Patent Grant
8409374
This invention relates to a method for the heat treatment of castings of high pressure die cast age-hardenable aluminium alloys.
High pressure diecasting (HPDC) is widely used to mass produce metal components that are required to have close dimensional tolerances and smooth surface finishes. One disadvantage, however, is that parts produced by conventional HPDC are relatively porous. Internal pores arise because of shrinkage porosity during solidification, and also the presence of entrapped gases such as air, hydrogen or vapours formed from the decomposition of die wall lubricants.
Castings made from HPDC aluminium alloys are not considered to be amenable to heat treatment. This follows because the internal pores containing gas or gas forming compounds expand during conventional solution treatment at high temperatures (eg. 500° C.) resulting in the formation of surface blisters on the castings. The presence of these blisters is visually unacceptable. Furthermore, expansion of internal pores during the high temperature solution treatment may have adverse effects both on the dimensional stability and mechanical properties of affected high pressure die castings.
As discussed in Altenpohl “Aluminium: Technology, Applications, and Environment”, Sixth Edition, published by The Aluminium Association and The Minerals, Metals and Materials Society, —see pages 96 to 98—there are techniques which allow high pressure die castings to become relatively pore-free and thus heat treatable in the absence of blistering. These techniques include vacuum die casting, pore-free die casting, squeeze casting and thixocasting, all of which involve cost penalties.
Of these techniques, vacuum systems are applied most frequently, with the aim of reducing porosity within the casting. In many cases the remaining level of porosity is still too high to allow heat treatment. However, there are some exceptions.
For example, in U.S. Pat. No. 6,773,666 to Lin et. al., an improved Al—Si—Mg—Mn alloy is disclosed as able to be high pressure die cast using Alcoa's AVDC die casting technique to produce extremely low porosity in resultant castings. The alloy composition contains less than 0.15Fe, less than 0.3Ti, less than 0.04Sr, and is substantially copper free, chromium free and beryllium free. It is similar to the casting alloy AA357 as well as the Australian casting alloy designations CA601 and CA603 (Aluminium Standards and Data—Ingots and Castings, 1997). The AVDC method uses very high vacuum pressure to produce components that are relatively pore free, and are reported as being weldable and heat treatable (see, for example, http://www.alcoa.com/locations/germany_soest/en/about/avdc.asp, 2005). In the prior art of Lin et. al., the castings were examined by X-ray analysis and found to be in excellent condition in regards to porosity contents. This high vacuum casting technique, followed by the heat treatment stages of solution treatment from 950-1020° F. (510-549° C.) for 10-45 minutes, quenching into water at 70 to 170° F. (ambient to 77° C.) and artificial ageing for 1-5 h at 320-360° F. (160-182° C.) was believed to achieve adequate properties for aerospace applications. Following the heat treatment schedules taught within this prior art, minor blistering was reported to have appeared upon the surfaces of the alloy examined, and was believed to have resulted from entrapped lubricant. However, the alloy was disclosed as being of high structural integrity and deemed suitable for aerospace applications.
Another example of a technique to reduce or remove porosity and thus facilitate heat treatment is disclosed in U.S. Pat. No. 4,104,089 to Miki wherein components produced from Al—Si—Mg—Mn alloy were able to be heat treated conventionally following a pore-free diecasting process. That diecasting process is based on earlier work evidently that of U.S. Pat. No. 3,382,910 to Radtke et al in which the die cavity is purged with a reactive gas that combines with the molten metal to reduce the level of porosity in resultant castings.
The conventional heat treatment procedure for aluminium alloys normally involves the following three stages:
The strengthening resulting from ageing occurs because the solute taken into supersaturated solid solution forms precipitates which are finely dispersed throughout the grains and which increase the ability of the alloy to resist deformation by the process of slip. Maximum hardening or strengthening occurs when the ageing treatment leads to the formation of a critical dispersion of at least one type of these fine precipitates.
An alternative to the heat treatment procedure mentioned above is what is known as a T5 temper. In this case, the alloy is quenched immediately following casting while it retains some of its elevated temperature, and then artificially aged to produce more moderate improvements in properties.
