zn may be present in the Al-containing grains.
Still in the foregoing alloy, a substantial amount of Cu present in the intergrain material is in the form of CuAl2. Substantially the entire amount of Cu present in the intergrain material may be in the form of CuAl2. In one embodiment, the substantial amount of Cu which is present in the intergrain material in the form of CuAl2 is about 5% to 100% of the total amount of Cu present in the intergrain material. In certain embodiments, Cu present in the intergrain material in the form of CuAl2 may be about 5, 20, 35, 50, 60, 70, 80, 85, 90, 95, 97, 98, 99, 99.5 or 100% of its total amount in the intergrain material. In some embodiments, Cu present in the intergrain material in the form of CuAl2 may be within a range defined by two of the foregoing amounts. The intergrain material may comprise a portion which comprises Cu in an amount from about 20 wt % to about 60 wt % with reference to the weight of the portion of the intergrain material.
Further in the foregoing alloy, the portion of the alloy may comprise Cu in an amount from about 6 wt % to about 8 wt % with reference to the weight of the portion. The portion of the alloy may contain Sn in an amount from about 0.01 wt % to about 0.05 wt % with reference to the weight of the portion. The portion of the alloy may contain Mg in an amount less than about 0.03 wt % with reference to the weight of the portion. The alloy may be produced by a method, which comprises: providing a powder mixture comprising Al, Cu and Zn; and heating the powder mixture to a temperature sufficient to melt at least part of the powder mixture.
Another aspect of the invention provides a method of making an aluminum alloy, comprising: providing a powder mixture comprising Al, Cu and Zn, wherein Cu is in an amount over 5.6 wt % and less than about 9 wt % with reference to the weight of the powder mixture, and wherein Zn is in an amount from about 1 wt % to about 5 wt % with reference to the weight of the powder mixture; and heating the powder mixture to a temperature sufficient to melt at least part of the powder mixture.
In the foregoing method, the powder mixture may comprise Al-containing particles, and wherein a substantial amount of Zn may be dissolved into at least part of the Al-containing particles. Upon heating at least part of the powder mixture is melt to form a liquefied state, and wherein a substantial amount of Cu may be present in the liquefied state. The method may further comprise cooling the heated powder mixture thereby forming an alloy comprising a portion which comprises Al-containing grains and an intergrain material disposed between and interconnecting neighboring grains, wherein a substantial amount of Cu is present in the intergrain material.
Yet another aspect of the invention provides an aluminum alloy produced by the foregoing method, wherein a portion of the alloy comprises Al-containing grains and an intergrain material disposed between and interconnecting neighboring grains, wherein a substantial amount of Cu is present in the intergrain material, and wherein a substantial amount of Zn is present in central areas of the Al-containing grains.
A further aspect of the invention provides a powder blend for use in making an aluminum alloy, the powder blend comprising Al, Cu and Zn, wherein the powder blend comprises: Cu in an amount over 5.6 wt % and less than about 9 wt % with reference to the weight of the powder blend; and Zn in an amount from about 1 wt % to about 5 wt % with reference to the weight of the powder blend.
In the foregoing powder blend, wherein the powder blend may comprise Cu in an amount from about 6 wt % to about 8 wt % with reference to the weight of the powder blend. The powder blend may comprise Sn in an amount from about 0.01 wt % to about 0.05 wt % with reference to the weight of the powder blend. The powder blend may comprises Mg in an amount less than about 0.03 wt % with reference to the weight of the powder blend.
One or more embodiments of the present invention provide an elementally mixed Al—Cu—Zn base powder blend, a method of fabricating an article of a sintered alloy using the powder blend, and an article fabricated using the powder blend.
An aspect of the present invention provides a powder blend comprising more than 5.6 wt % Cu added to a balance Al to form a mixed powder blend. Thus, considerable amount of a liquid phase, which is formed over the eutectic temperature of Al—Cu, i.e. 548° C., persistently presents at a sintering temperature (about 600° C.), though a portion of the liquid phase is solutionized into a matrix. The persistent liquid phase fills boundaries and pores between powders as well as accelerates the sintering of the solid powders. Thus densification of the sintered alloy is improved.
