Patent Application
20090252643
The present invention relates generally to improved high strength aluminum base alloys having improved hot tear resistance when solidified into cast products.
It is well-known that alloying ingredients improve the properties of an aluminum-based material. High-strength aluminum alloys (which typically contain less than ninety five percent by weight aluminum, and possess a heat-treated yield strength in excess of approximately thirty thousand pounds per square inch) can be made compatible with low-cost fabrication techniques such as casting. For example, the addition of silicon (Si) to aluminum results in an improved ability of the molten metal to flow, thereby improving castability. Other alloying agents, such as copper, can be used to improve the strength characteristics of aluminum-based alloys, although traditionally at the expense of castability. Others may include both.
One such alloy, created by the Assignee of the present invention, is known as GM220, an alloy that can be easily cast into engine blocks, heads and other components for high-strength, weight-sensitive applications. Nevertheless, many aluminum alloys such as GM220 are not able to fully realize the efficient, low cost manufacturing that casting has to offer because of the tendency to form a defect known as hot tearing. In hot tearing, the strength of the solidifying melt is insufficient to withstand the stress related to thermal contraction of the metal due to solidification shrinkage and thermal expansion. Such phenomenon is exacerbated when the mold is more rigid, such as in metal molds.
Modified fabrication techniques, such as mold thermal control and higher pouring temperatures, can be used to reduce this problem; however, such approaches are often limited by the geometry of the molded part, as well as the complexity and concomitant expense. Likewise, casting simple billets with extensive post-cast machining significantly increase the cost, and therefore defeat the purpose of low-cost casting operations.
Grain refining is a preferred way to reduce the likelihood of hot tearing in castable aluminum alloys. In grain refining, a master alloy (for example, a titanium (Ti)-containing or related agent) is added to the molten metal just prior to pouring. In one well-known form, such master alloy may be a metal carbides, nitride and boride, such as titanium diboride (TiB2). It is believed that these master alloys introduce microscopic particles as nucleating agents into the melt.
Despite the addition of such master alloys, it is still desirable that an improved method of grain refinement of high strength, aluminum-based casting alloys be developed. It is further desirable that such grain refinement be useful in the production of an aluminum-based alloy without the attendant problem of hot tearing.
These desires are met by the present invention, wherein an improved high strength aluminum alloy substantially free from hot tearing is disclosed. By the disclosed invention, hot tear-inhibiting materials are introduced into an aluminum alloy such that smaller grain sized cast parts are produced. In accordance with a first aspect of the present invention, a method of increasing hot tear resistance in a cast aluminum alloy is disclosed by providing one or more of Si, copper (Cu), zinc (Zn), magnesium (Mg), iron (Fe), manganese (Mn) and strontium (Sr) as alloying ingredients to an aluminum-based mixture, adding at least one grain refining agent to the mixture, forming the mixture and the grain refining agent into a castable melt, casting the melt into a mold; and solidifying the melt in the mold to provide a substantially hot tear-free cast aluminum alloy product. The grain refining agent includes one or more of scandium (Sc), zirconium (Zr), metal carbides, metal aluminides, metal borides and at least one rare earth element. It will be understood by those skilled in the art that trace quantities of impurities, such as calcium (Ca) and phosphorous (P), may also be present in the alloy.
Optionally, particular concentrations of the grain refining agents are used. In one form, the Sc is included in an amount of up to 0.15 weight percent of the mixture. Likewise Zr can be used, either with or in place of the Sc. Concentrations may include up to 0.7 weight percent of the mixture, with more particular levels being 0.5 weight percent, 0.3 weight percent and 0.15 weight percent. Of the metal borides, carbides and nitrides, the present inventors have found that TiB2 is a particularly suitable grain refining agent, preferably in conjunction with one or both of the Sc or Zr. Silver (Ag), in an amount of up to 0.7 weight percent of the mixture, may also be included. In one particular form, Ag can be added with the TiB2, Zr and an optional amount of Sr. In a particular form, the grain refining agent further comprises TiB2 and Ag. Instead of, or in addition to, the Sc, Zr and TiB2, up to approximately 1.5 weight percent of at least one rare earth element can be added to the mixture. In the present context, the terms “approximately” and its variants, Likewise, the term “about” is construed in the present context to be equivalent to “approximately” or “nearly”. In this way, limits that use such modifiers indicate that exactitude is not claimed, but rather a contemplated variation between chemically desirable lower and upper operative limits.