Solution treatment conditions differ for different alloy systems. Typically, for casting alloys based around Al—Si—X, solution treatment is conducted at 525° C. to 540° C. for several hours to cause appropriate spheroidisation of the Si particles within the alloy and to achieve an appropriate saturated solid solution suitable for heat treatment. For example, Metals Handbook, 9th ed. vol. 15 p. 758-759 provides times and temperatures typical for solution treatment of casting alloys to provide these changes. Typically, the time of solution treatment for the alloys based on Al—Si—X is given as being between 4 and 12 hours, and for many alloys 8 hours or more, depending on the specific alloy and temperature of solution treatment. The time of solution treatment is normally considered to commence once an alloy has reached within a small margin of the desired solution treatment temperature (eg. within 10° C.), and this can vary with furnace characteristics and load size. However, this process, if applied to conventional aluminium alloy high pressure diecastings, is unsuitable because it will cause substantial unacceptable surface blistering on the diecastings.
The present invention provides a method for the heat treatment of age hardenable aluminium alloy high pressure die castings (HPDC) which obviates the need to use the more expensive alternative component production techniques as discussed in Altenpohl and other sources. The invention is applicable to all age hardenable aluminium alloy HPDC castings but is particularly applicable to those that contain internal pores residual from the die casting process. The castings may be produced by what can be regarded as a conventional or usual HPDC technique, such as with a cold-chamber die casting machine, and without a need to determine the level of porosity in resultant castings in order to select those that are sufficiently pore-free as to be amenable to conventional heat treatment. That is, the alloys are cast under pressure to fill one or more mould cavities in a die without the application of a high vacuum by which air is withdrawn from the die cavity and without the use of a reactive gas to purge air from the die cavity. Thus, the alloy is able to be cast in a die which, at the onset of casting, is exposed to the natural, surrounding atmosphere and is at ambient gas pressure. As a consequence, the casting to which this invention is applicable can be characterised by the presence of porosity. The presence of porosity can be determined by several techniques. For example, optical microscopy of a cross section of as-cast alloy will reveal porosity. X-ray radiography will also reveal porosity, but only that which is resolvable or large enough to be easily seen.
The present invention provides a method for the heat treatment of a casting produced by high pressure die casting of an age-hardenable aluminium alloy, wherein the method includes the steps of:
In one form, the present invention provides a method for the heat treatment of a high pressure die casting of an age-hardenable aluminium alloy typically exhibiting porosity, wherein the method includes the steps of:
The quenching in step (b) may be to a temperature suitable for the strengthening of step (c). The ageing in step (c) may be natural ageing or artificial ageing. Thus, in the former case, the alloy may be held at ambient temperature, that is at the prevailing atmospheric temperature which may range from 0° C. to 45° C., for example from 15° C. to 25° C., such that heating is not necessary. Alternatively, the casting may be artificially aged by heating above ambient temperature. Artificial ageing preferably is by heating in the range of from 50° C. to 250° C., most preferably in the range of from 130° C. to 220° C.
The duration of the heating in step (a) may include the time for heating to the lower limit of the range of 20 to 150° C. below the solidus melting temperature. On reaching that range, the casting can be held at one or more temperature levels within the range for a duration of time of less than 30 minutes. Alternatively, the heating of the casting in stage (a) can be non-isothermal within the specified range of temperatures.
Step (a) may be conducted, at least in part, non-isothermally, or conducted substantially completely non-isothermally. Alternatively, step (a) may be conducted substantially isothermally.
In step (c), where the casting is subject to artificial ageing, the casting may be held at one or more temperature levels within the artificial ageing temperature range, or the ageing can be conducted non-isothermally such as by the temperature of the casting can be ramped up to a maximum within the range.
Step (c) may be conducted such that the age-hardened casting is in an underaged condition, peak aged condition, or an overaged condition, compared in each case to a full T6 temper. In the process of the invention, the casting may be cold worked between step (b) and step (c). Cooling of the casting from the ageing temperature for step (c), where step (c) provides artificial ageing, may be by quenching. Alternatively, the casting may be slowly cooled from an artificial ageing temperature in step (b), such as by slow cooling in air or another medium. The casting following step (c) typically is devoid of dimensional change from its as cast condition.
For a conventional heat treatment, the time at the solution treatment temperature is to provide alloy homogenisation and development of a maximum solute content solid solution. In contrast, in step (a) of the present invention the alloy is not fully homogenised or equilibrated due to the short time-frame used, and the solid solution formed is not expected to be completely at equilibrium at that temperature for its given duration. That is, the solution treatment in effect is partial with respect to current practice in heat treatment of aluminium alloys.
The heat treated casting that results from the present invention may be produced by a conventional or usual high pressure die casting technique in which die cavity fill is by substantially fully molten alloy. As a high vacuum is not applied in that technique to withdraw air from the die cavity, turbulence in the alloy can result in entrapped gases and internal porosity. Castings may also be produced by a variant of that technique disclosed in International patent application WO026062 by Cope et al and assigned to the assignee in respect of the present invention. In the technique of Cope et al, die cavity fill is by an advancing front of semi-solid alloy, and resultant porosity is more finely distributed within the alloy. However, heat treatment of a casting produced by this variant of conventional or usual HP die casting also can result in blistering in some instances, such that the casting of this variant also benefits from application of the present invention.