Maximal solid solubility of Cu in an Al matrix is about 5.5˜5.6 wt % at the eutectic temperature 548° C. Thus, when more than 5.6 wt % Cu powder is added to Al powder, the persistent liquid phase always presents during the sintering, without relation to a temperature elevation rate, etc. The liquid phase which persistently presents during the sintering is solidified into a mixed phase of α-Al and CuAl2 (θ) phase, as the liquid phase is cooled to the room temperature, and the solidified liquid phase incorporates Al and 35 wt % Cu. (See Journal of Materials Science, 40, p 441, 2005 by Sang Chul et al.) The CuAl2 phase has vickers hardness (HV) of 980 higher than that of α-Al. (See Intermetallics, 7 p 1001, 1999 by D. Moreno et al.) The CuAl2 phase contributes to improve both of the strength and the wear resistance of the sintered alloy. However, increment of the amount of the CuAl2 phase may result in decrease of ductility of the sintered alloy. Thus, the amount of Cu may be decided by way of considering the strength, ductility, productivity of a sintered article, required shape of the article, required dimensions and tolerance of the article and deformation of a product shape during sintering. It is preferable to limit the amount of Cu to less than 9 wt %.
Meanwhile, since the solid solubility of Cu at a temperature of about 600° C. for sintering Al base powder blends is less than 3 wt %, the effect of solid solution strengthening is limited. Thus, Zn powder which has a higher solid solubility is added to improve the solid solution strengthening effect. Zn reacts with Al during a heating period of sintering, and forms a eutectic liquid phase over 382° C., and then is all solutionized into the Al matrix to enhance the strength of the matrix. In addition, Zn together with Cu can improve a hardening effect resulting from an age-hardening treatment after a solution-treatment, thus enhancing the strength of a sintered alloy. Increment of addition of Zn increases the strength of a sintered alloy and a thermal treated sintered alloy. However, an excessive transient liquid phase formed due to the surplus increment of Zn makes it difficult to maintain the shape of a compact. Therefore, it is preferable to limit the addition of Zn to not more than 5 wt %.
An aspect of the present invention provides a method of fabricating an article of a sintered Al—Cu—Zn base alloy. The method comprises mixing more than 5.6 wt % and less than 9 wt % Cu, 1˜5 wt % Zn, and a balance Al to form a mixed powder blend. The mixed powder blend is compacted, and then the compacted powder is sintered to form a sintered alloy. As a result, an article of a sintered Al—Cu—Zn base alloy can be fabricated.
The sintered alloy may be easily re-pressed because it has low yield strength (YS). With the re-pressing, a sintered alloy having a theoretical density of over 95% can be obtained, and the increase of the density improves the strength of the article of the sintered alloy. Also, deformation occurred during the sintering can be corrected using the re-pressing. Thus, the article having precise dimensions can be fabricated. In addition, the re-pressed alloy can be heat-treated to fabricate an article of a sintered alloy having a better combination of strength and wear resistance. The heat treatment may comprise solution-treating the re-pressed alloy, and heat-treating the solution-treated alloy for an age-hardening. Usually, the solution-treated alloy is water-quenched before the heat treating for an age-hardening. With the solution treatment, Cu in the liquid phase is solutionized into an Al matrix, and then CuAl2-x (θ″, θ′, or θ) phases are precipitated by the age-hardening treatment. As a result, the strength and hardness of the sintered alloy are improved.
An aspect of the present invention provides an article of a sintered Al—Cu—Zn base alloy. The article includes more than 5.6 wt % and less than 9 wt % Cu, 1˜5 wt % Zn, and a balance Al. The article of the sintered Al—Cu—Zn base alloy may be fabricated using the fabricating method described above. Meanwhile, the article of the sintered Al—Cu—Zn base alloy comprises an Al matrix (α-Al) and a CuAl2 phase. The CuAl2 phase may present at boundaries of α-Al and/or in the Al matrix. The CuAl2 phase serves as a reinforcing phase and improves both of the strength and the wear resistance of the article of the sintered Al—Cu—Zn base alloy. In several embodiments of the present invention, additive powders may be incorporated in the Al—Cu—Zn base powder blends. Mg added to the Al—Cu—Zn powder blends decreases the strength and ductility of its sintered alloy, whereas a small amount of Sn added to the Al—Cu—Zn powder blends increases the ductility of its sintered alloy. Preferably, 0.010.05 wt % Sn may be added to the powder blends, and less than 0.01 wt % Mg may be incorporated in the powder blends.