The rare earth element or elements may be produced from mischmetal that is added to the mixture. In one particular form, the aluminum alloy is a high-strength, low-silicon alloy, comprising no more than about 2.0 weight percent Si. In a more particular variant, there is no more than about 1.0 weight percent Si. The metal carbide, metal aluminide and metal boride preferably include a titanium-containing compound that produces between about 0.02 and about 0.2 weight percent of Ti in the mixture. As stated above, such Ti can be in the form of TiB2. While the present method may be applied to numerous casting operations, the present inventors have found it to be particularly well-suited to permanent mold casting.
Various ranges may be used, depending on the particulars of the alloy. For example, a silicon content of up to about 1.3 weight percent Si may include a narrower range of between about 1.1 and about 1.3 weight percent (in the case of GM220), and about 0.1 weight percent (in the case of Alloy 206). Likewise, the range of between about 0.1 and about 0.5 percent Fe is between about 0.4 and about 0.5 percent iron (in the case of GM220), and about 0.1 percent (in the case of Alloy 206). Similarly, the Cu content is between about 1.9 and about 2.5 weight percent (in the case of GM220), and about 4.2 and 5.0 weight percent (in the case of Alloy 206). Mn is present in an amount between about 0.5 and about 0.75 weight percent (in the case of GM220, with a particularly useful upper limit of about 0.73 weight percent), and between about 0.2 and about 0.5 weight percent (in the case of Alloy 206). Mg is present in an amount between about 0.3 and about 0.5 weight percent (in the case of GM220), and between about 0.2 and about 0.35 weight percent (in the case of Alloy 206). Up to about 0.04 weight percent Ca and about 0.04 weight percent P is present (in the case of GM220. The other alloying ingredients that make up to about 0.05 weight percent of either GM220 or Alloy 206 may include nickel (Ni), tin (Sn), Ti alone or in combination.
According to another aspect of the invention, a method of improving hot tear resistance in an aluminum alloy is disclosed. The method includes adding TiB2 in addition to at least one grain refining agent to an aluminum-based mixture that comprises Si, Cu, Mg, Fe and Mn. The additional grain refining agent may include Sc, Zr and at least one rare earth element. The method further includes forming the mixture with the TiB2 and the grain refining agent (or agents) into a castable melt, placing the melt into a mold and solidifying the melt in the mold. In another form, elemental boron can be added to a titanium-containing melt to form the TiB2.
According to yet another aspect of the invention, an aluminum alloy is disclosed. The alloy includes up to about 2.0 weight percent Si, between about 1.9 and about 5.0 weight percent Cu, between about 0.2 and about 0.5 weight percent Mg, between about 0.1 and about 0.5 weight percent Fe, between about 0.2 and about 0.75 weight percent Mn such that a weight ratio of Mn to Fe is between about 1.25 and about 1.45, between about 0.02 and about 0.2 weight percent Ti, up to 0.01 weight percent Sr, up to 0.05 weight percent Zn, up to 0.05 weight percent Ca, up to 0.05 weight percent P, one or more grain refining agents from the group selected from Sc, Zr, rare earth element and TiB2, and a balance substantially made up of aluminum. In an optional form, the Sc, if present, is in a concentration of up to about 0.15 weight percent of the alloy. Likewise, the Zr, if present, is in a concentration of up to about 0.7 weight percent of the alloy. Lastly, the rare earth element, if present, is in a concentration of up to about 1.5 weight percent of the alloy.