The method of the invention can be applied to a high pressure die casting produced from any age-hardenable aluminium alloy. However, alloys for which the present invention is most suitable are Al—Si alloys having 4.5 to 20 wt % Si, 0.05 to 5.5 wt % Cu, 0.1 to 2.5 wt % Fe and 0.01 to 1.5 wt % Mg. The alloys optionally may contain at least one of Ni up to 1.5 wt %, Mn up to 1 wt % and Zn up to 3.5 wt %. In each case, the balance apart from incidental impurities comprises aluminium. The incidental impurities which may be present include, but are not restricted to Ti, B, Be, Cr, Sn, Pb, Sr, Bi, In, Cd, Ag, Zr, Ca, other transition metal elements, other rare earth elements and rare earth compounds, carbides, oxides, nitrides, anhydrides and mixtures of these compounds. The incidental impurities may vary from casting to casting and their presence is without significant detriment to the current invention.
Particularly with castings of those Al—Si alloys, the castings may be pre-heated to a temperature in the range of 100° C. to 350° C. before step (a) so that the time required to heat into the appropriate temperature range for step (a) is minimised.
With those Al—Si alloys, the silicon plays an important role in the method of the present invention, as detailed later herein.
As indicated, a casting heat treated by the method of the present invention is subjected to solution treating for a period of less than 30 minutes in a temperature range that is 20 to 150° C. below the solidus melting temperature of the alloy of the casting. The solution treatment period within that temperature range may be less than 20 minutes and preferably not more than 15 minutes, such as from 2 to 15 minutes.
With quenching the casting into water at higher temperatures in the range between 0 and 100° C., the casting can have substantial heat energy content. In that case, the alloy may be cooled rapidly from the higher temperature, if required.
Prior to the commencement of step (a) of the method of the invention, the casting is referred to as “as cast”, meaning it has been high pressure die cast in a conventional high pressure die-casting machine, without the need for a use of an applied high vacuum or a reactive gas. Prior to the commencement of step (a) the alloy can be at ambient temperature or at a higher intermediate temperature, such as 200° C.-350° C. if it is pre-heated or if it has retained some thermal energy from the casting process. During step (a), the alloy is heated into the appropriate temperature range for the appropriate time according to the current invention for the solution treatment step. Following step (b), the casting can be referred to as “solution treated” or “solution treated and quenched”. Following step (c), the casting is referred to as “precipitation hardened” or “age hardened”.
With the application of the heat treatments described for the present invention to HPDC's exhibiting normal porosity, there is a surprising minimisation or total absence of surface blistering. Components remain dimensionally stable and may exhibit large increases in mechanical properties.
The plots of
For the samples tracked in
As is apparent from
The castings 3(a) to 3(i) shown in
Castings of CA605 alloy composition are considered not to be amenable to age hardening heat treatments when produced by the HPDC technique used for the castings of
The castings shown in
Casting 3(a) exhibits a quality finish characteristic of high pressure die castings of aluminium alloys. Each of castings 3(b) to 3(i) in the as cast condition, exhibited the same high quality surface finish and were chosen randomly from the same casting batch as that shown in
Table II summarises the tensile properties of CA605 alloy in castings that were prepared by conventional HPDC techniques, without application of a vacuum or use of a reactive gas and containing typical porosity levels, and then subjected to various heat treatments. For the castings, a slow shot velocity of 26 m/s, a high shot velocity of 82 m/s or a very high shot velocity of 123 m/s were used where these speeds were the velocity of the metal at the in-gate.
In Table II, abbreviations have the following meanings:
As shown by Table II, the tensile properties obtainable by using the present invention reveal the highly beneficial effect of age hardening. The property levels do not reflect any significant compromise when compared with conventional ageing treatments, and yet they have been obtained with castings produced by conventional HPDC without the heat treated castings exhibiting blistering. Table II also indicates that there is no benefit to the current invention in quenching from the casting process, before solution treatment, quenching and ageing according to the present invention.
Castings of this CA313 alloy also are not considered to be amenable to heat treatment when produced by the conventional HPDC casting technique used for castings 8(a) to 8(j), again due to the incidence of surface blistering and loss of dimensional stability.