The above features and advantages of the present invention will become more apparent by describing the embodiment thereof with reference to the accompanying drawings, in which:
Hereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings.
Table 1 lists typical compositions and tensile properties of commercially available aluminum base mixed elemental powder blends. 2xxx blends contain Cu as a main additive element, however, Cu is less than 5 wt %, and 7xxx blends contain Zn as a main additive element. All of the commercially available aluminum base powder blends show a transient liquid phase sintering behavior, that is, the liquid phase formed during the sintering is almost solutionized or absorbed into a matrix. Among these powder blends, 7xxx series possess the highest tensile strength at both as-sintered and heat-treated conditions. Wherein, T1 indicates the as-sintered condition, and T6 indicates the heat-treated condition, that is, aged to peak hardness condition.
Referring to Table 2, the solidified liquid phase (A) includes Al and Cu and does not incorporate Zn. The solidified liquid phase was identified to consist of α-Al and CuAl2 (θ) phase by XRD analysis of
Referring to
In certain embodiments, the Cu content of the Al matrix can be slightly increased after the heat treatment, but Zn content was almost unchanged. Referring to Table 3, the solidified liquid phase was found to contain about 34 wt % Cu. Therefore, the Al matrix is strengthened by fine precipitates after aging and also by the presence of a Cu-rich hard phase in. The Cu-rich hard phase consists of about 40 wt % of a-Al and 60 wt % of CuAl2 (θ) phase. The CuAl2 phase functions as a reinforcing phase, thus improving the strength and wear resistance of the sintered alloy.
Referring to
Table 4 lists hardness, transverse rupture and tensile properties of Al-6Cu-3Zn and Al-6Cu-5Zn mixed elemental powder alloys compacted to 90% theoretical density (T.D.) at room temperature using double action die, sintered at 610° C. for 1 hour under flowing N2 atmosphere to about 96% T.D., further re-pressed to 98% T.D., and finally heat-treated for age-hardening. All samples were maintained at 540° C. for 1 hour and then water-quenched before aging.
Referring to Table 4, as-sintered samples usually show very low hardness and yield strength (YS) values and significant ductility, facilitating further plastic working process such as re-pressing and other cold working processes. Heat-treated samples usually show significant increases in hardness and strength with accompanying decrease in ductility. This is caused by precipitation of fine θ′ phase in the α-Al matrix. Strength and ductility slightly decreased by increasing Zn contents from 3 to 5 wt %. Thus, it is preferable to limit the Zn contents within 5 wt %.
Table 5 details wear resistance characteristics of various Al-base alloy systems, commercial and developed ones alike, compacted to 90% T.D., sintered for 1 hour at 610° C. under flowing N2 atmosphere to about 96% T.D., further re-pressed to 98% T.D., and finally heat-treated for age-hardening. For age-hardening, sintered samples were solution-treated for 1 hour at 540° C. and water-quenched immediately afterward, followed by artificial ageing for 22 hours at 150° C. Wear resistance was characterized by weight loss of pins after sliding 2000 m against rotating disk at 100° C. in a commercial engine oil. The pins were pressed against the rotating disk with the force of 500 N. Both the pin and disk were made with same blend.
Referring to Table 5, Very high coefficients of friction were observed for commercially available 7xxx series alloys and the 7xxx alloys with reinforcing SiC particles, resulting in significant amounts of wear. Al-4Cu alloy system shows still significant amount of wear, although somewhat reduced compared to commercially available 7xxx series alloys. Al-6Cu alloy system, on the other hand, caused marked reduction in both coefficient of friction and amount of wear. This is attributed to the solidified liquid phase which acted as reinforcing phase upon solidification as illustrated in
It will be appreciated that many changes and modifications can be made to the discussed embodiments without departing from the scope of the present invention, which is defined in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2006-0010896 | Feb 2006 | KR | national |
