Patent Application
20080060723
The invention provides a castable aluminum alloy having alloy compositional features controlled for reducing casting porosity and improving mechanical properties in the chilled or non-chilled cast condition and the heat treated (precipitation strengthened) condition of the alloy. The invention is particularly suitable for making cylinder head castings for reciprocating piston, internal combustion engines, although the invention is not limited in this regard since the alloy may find use for making cylinder block castings and other engine component castings as well as for making other cast articles of manufacture. The alloy can be cast by conventional casting processes and heat treated using conventional heat treatments to produce the well known T5, T6, etc. precipitation strengthened condition. The alloy also can be heat treated by a so-called post-solution, 3-step sequential aging heat treatment described below.
An illustrative cylinder head 10 for a V-6 internal combustion engine is shown in
An aluminum alloy pursuant to an illustrative embodiment of the invention for achieving the dual benefits of reduced casting porosity (e.g. macroporosity and/or microporosity) and improved mechanical properties comprises, in weight %, about 0.4% to about 2.5% Si, up to about 5% Cu, up to about 1% Mg, up to about 1% Fe, up to about 2% Mn, up to about 0.3% Ti, up to about 2.5% Ni, up to about 3% Zn, and the balance aluminum. Copper preferably is present in an amount of about 1% to about 4% by weight. Strontium (Sr) may be present in the alloy in an amount up to about 0.03% by weight and preferably is present in higher Cu alloys including 3% to 5% by weight Cu.
The Si concentration of the alloy preferably is selected within the range set forth to be insufficient to form a substantial amount of eutectic Si phase in the cast condition of the alloy and yet to be sufficient to precipitate typical Fe-rich intermetallic compounds commonly found in the 300 series aluminum alloys in the cast and heat treated condition. The Fe-rich intermetallic compound precipitates include, but are not limited to, α-Al15(Fe,Mn)3Si2 and Al15Mn3Si2, β-Al5FeSi. As a result, the aluminum alloy is substantially free of any separate Si phase to the extent that less than about 3 volume % of eutectic Si phase typically is present in the cast microstructure.
Iron (Fe) is usually present in aluminum alloys as a tramp element contained in aluminum produced from bauxite, which often contains ferric oxide. The ratio of Mn/Fe of the alloy preferably is selected to be 0.6 to 1.8 when the Fe concentration is greater than about 0.3% by weight for reducing microporosity and shrink porosity (macroporosity) in castings made from the alloy. Aluminum alloys containing less than 0.3% to 0.4% by weight iron may command a premium price. For most casting methods, it is preferred that the iron content not exceed about 0.8% by weight of the alloy. However, in die casting the iron content may be as high as about 1.5% to prevent the cast metal from sticking to the metal die surface.
Copper (Cu) preferably is present in the alloy composition as a solid solution strengthening alloying element in an amount of amount 1% to about 4% by weight. Magnesium (Mg) can be present in the alloy composition as a solid solution strengthening alloying element. Nickel (Ni) is not a necessary constituent of the alloy, but it is often present in available aluminum alloys and can be tolerated in amounts up to about 2.5% by weight. Zinc (Zn) is often a tramp element and can be tolerated within the specified maximum value. Titanium (Ti) is often a constituent of scrap aluminum alloys and reduces grain size when present in the range of 0.04 to 0.2% by weight. Strontium (Sr) may be added to modify the eutectic aluminum-silicon phase and the iron-rich intermetallics. Alternatively, these intermetallic and eutectic phases may be modified by the addition of sodium or rare earth metals, especially cerium, lanthanum and neodymium, either individually or in combination.
A preferred aluminum alloy pursuant to an illustrative embodiment of the invention comprises, in weight %, about 0.8% to about 2.5% Si, about 1.5% to about 2.5% Cu, about 0.1% to about 0.5% Mg, about 0.3% to about 0.6% Fe, about 0.4% to about 0.8% Mn, about 0.04% to about 0.2% Ti, up to about 2.5% Ni, up to about 1% Zn, and the balance aluminum. A more preferred alloy pursuant to this illustrative embodiment includes about 0.8% to about 1.5% Si and even more preferably about 1.0% to about 1.4% by weight Si. A particularly preferred alloy pursuant to this illustrative embodiment comprises, in weight %, about 1% Si, about 2% Cu, about 0.4% Mg, about 0.5% Fe, about 0.65% Mn, and the balance aluminum. These alloys exhibit reduced casting porosity and improved combination of tensile strength and ductility in the cast condition and heat treated (precipitation strengthened) condition.