The castings shown in
Casting 8(b) exhibits dimensional instability due to the solution temperature being slightly too close to the solidus, although there is little if any evidence of such instability in casting 8(c) at the next lower solution temperature, or in other castings. However, each of castings 8(b) and 8(c) exhibit unacceptable blistering. Both castings 8(d) and 8(e) show one large blister and several smaller ones, indicating an unacceptable reject rate, whereas castings 8(f) to 8(j) show good finish quality following solution treatment and no evidence of blistering.
A comparison between castings 8(b) to 8(j) on the one hand and castings 3(c) to 3(i) of
The arrows shown for castings 12(d) to 12(h) point to blisters that have formed on the surfaces of those castings. As the solution treatment time is increased, beginning at about 20 minutes, the prevalence of blisters increases from a few on casting 12(d) up to a larger number at the longer time of 120 minutes.
Table V shows the tensile properties of the HPDC CA313 alloy prepared to either T6, T4, T6I4 or T6I7 conditions. Each alloy was solution treated at a maximum temperature of 490° C. for 15 minutes (including time of heating up to temperature), cold water quenched and then aged. Artificial ageing for the T6 temper was conducted at 150° C. For the T4 temper, the alloy was solution treated as above, then exposed at ˜22° C. for a period of 14 days.
For the T6I7 conditions, represented are samples that have been underaged 2 or 4 hours, and then slowly cooled in oil at approximately 4° C./min. to suppress subsequent secondary precipitation. T6I4 tempers have been designed to retain elongation, rather than to gain equivalent to T6 tensile properties as in the example shown in Table 1. These were artificially aged 2 hours at 150° C., quenched, then exposed at 65° C. for 4 weeks. The alloy samples were from castings of the same form and dimensions as the castings shown in
Table VI shows the tensile properties of a conventional CA313 HPDC alloy, recorded for further castings of the form and dimensions for the castings of
Table VII shows tensile property data for alloy CA313 which was HPDC, without application of a vacuum or use of a reactive gas and containing typical porosity levels, to produce both cylindrical and smaller, flat specimens in order to examine possible effects of specimen sizes arising from age hardening treatments. The cylindrical test specimens included for comparison are the same size and dimension as those shown in
These specific flat castings had dimensions of 70 mm long, and 3 mm thick with a head width of 14 mm, head length of 13 mm, parallel gauge length of 30 mm, and a gauge width of ˜5.65 mm. The castings were prepared by conventional HPDC at slow shot in-gate velocities of 26 m/s and fast in-gate shot velocities of 82 m/s. The terminology of “slow” and “fast” in Table VII is the same as in Table II. Solution treatment temperatures ranging from 490° C. down to 440° C. were examined for both the slow speed and high speed high pressure die-castings. Five or more specimens were tested in every condition and the total immersion time for solution treatment was 15 minutes. The surface quality was also noted, as this was found to differ slightly from the cylindrical tensile bars examined. However, the tensile results showed a good correlation between the different specimen dimensions. The results of Table VII are summarized in
Suprisingly, the CA313 alloy of the HPDC casting both hardens faster and to higher levels than the same alloy of the gravity casting. Both castings were given a total time of immersion in a furnace pre-heated to 490° C. of 15 minutes.
“B” in
“C” in
In an alternative to, or a combination of, the processes of “B” and “C” shown in
The process of the present invention is not limited to current composition ranges of aluminium HPDC alloys. Composition ranges for specifications on HPDC alloy vary from country to country but most alloys have equivalent or overlapping alloy compositions. The effect of alloy chemistry on tensile properties was examined using a range of 9 different alloys, some of which fall within current alloy specifications and some which are experimental compositions. Results shown in Tables VIII-XVI are presented for the as-cast condition, the as-solution treated condition (solution treated according to the present invention and immediately tested) the T4 temper (2 weeks natural ageing at 25° C.) and the T6 temper (24 h ageing at 150° C.). For all of Tables VIII to XVI, the shot in-gate velocity was kept constant at 82 m/s.
In addition, in Table VIII, the effect of a T8 temper is shown where the as-solution treated alloy was cold worked 2% by stretching prior to artificial ageing for the same duration as the T6 alloy. For Table VIII, all quenching from solution treatment was conducted into cold water with the exception as noted where the alloy was aged to a T6 temper following a quench from solution treatment into hot water at 65° C. The T8 temper shown in Table VIII reflects the possibility that a forming operation such as straightening may be required during fabrication of the alloy. The example provided for quenching into hot water and holding such as at 65° C. reflects a common industry practice in heat treatment of Al—Si based casting alloys.