The following detailed description of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
Referring initially to
In one form, the alloy is made up of between 1.0 and 2.0 weight percent silicon (Si) with a particularly beneficial upper range of about 1.3 weight percent, 1.9 and 3.5 weight percent Cu, 0.3 and 0.5 weight percent Mg, 0.4 and 0.5 weight percent Fe, 0.5 and 0.73 weight percent Mn, 0.02 to 0.2 weight percent Ti, up to 0.05 weight percent Sr, up to 0.5 weight percent Zn, up to 0.04 weight percent Ca, up to 0.04 weight percent P, (all numbers approximate) and a balance of aluminum and minute quantities of incidental impurities. In addition, a preferred (but not required) ratio of Mn to Fe is between approximately 1.25 and 1.45.
GM220 includes enough Si (between approximately one and two percent by weight) to form intermetallic compounds, but not so much as to form a separate eutectic Si phase that if present would undesirably lead to hot spots, as well as decrease the overall thermal conductivity and mechanical properties of the casting. The inclusion of such Si quantities can be beneficial in improving thermal fatigue resistance. Likewise, the inclusion of Cu helps to strengthen the alloy. For these reasons, GM220 exhibits high strength (tensile strengths of between approximately 250 and 400 MPa, depending on the casting technique), good elongation (between approximately 0.5 and 4.5 percent, again, depending on the casting technique) and low microporosity. The alloy includes Cu for its ability to increase strength and other mechanical properties, although such level is kept relatively low to avoid reductions in corrosion resistance. Mn is included to control the grain structure, while Zn improves the precipitation hardening response and corrosion performance, as long as levels aren't so high that they harm welding performance.
GM220 is also easy to cast, and is amenable to multi-step heat treating that can allow tailoring of the desired strength levels. These attributes make it useful in the production of high quality automotive castings. For example, higher strength alloys allow for the formation of components that can be made lighter for the same load bearing properties so that increased specific properties (an important consideration for automotive, aeronautical and other transportation applications) can be realized.
Nevertheless, during certain casting operations, aluminum alloys such as GM220 are susceptible to hot tearing where, referring next to
HTS=Σ(Ci×Li)
where C is the numerical value used to represent the degree of the crack severity in a test sample (such as a bar), and L is the numerical value corresponding to sample (bar) length. This latter factor takes into account the fact that the longer bars were less resistant to hot tearing than the shorter ones. For example, as shown with reference to
Referring next to
This method of adding Zr, Sc or both before adding a Ti-containing grain refiner is an improvement over merely adding Ti, boron (B) or a combination of the two prior to pouring the casting. It was also found that adding just over one percent rare earth elements via mischmetal achieved similar results. Sc improves strength and weldability of the cast alloy by reducing the sensitivity to hot tear formation. Likewise, it increases the recrystallization temperature and improves the ability to control the grain structure. Zr acts as a recrystallization inhibitor, as does TiB2.
To test the effects of the various grain refining agents being added to alloy GM220, the present inventors conducted a series of fifty castings. Table 1 shows the results of those castings, showing with particularity the number of hot tears that were produced.
As can be seen from the above table, the base alloy suffered thirty hot tears in fifty castings. Adding 150 ppm Sr as a eutectic modifier exacerbated the hot tear incidence to between forty and fifty. Grain refining with TiB resulted in lowering the hot tears to eighteen. Adding both Sr and TiB reduced it to ten hot tears. The addition of silicon to 2.0 and 3.0 percent by weight, in addition to improving castability, eliminated the hot tears, although at higher levels, the higher Si resulted in higher porosity (due to the presence of the eutectic Si phase that interferes with feeding during solidification) and lower strength. Adding Zr at 0.15 to 0.7 weight percent eliminated the hot tears, as did adding Sc. Furthermore, all combinations of Sr, TiB with one or both of Zr and Sc also resulted in the elimination of hot tears. Furthermore, adding mischmetal that produces 1.2 weight percent rare earths eliminated the hot tears.