A further preferred aluminum alloy pursuant to another illustrative embodiment of the invention comprises, in weight %, about 0.8% to about 2.5% Si, about 3.0% to about 4.0% Cu, about 0.1% to about 0.5% Mg, about 0.3% to about 0.6% Fe, about 0.4% to about 0.8% Mn, about 0.04% to about 0.2% Ti, up to about 2.5% Ni, up to about 1% Zn, about 0.008% to about 0.02% Sr, and the balance aluminum. A more preferred alloy pursuant to this illustrative embodiment includes about 0.8% to about 1.5% Si and even more preferably about 1.0% to about 1.4% by weight Si. A particularly preferred alloy pursuant to this illustrative embodiment comprises, in weight %, about 1% Si, about 3.5% Cu, about 0.4% Mg, about 0.5% Fe, about 0.65% Mn, about 0.010% Sr, and the balance aluminum. These alloys exhibit reduced casting porosity and improved combination of tensile strength and ductility in the cast condition and heat treated (precipitation strengthened) condition. The alloy preferably is heat treated using a heat treat method that ends with a 3-step sequential aging (SA) treatment where, after solutioning and rapid cooling from the solution temperature, the alloy is initially aged at a first aging temperature, quenched to ambient or room temperature, and aged at a second aging temperature less than the first aging temperature to obtain even more improved tensile strength and ductility as described below.
As mentioned above, aluminum alloys pursuant to the invention can be cast by conventional casting processes. A typical melt of the aluminum alloy can be prepared by melting aluminum ingot with suitable aluminum based master alloys such as Al-50% Si, Al-25% Fe, Al-50% Cu, Al-20% Mn, (where % is weight %) and pure magnesium metal to a desired composition as described above.
The melt is prepared in a suitable furnace, such as a coreless induction furnace, electric resistance furnace, reverberatory furnace, or a gas-fired crucible furnace of clay-graphite or silicon carbide. A flux is required only with dirty or drossy charge materials. Usually, no special furnace atmosphere is necessary. The charge can be melted in ambient air. Once molten, the melt is degassed using common aluminum foundry practice, such as purging the melt with dry argon or nitrogen through a rotary degasser. The degassing operation can also contain a halogen gas, such as chlorine or fluorine, or halogen salts to facilitate impurity removal. Preferably, the melt is handled in a quiescent manner so as to minimize turbulence and hydrogen gas pickup.
Once degassed and cleaned, the melt is treated with an agent to effect eutectic aluminum-silicon phase and/or intermetallic phase modification. A preferred agent to this end comprises Sr and/or one or more rare earth metals. The preferred method is to use Al-10% Sr or Al-90% Sr (% is weight %) master alloys, plunged into the melt during the last stages of degassing, provided no halogen material is used. The gas level of the melt is assessed via any of the common commercially available methods, such as the reduced pressure test or an AISCAN™ instrument.
Finally, just prior to pouring, the melt optionally may be grain refined to reduce grain size using a titanium-boron master alloy with a typical addition of titanium being about 0.02 to 0.1 weight % of the alloy. Some applications may not require grain refining, however. This grain refiner addition could be made prior to the modifier addition described in [0028].
Melt superheat can be varied from less than 50 degrees F. to well over 500 degrees F. Lower levels of superheat are recommended to minimize micro-porosity. However, higher levels of superheat have resulted in a refinement of the intermetallics in the cast microstructure such that this method may be preferred in some circumstances. The Examples below provide illustrative melt pouring temperatures. The melt is poured into a suitable mold that can be made by a number of known mold making practices. Such molds include, but are not limited to, bonded sand molds, metal molds, die casting molds, permanent molds, lost foam, or investment molds. Sand molds can contain metal chills to facilitate directional solidification or to refine cast microstructures locally in certain critical regions of the casting. In the case of sand molds, post-cast operations typically include removing excess sand from the casting by shot blasting. Post-cast operation also typically include removal of gating portions of the casting.
The aluminum alloys described above can be cast by conventional casting processes such as precision sand casting, permanent mold casting, semi-permanent mold casting, bonded sand casting, lost foam casting, investment casting, die casting, centrifugal casting, green sand casting, and other casting processes to make cast components of myriad types. For certain cast components, the casting is heat treated to develop mechanical properties appropriate for the intended service application. The invention is advantageous in that unexpectedly both microporosity and macroporosity can be reduced in cast components made from the above alloys. In addition, the invention unexpectedly can improve mechanical properties of the cast component while achieving a reduction in casting porosity.
Castings can be evaluated by commonly used nondestructive tests, such as X-ray inspection, dye penetrant inspection, or ultrasonic inspection. These tests are typically conducted to determine whether the casting has formed porosity due to shrinkage during solidification. Such shrinkage can be due to the composition of the cast alloy and/or to the shape of the casting.
Castings made from the aluminum alloys of the invention can be heat treated to enhance the mechanical properties by known precipitation hardening mechanisms for aluminum alloys. Such precipitation hardening heat treatments include, but are not limited to, the T5 temper, T6 temper, T7 temper and the 3-step sequential aging (SA) heat treatment. The T5 temper involves artificially aging the casting at an intermediate temperature typically from 300 to 450 degrees F. for up to 12 hours or more. More demanding casting applications may require the peak strength of the T6 temper which involves a solution heat treatment at a temperature near, but less than, the alloy solids temperature, for times typically ranging from 4 to 12 hours. The casting is quenched from the solution temperature in a suitable quenchant such as water, oil, or polymer or rapidly moving air. Such quenching rapidly cools the heat treated casting through the critical temperature regime, usually 850 degrees F. to 450 degrees F. Once cooled, the casting usually resides at room temperature for 1 hour to 24 hours and is then reheated to an intermediate temperature, similar to the T5 temper. In applications where dimensional stability is of importance, the T7 temper may be specified. This temper is similar to the T6 temper except that the artificial aging cycle is either conducted at higher temperatures or longer times, or both, to achieve a somewhat softer condition with greater dimensional stability. The SA temper is generally described above and in detail below.