In each instance changes to the tensile properties of the alloys are evident. Characteristically and quite surprisingly, the as-solution treated alloy in every condition shows effectively double or greater, the elongation of the as-cast alloy. In the T4 temper the elongation is characteristically higher than the as-cast condition, and the 0.2% proof stress and tensile strength of the alloys are improved. In the T6 temper, the elongation is typically only slightly lower than the as-cast condition, but the 0.2% proof stress and tensile strength are significantly improved.
Table XVII shows the reduction to practice of the invention as it was applied to a statistical number of high pressure die-castings produced industrially. The castings had the following characteristics:
Casting A: Alloy CA313: Complex part, thin walls, constant thickness and weight about 54 g
Casting B: Alloy CA313: Simple part, maximum thickness about 8 mm, minimum thickness about 2 mm, and weight about 49 g
Casting C: Alloy CA313: Complex part, thin and thick sections in same casting, maximum thickness about 7 mm, minimum thickness about 2 mm, and weight about 430 g
Casting D: Alloy CA605: Simple part, thick walled, constant thickness sections. maximum thickness about 15 mm, and weight about 550 g
Casting E: Alloy CA605: As D, but different part, and weight about 515 g
Casting F: Alloy CA605: Highly complex part, multiple thickness sections in same casting, minimum wall thickness 1.4 mm, maximum wall thickness about 15 mm.
It should be noted that alloy CA313 has a nominal specification of Al-(7.5-9.5)Si-(3-4)Cu-<3Zn-<1.3Fe-<0.5Mn-<0.5Ni-<0.35Pb-<0.3Mg-<0.25Sn-<0.2Ti-<0.1Cr<0.2 other elements; while alloy CA605 has a nominal specification of Al-(9-10)Si-(0.7-1.1)Fe-<0.6Cu-(0.45-0.6Mg)-<0.5Ni-<0.5Zn-<0.15Sn-<0.25 other elements.
As each of the castings A through F was produced at different times, the compositions are expected to vary within these given ranges.
Castings A through F were all produced under industrial conditions. All castings were x-rayed before heat treatment. The 75 castings of A were relatively free of large porosity as determined by x-ray inspection, although fine porosity was still observable upon close scrutiny at higher magnifications. However, all of the 500 castings B to F showed substantial quantities of fine and large porosity, of sizes up to 10 mm in dimension. An example of this porosity is shown in
Castings D and E were received in a condition wherein the surface of the castings had been shot blasted to remove a thin layer of material and create a rough finish.
For each part, a heat treatment schedule according to the routine determination of process window according to the current invention was determined, and all parts were heat treated to a T6 temper in air and then air cooled.
A visual rating was given to each part based on a quality inspection. This was based on the following criteria: a “perfect” rating was given to parts displaying equal to or better than the as-cast surface finish, no blistering and no dimensional instability.
An “acceptable” rating was given to parts displaying one small surface blister, of the order of 1 mm or less in size and typically requiring significant scrutiny to detect.
A “reject” rating was given to parts displaying one large blister, multiple small blisters, or a cluster of blisters.
Therefore, nearly 89% of all parts heat treated showed a perfect surface finish with no blisters or dimensional instability, 10% showed one minor blister that took close examination to detect, and 1.4% showed a large blister or blister cluster that led to it being classed as a reject.
The invention has the following main benefits over known conventional processes. Conventionally produced HPDC alloys are not known to be heat treatable, due to the occurrence of blistering. Without recourse to an applied high vacuum or the use of a reactive gas, age hardenable aluminium alloy castings produced from conventional HPDC can be suitably solution treated without blistering provided the time at temperature is kept within the appropriate process parameters described herein. The castings thus are able to be visibly sound for automotive and other consumer applications. The alloy of the castings can be precipitation hardened or strengthened producing substantially higher properties than the as-cast material. In many instances, the T4 temper produces improvements in the ductility. These benefits to mechanical properties are also summarised by
The invention also can be applied to age-hardenable aluminium alloys which have not previously been designated or regarded as casting alloys, as a means to develop superior mechanical and/or chemical and/or physical and/or processing attributes.
The invention also relates to alloys having additions of trace elements whose addition modifies the processing routes or precipitation processes, as a means to develop superior mechanical and/or chemical and/or physical attributes.
Each of
With reference to the data of
The results illustrated by
Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004907329 | Dec 2004 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU2005/001909 | 12/19/2005 | WO | 00 | 5/19/2008 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2006/066314 | 6/29/2006 | WO | A |
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| Number | Date | Country | |
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| 20090038720 A1 | Feb 2009 | US |