Referring next to
The present inventors have discovered that various ranges could be employed in the grain refining agents added to the base alloy. For example, Sc can be added in a broad range of 0 to about 1 weight percent, with a more particular range of 0.1 to 0.75 weight percent and an even more particular range of 0.2 to 0.5 weight percent. Likewise, Zr can be added in a broad range of 0 to about 1 weight percent, with a more particular range of 0.1 to 0.75 weight percent and an even more particular range of 0.15 to 0.25 weight percent. Ti can be added in a broad range of 0 to 0.25 weight percent, with a more particular range of 0.02 to 0.15 weight percent and an even more particular range of 0.04 to 0.08 weight percent. B can be added in a broad range of between 0 and 100 ppm, in a medium range of between 5 and 50 ppm, and in a narrow range of between 5 and 20 ppm. In addition, rare earth elements can be added in a broad range of 0 to about 3 weight percent, with a more particular range of 1 to 2.5 weight percent and an even more particular range of 1.25 to 1.75 weight percent.
The addition of Zr in amounts at least up to 0.7 weight percent is not harmful to tensile properties; in fact, a low amount of Zr improves tensile properties over both no Zr and 0.7 weight percent Zr, implying an optimal range for tensile properties. Considering only the impact on hot tears, a certain minimum amount of Zr is required; such minimum will depend on die temperature and solidification rate as well as part geometry. The inventors believe that zirconium oxide (ZrO) dispersoids could improve the high temperature strength retention of GM220 alloy. Likewise, in chilled sections, the addition of Zr plus TiB improves the mechanical properties of the cast product.
The T6 (i.e., fully aged) tensile strength is higher than the chilled sand SA cast product, whereas elongation is higher for SA. Lost foam properties are somewhat lower than sand cast for both strength and elongation.
It will be appreciated by those skilled in the art that the Ti present in an alloy according to the present invention may be present in both the dissolved and undissolved form. Generally, maintaining relatively low levels of dissolved Ti contributes to the desirable smaller grain size and concomitant reduction in hot cracking. The smallest grain sizes are generally produced by the lower Ti levels. By way of example, an important commercially-available grain refiner is the master alloy discussed above that has the composition Al-3% Ti-1% B, which contains both soluble Ti and numerous microscopic particles of TiB2 that are suspended in the master alloy, and released into the melt when the master alloy is added to a bath of liquid aluminum. The particles are typically about one micron in diameter, and are therefore easily suspended in the molten metal. They are also insoluble in molten aluminum at normal casting temperatures. The amount of addition of insoluble and soluble titanium present in boride particles may be both varied and calculated. Furthermore, by allowing the melt to sit for an extended period of time (for example, two hours), all of the Ti added will go into solution, at which time no additional grain refinement would be produced.
A constant addition of a grain nucleating agent comprised of Ti and B can be accomplished by adding a quantity of commercial Al-3% Ti-1% B (3 wt. % Ti, 1 wt. % B, remainder aluminum) master alloy to the melts, where at least a portion of the Ti is present in the form of insoluble borides, with the remainder in dissolvable form.
In addition to the grain refining agents discussed above, processing conditions can be varied to improve hot tear resistance through smaller small grain size. For example, solidification times can be varied, where the casting method may influence the grain size. For example, the slower cooling rate of sand casting tends to produce larger grains than that of permanent mold castings.
Referring next to
The alloy depicted in
A typical Alloy 206 heat treatment may proceed as follows. The solutionizing stage is 480 degrees Celsius for one hour, followed by a ramp up to 495 degrees Celsius over a half an hour, after which it is held at 495 degrees Celsius for two hours, followed by a ramp up to 528 degrees Celsius over a half an hour, then holding at 528 degrees Celsius for ten hours. Next, a quenching stage followed by an aging stage at 200 degrees Celsius for four hours. By comparison, GM220 can be heat treated with more conventional T6 process, where (for example) the solutionizing stage takes place at 495 degrees Celsius for six hours, followed by a quench and subsequent aging at 190 degrees Celsius for six hours.
While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention, which is defined in the appended claims.