Certain aluminum alloys pursuant to the invention are especially useful for making engine cylinder head castings and cylinder block castings which are machinable and which exhibit reduced casting porosity defects and improved mechanical properties in the as-cast condition as well as in the heat treated condition. For example, cylinder block castings and cylinder head castings for use in gasoline fueled, reciprocating piston internal combustion engines can be made of an alloy pursuant to the invention.
The following Examples are offered to further describe the invention but are not intended to limit the invention:
A master heat of an alloy of the invention nominally comprising, in weight %, 1.1% Si, 2.0% Cu, 0.4% Mg, 0.5% Fe, 0.6% Mn, and balance Al and incidental impurities was made by the following steps. The proper amounts of Al-36Si, Al-50Cu, Al-25Fe, Al-25Mn master alloys and pure magnesium metal were carefully weighed and melted in a clay-graphite crucible in an electric resistance furnace. Once molten the alloy was stirred and degassed, the alloy composition and gas content were checked and the alloy melt was gravity cast into sand molds to form five test bars having the dimensions of rectangular bars, 1 inch×1.5 inches in cross-section and 14 inches long, connected at both ends and at the midpoint by in-situ cast cross bars. The test bars each included an unchilled section on one side of the cast center cross-bar and a chilled section as a result of a 2.5×1 inch×8 inch long steel chill bar being located in the sand mold transversely across the center sections of the five parallel bars on the other side of the cast cross-bar. The chilled ends of the bars were located at the far end of the casting away from the in-gates.
The sand cast test bars then were subjected to the T6 heat treatment (solution treated at 910 degrees F. for 8 hours, then hot water (120 degrees F.) quenched, and then aged at 380 degrees F. for 8 hours). Tensile test specimens then were machined from the unchilled and chilled sections of the test bars for mechanical property testing using ASTM procedures B557.
For comparison, heats of conventional aluminum alloy 319 and aluminum alloy A356 were made and sand cast in similar manner to provide test bars with unchilled and chilled sections, which were heat treated to the T6 condition. Tensile test specimens then were machined from the unchilled and chilled sections of the test bars for mechanical property testing in similar manner.
Table 1 sets forth the results of the mechanical property testing where UTS is ultimate tensile strength (MPa) and YS is yield strength (MPa) at 0.2% offset.
With respect to conventional alloys A356 and 319, it is apparent that the unchilled test specimens exhibited very poor mechanical properties compared to the mechanical properties of the chilled test specimens of the same alloys. Chilling has been conducted in commercial casting operations to enhance mechanical properties.
With respect to the Invention Alloy, it is apparent that the chilled test specimens of the Invention Alloy exhibited a better combination of tensile strength and elongation compared to the chilled test specimens of the conventional alloy A356 and alloy 319.
Moreover, importantly, the unchilled test specimens of the Invention Alloy exhibited only slightly reduced mechanical properties compared to the mechanical properties of the chilled test specimens of the same alloy. The unchilled test specimens of the Invention Alloy exhibited mechanical properties that exceed those of unchilled test specimens of alloy A356 and alloy 319. As a result, alloys of the invention may enable the design of castings of lower weight since the unchilled regions of the castings will have improved mechanical properties and can be designed with reduced section thickness as a result. Furthermore, alloys of the invention may find use in lost foam casting processes where chilling of casting regions typically is not possible or practical. Alloys of the invention may allow lost foam castings to meet more stringent mechanical property requirements without the need for expensive post-casting operations, such as hot isostatic pressing.
Master heats of other alloys pursuant to the invention were made having the alloy compositions set forth in
The alloy melts were gravity cast into sand molds to form test bars having the dimensions of 1 inch×1.5 inch×14 inches long. The test bars each included a non-chilled section and a chilled section as a result of a 2.5 inch×1 inch×8 inch long steel chill being located in the sand mold transversely across the center sections of the five parallel bars as described above for Example 1.
Some of the sand cast test bars then were subjected to the T6 heat treatment or to a sequential aging (SA) heat treatment where as-cast test bars were solution heat treated at 910 degrees F. for 8 hours, then hot water (120 degrees F.) quenched, then aged at 480 degrees F. for 1 hour, then air cooled to room temperature, and finally aged at 360 degrees F. for 4 hours. The SA heat treatment involves a double age cycle utilizing a brief precipitate nucleation treatment followed by an abbreviated conventional artificial aging cycle.
Tensile test specimens then were machined from the non-chilled sections and from the chilled sections of the test bars for mechanical property testing using ASTM procedures B557.
The mechanical properties set forth in
It should be understood that the invention is not limited to the specific embodiments or constructions described above but that various changes may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.
