Definitions

• the present invention relates to a method and apparatus for making aluminum alloys castings, wherein the heat-treatment processes of the prior art are simplified, by actually eliminating some traditional steps and equipment.
• the invention is applicable for example in the production of cylinder heads, motor blocks and the like, for automotive engines.
• the invention provides many advantages over the prior art heat- treatments, with an increased productivity of the casting plants, and lower capital and operation costs as well.
• the invention is particularly useful for producing aluminum alloys of the 3xx.x series of the classification of the Aluminum Association (AA), especially for T6 and T7 properties.
• AA Aluminum Association
• This invention is broadly applicable to the production of any aluminum alloy casting which in the past has derived meaningful benefit from quenching and artificial aging in an aging furnace.
• the invention eliminates the need for an aging furnace, while retaining the benefits thereof.
• This improvement has been styled herein as artificial self-aging (to distinguish from natural aging at ambient temperatures and from prior art artificial aging, which requires an aging furnace).
• the present invention uses a selectively directed spray quench in a manner which eliminates expensive equipment and reduces significantly the overall production time.
• the castings are preferably so quenched promptly after demolding in accordance with applicants' own recent patent to obtain the properties of a conventional "solution” heat treatment (such as the properties required by a T6 temper) but without the usual "solution” heat treatment in a furnace.
• This invention is broadly applicable to the production of any aluminum alloy casting of the type having significant precipitation hardening with meaningful benefit from "solution” heat treating and aging.
• the production process currently followed in the industry comprises filling a mold with liquid aluminum alloy, cooling the cast part in the mold in order to obtain a solidified casting, extracting the casting from the mold, and allowing the cast part to cool-down naturally to ambient temperatures, and then subjecting batches of such cooled castings to the aforementioned "solution" heat-treating process.
• One way to reduce the heat load in the solution heat treatment furnace has been to remove the sand cores and riser portions of the castings after natural cooling and before the "solution" heat treatment.
• the heat treating of the prior art comprises heating the preferably trimmed castings in a furnace to temperatures above about 470°C (typically in the range between 480°C and 495°C) for a certain period of time, usually in the range between at least 2 to 7 hours.
• This treatment is performed in order to bring back into solid solution the copper and or other alloying elements that give the castings their hardness. It is known that, while the casting metal is in the molten state, the alloying elements are in solution in the aluminum substrate.
• the cooling process particularly if the cooling is carried out at a slow rate, there is a tendency for the different elements to become segregated.
• the casting is re-heated in a "solution" heat treatment furnace for several hours, and only then is quenched, i.e. rapidly cooled down by a fluid quench from a temperature for example about 480°C to around 85°C, so that the solid solution is preserved (before segregation can occur).
• a fluid quench from a temperature for example about 480°C to around 85°C, so that the solid solution is preserved (before segregation can occur).
• Such post solution- treatment quench cooling may commonly be continued in a manner sufficient to bring the castings down to any of a number of different temperatures and at different rates according to the final properties of the alloy to be emphasized.
• This quenching step produces a supersaturated solid solution that causes the alloy to harden naturally as time passes.
• the quenched castings are maintained at temperatures of about 200°C in an "aging" furnace for about 2 or more hours. The time spent in the "aging" furnace at elevated artificial aging temperatures brings the alloy to at least a partial coherency in its structure giving the required hardness and strength properties.
• U.S. patent No. 5,788,784 to Koppenhoefer et al. discloses a process for heat- treating light metal castings that requires "a solution heat treatment furnace 2, an adjoining quenching device 3, as well as an aging furnace 4", all particularly for cylinder heads of piston engines.
• a solution heat treatment furnace 2 an adjoining quenching device 3
• an aging furnace 4" an aging furnace 4"
• the castings are quenched with an air/water mixture down to 130°C to 160°C, and then aged in a furnace at approximately 170°C to 210°C (thus taking advantage of some relatively minor residual heat carryover into the aging furnace), and are then finally cooled to room temperature after, for example, four hours of furnace aging.
• the castings are individually quenched with a mist-type fine mixture of air and water, which is "nozzle sprayed on all sides" of the casting.
• Koppenhoefer asserts a number of advantages by reason of quenching the castings with an air-water spray, for example that a uniform and low-distortion cooling is achieved, that the adhering core sand is not wetted at the elevated quenching temperatures and can be collected clean and reused after regeneration, and that the residual heat of the casting remaining at 130° to 160°C can be used to aid in the subsequent furnace aging step (by not cooling down the casting too much and leaving some heat in said casting). Quenching the casting by directing the sprayed water on all sides of the casting suggests that most of the residual heat is lost, with that amount retained being mainly in the inner portion of the casting.
• U.S. patent No. 5,112,412 to Plata et al. teaches a process for cooling large cast billets of aluminum after a temperature homogenization (re-heat) annealing step.
• Annealing is a softening process for aluminum (just the opposite of the strengthening and hardening heat treatment of the present invention), and this Plata patent is silent on how the cooling is to be done to accomplish a particular result (mainly mentioning only that it be "in accordance with the alloy composition" and describing how the "automated and controlled manner of spraying can be adjusted to different shaped billets, as they may differ from the usual round shape”).
• This patent first describes cooling the annealed billet with a spray on all sides.
• Plata et al describes a process modification in the case of a so-called (but otherwise unidentified) "hard” alloy to continue spraying until the billet has achieved an equalized temperature.
• An example of this temperature is given as "310 C.-350 C. in AlMgSi alloys" (a range above most age hardening but typical of softening annealing).
• the teaching includes the possibility of varying the intensity of the continuous spray, but only for the purpose of achieving a "better balanced heat flow” and a temperature zone "preferably distributed homogeneously during cooling so that no or only minimal deformations, stresses or cracks form".
• the patent states that circular billets are sprayed evenly, but a rectangular billet may be sprayed with different intensity along it periphery. This difference in spray intensity is to achieve uniformity of cooling during the quench (just the opposite of subjecting the casting to a significant differential or complete absence of quench cooling of a specific waste portion of the casting in order to maintain such portion at a significantly higher temperature during the quench of the work portion (and much less to identify such a waste portion which is accessible to the spray, but is not to be so spray cooled).
• one of the embodiments discloses a spray process involving a difference in the temperature between certain portions of the billet which later reach an equalized temperature
• differential quenching of selected portions of the casting to promote rather than minimize an initial significant heat differential between selected different portions of the casting (particularly with the division being between equally exposed waste and workpiece portions).
• Plata et al. teaches a process of cooling the surfaces of the workpiece (billet) on all sides, while the inner portion of the workpiece remains hot.
• the unquenched portion is an existing waste portion that is put to a useful interim purpose but whose ultimate alloy and physical properties are irrelevant. Engine castings, if made by the Plata process would be rejected.
• the present invention improves upon the applicants' aforementioned 5,922,147 patent by simplifying even further the overall heat-treatment of the castings, although broadly it can be applied separately.
• the invention dispenses with the aging furnace, in addition to preferably also dispensing with the solution furnace. Therefore, it provides a method and apparatus for producing the casting in a considerably shorter time, with less capital, and lower production costs, while maintaining and even improving on required mechanical properties of the castings.
• riser portion is a reservoir of liquid metal used to largely compensate for shrinkage of a casting as it cools in its mold.
• the term “riser” also commonly has the meaning used in this application, namely the solidified metal portion of the casting remaining in the reservoir after the casting is cooled.
• Rigger portion is intended to include at least the riser and additionally in its broader sense can include other similar waste attachments such as sprue, runners, gates, etc. formed as part of the original casting.
• the workpiece When the casting is demolded from the typical water-cooled mold, the workpiece commonly has a temperature of about 400°C and the riser one of about 500°C.
• the invention achieves its advantages by a selective quench of only the workpiece portion to surface temperatures preferably in the range from above about 100°C to about 130°C, at a rate sufficient to achieve a supersaturated solution of the hardening element (typically copper) in the aluminum alloy of the workpiece at the atomic level.
• spray nozzles are set to direct the water spray or mist on the workpiece and minimize any impingement on the riser.
• This workpiece-directed quench permits the riser (subjected only to natural or at most a minimized indirect cooling) to maintain a significantly higher temperature typically above about at least 300°C to 350°C during the workpiece quenching step. Thereafter, when the quenching is finished, the residual heat in the sprue portion is used as a heat reservoir to slightly re-heat the workpiece and maintain it (by conductive phenomena) in the artificial aging temperature range of between 140°C and 250°C, and preferably about 180°C to about 220°C, for an adequate time period, to thus achieve the desired properties for the workpiece.
• the invention dispenses with the need to supply furnace heat for re-heating and maintaining the whole casting in the artificial aging temperature range and simplifies the casting plant by thus rendering the aging furnace unnecessary.
• the final quench temperature should not be so low that the residual heat from the riser is too little to maintain the workpiece in the required aging temperature range for the necessary length of time.
• the workpiece surface temperature is maintained high enough above the boiling point of the spray liquid (typically water) throughout the quench, then liquid overflow onto the riser can be more easily minimized or avoided altogether and the latent heat of can be utilized and concentrated on the workpiece.
• a copious flow of a fine water mist is especially effective, since the mist particles evaporate immediately and there is no liquid wetting of the hot workpiece surface that can flow over onto the riser .
• the quenching temperature can be regulated to achieve the advantages of the invention at different temperature paths.
• the aging position of the casting plant can be insulated to prolong the artificial aging step at elevated temperatures for a more extended time period as may be needed (or even make use of a heat exchanger to take advantage of other residual or excess heat sources that may be available elsewhere in the casting plant system), all as an aid to avoid the need for the added expense of an aging furnace.
• Figure 1 is a graph showing the different casting temperature paths followed over time during the heat treatment according to the conventional prior art, according to applicants' most recent prior art process (shown on U.S. Patent 5,922,147 to Naltierra et al); and according to the present invention with respect to the workpiece portion and also to the riser portion.
• Figure 2 is a schematic side elevational view of a preferred embodiment according to the present invention illustrating a series of stations making up the quench portion of a casting production line, showing some castings (each comprised of both the workpiece portion and the riser portion) and the spray nozzles used for quenching only the workpiece portion oriented uppermost.
• Figure 3 is a schematic frontal view of the embodiment shown in Figure 2, showing the nozzles directing the spray or mist selectively onto the workpiece portion of the casting. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION.
• Figure 1 is a graph showing the different temperature paths vs. time of various castings; with the prior art processes in dotted lines and the present invention in continuous lines.
• the most common practice of the prior art includes, after demolding, the steps of subjecting the casting to: natural cooling, reheating and maintaining in a solution heat furnace, quenching, and reheating and maintaining in an aging furnace.
• Another illustrated prior art temperature path is the heat treatment disclosed in applicants' own very recent U.S. Patent 5,922,147 (wherein the solution treatment is omitted entirely, with the quenching of the casting occurring without natural cooling after demolding, and preferably immediately).
• the casting temperature paths of the workpiece portion and of the riser portion according to the present invention are shown in respective continuous lines.
• the riser remains essentially unquenched, with any cooling typically occurring only naturally and at a much lower rate; so that when the quenched workpiece has a temperature of about 120°C, the riser still has a temperature about 350°C.
• the quenching step is stopped and the casting (riser and workpiece together) is allowed to homogenize its temperature with the workpiece being mainly in the range of from about 160°C to about 220°C (initially towards the high end of the range, preferably).
• This essentially duplicates the conventional temperature profile of a casting maintained in an aging furnace after the quenching step, while surprisingly eliminating the need for any aging furnace. This is possible, because there is sufficient mass in the riser to function as an adequate reservoir of heat available for a sufficient duration to achieve complete aging.
• FIG. 2 schematically shows a preferred embodiment of the apparatus used for the quenching step to perform the invention.
• the casting broadly comprising riser 24 and the workpiece 22 is placed on a conveyor 16 by means of a feeding robot 12.
• the conveyor 16 has structural supports 18 (such as rollers) located through the quenching unit 10.
• an air header 26 and a water header 28 both being connected to spray nozzles 30.
• Spray nozzle 30 projects a water spray or an air driven mist 32 that is directed to impinge mainly on the workpiece portion 20 of the casting 22 positioned on the conveyor 16.
• the conveyor 16 is operated discontinuously in order to transport castings 20 from one quenching station 33 to the next in a step-wise mode (over a distance 34).
• the residence time needed for quenching the workpieces 22 to the desired temperatures is completed.
• a withdrawing robot 14 transports the quenched castings to a place to be aged artificially at still elevated temperatures over an extended time utilizing the reservoir of heat remaining in the riser 24.
• a fan 13 can be supplied to extract the vapor produced by the evaporation of the sprayed water while quenching the workpieces 22 of castings 20.
• Figure 3 shows an end view of the quenching unit 10. The same elements bear the same reference numerals of Figure 2. Additionally shown is an air supply 25 for air header 26, preferably at high pressure in order to achieve a better water spray or mist. Liquid supply 27 feeds header 28, which can handle water or any other suitable liquid cooling medium.
• the invention in its broader aspects can be applicable to other aluminum alloys and heat treating processes wherein the aging furnace step is normally used, including those prior art systems still using a conventional solution heat treatment with a subsequent quenching step.

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• Heat Treatments In General, Especially Conveying And Cooling (AREA)
• Cylinder Crankcases Of Internal Combustion Engines (AREA)
• Heat Treatment Of Articles (AREA)

Applications Claiming Priority (1)

Publications (3)

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• 2000
  • 2000-12-14 JP JP2002550130A patent/JP2004515655A/ja active Pending
  • 2000-12-14 CA CA002398600A patent/CA2398600A1/en not_active Abandoned
  • 2000-12-14 EP EP00987590A patent/EP1530651B1/de not_active Expired - Lifetime
  • 2000-12-14 BR BRPI0017079-8A patent/BR0017079A/pt not_active Application Discontinuation
  • 2000-12-14 MX MXPA02007829A patent/MXPA02007829A/es active IP Right Grant
  • 2000-12-14 DE DE60045291T patent/DE60045291D1/de not_active Expired - Lifetime
  • 2000-12-14 AU AU2001223891A patent/AU2001223891A1/en not_active Abandoned

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