Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/043—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with silicon as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent

Definitions

• the present invention relates to an improved foundry alloy and to a method of producing an improved foundry alloy.
• the improved foundry alloy is an aluminium-based alloy.
• Primary metal based foundry alloys are largely used for structural or safety type applications where there is a requirement for high and consistent mechanical properties.
• the majority of components made from aluminium foundry alloys are made from hypoeutectic aluminium- silicon-magnesium alloys containing a nominal silicon level of 1% by weight (601 and 603 designations) .
• these alloys are a composite of hard, discontinuous silicon particles and large, brittle iron intermetallics embedded in a ductile aluminium matrix.
• the main impurity found in these alloys is iron.
• the iron solidifies from the eutectic liquid into a number of brittle phases.
• the two major iron-containing phases found in these alloys are the ⁇ phase (Al ⁇ Si 6 Mg 3 Fe) which is the predominant phase formed in high Mg content alloys and the ⁇ phase (Al 5 SiFe) which forms in low magnesium content alloys.
• the ⁇ phase forms into a script morphology while the ⁇ phase is less voluminous and forms into acicular plates . Both phases are detrimental to mechanical properties.
• High Mg contents ie greater than 0.6 wt% Mg) are desirable to provide higher strength, but the presence of ⁇ phase at high Mg contents causes the ductility of the alloys to unfavourably decrease.
• the alloys do not achieve the maximum possible strength consistent with their magnesium content.
• the magnesium content of an alloy increases the magnesium content of the ⁇ phase may change leading to even greater volume fractions of the phase for a given Fe content.
• beryllium additions can be used to precipitate the iron impurity as part of the BeSiFe 2 Al 8 phase.
• This beryllium-containing phase forms in preference to the ⁇ phase, leading to alloys with improved mechanical properties.
• Unfortunately there are serious health hazards associated with using beryllium. Consequently, beryllium modification is not widely practised and the deleterious effect of the ⁇ phase on alloy quality remains.
• this object is achieved by an alloy having a microstructure in which ⁇ phase that forms during heat treatment as a transformation product of ⁇ phase is the sole or predominant iron-containing phase.
• the reduction in ⁇ phase results in an improvement in ductility.
• the ⁇ phase that forms as the transformation product has a fine structure that improves ductility.
• the reduction in ⁇ phase means that there are higher levels of Mg in solution which are available for precipitation during ageing to improve the strength of the alloy.
• the present invention provides an alloy which comprises:
• the other components comprise a total of not more than 0.15 wt% and any single component of the other components does not exceed 0.05 wt%, the alloy having a microstructure which includes a primary aluminium-containing matrix and one or more iron-containing phases dispersed in the matrix, and wherein the sole or predominant iron-containing phase is ⁇ phase that formed as a transformation product of ⁇ phase.
• the dendrite arm spacing of the matrix be 10- 5 ⁇ m.
• the iron-containing phases also include ⁇ phase .
• the iron-containing phases include ⁇ phase in an amount up to 30 vol° ⁇ of the iron-containing phases .
• the amount of ⁇ phase may be higher if the Mg content is in the upper end of the range.
• the Mg content of the alloy is preferably 0.40- 0.45 wt%. Within this Mg range, the alloy is a variant of the 601/603 type foundry alloy. It has been realised by the applicant that close control of the magnesium content to be between 0.40 and 0.45 wt% can lead to an increase in alloy quality and improved mechanical properties. In particular, when the magnesium content is controlled to be between 0.40 and 0.45 wt% the variation in alloy quality for a small change in magnesium level is minimal. Thus, the consistency in the mechanical properties of the alloy is maximised.
• the present invention also provides a method for manufacturing an alloy article.
• the present invention provides a method for manufacturing an alloy article which comprises :
• the cooling rate be sufficient to produce a dendrite arm spacing in the matrix in the casting of 10-45 ⁇ m.
• the sole or predominant iron- containing phase in the alloy article is ⁇ phase.
• the iron-containing phases also include ⁇ phase. More preferably, the iron- containing phases in the alloy article include ⁇ phase in an amount of up to 30 vol% of the iron-containing phases. Higher levels of ⁇ phase may be present if the Mg content is at the upper end of the above range.
• the step of solidifying the casting produces iron-containing phases that include a substantial proportion of the ⁇ phase and the subsequent solution heat treatment step is effective to convert at least some and preferably a majority of the ⁇ phase to ⁇ phase to give a microstructure in the alloy article that includes iron-containing phases which are predominantly ⁇ phase.
• the melt prior to casting may be at a temperature above the liquidus temperature of the alloy, with the melt having sufficient superheat to fill the mould, that is at a temperature of 680-720°C.
• the solution treatment of the casting may be carried out at any suitable temperature and for any suitable time to achieve a desired level of transformation of ⁇ phase ⁇ phase.
• the selection of the parameters of temperature and time will depend on variables, such as the concentrations of magnesium and other elements in the casting.
• concentrations of magnesium and other elements in the casting such as the concentrations of magnesium and other elements in the casting.
• solution treatment at 540°C for 2 or more hours produced desired levels of transformation of ⁇ to ⁇ phase
• the casting is preferably quenched, more preferably quenched in hot water, such as hot water having a temperature of 70-80°C.
• the alloy article After quenching, the alloy article is cooled to room temperature and optionally subjected to an ageing heat treatment .
• the ageing heat treatment may include heating the alloy article to a temperature of 140-170°C and holding at that temperature for 1-10 hours. After the ageing heat treatment, the alloy article may be air cooled to room temperature.
• Results to support the present invention are given in Figure 1, in which plots of typical response surfaces derived from experimentally determined quality index data are shown.
• the three surfaces correspond to alloys that were cast at different solidification rates and thereafter solution treated and aged.
• Solidification rate is commonly measured by the as-cast dendrite cell size or secondary dendrite arm spacing (DAS) but other methods exist .
• DAS secondary dendrite arm spacing
• the results here use secondary dendrite arm spacing to indicate solidification rate, with a small dendrite arm spacing corresponding to a high solidification rate.
• the magnesium level for the peak quality is independent of the iron level for the iron levels examined. Also, the rate of change of the response surfaces with magnesium is least near the peak in quality index. This means that the alloys at the peak are less sensitive to changes in magnesium than other alloys.
• the peak quality from Figure 1 corresponds well with microstructural evidence for small amounts of ⁇ phase in the alloy. By increasing the magnesium content of the alloy, it can be seen that in some circumstances improved quality results.
• Figures 2(a) to 2(c) are photomicrographs of hypoeutectic alloys having a Si concentration of 7 wt% and various Mg concentrations which were cast at the same solidification rate (60 ⁇ m DAS), solution treated, and aged.
• Figure 2(d) is a photomicrograph of the as-cast alloy of Figure 2(c), ie before heat treatment.
• the Mg content of the alloy is higher than the Mg content of the alloy of the present invention.
• the main phases shown in Figure 2(a) are spheroidal silicon-containing phase and the iron-containing ⁇ phase.
• Figure 2(b) shows the microstructure of an alloy containing less Mg than the alloy of the present invention.
• the phases present include spheroidal silicon-containing phase and iron-containing ⁇ phase.
• the ⁇ phase is present as structures of high aspect ratio dispersed throughout the matrix.
• Figure 2(c) shows the microstructure of an alloy of the present invention.
• the phases include spheroidal silicon-containing phases, a small amount of ⁇ phase and ⁇ phase.
• the ⁇ phase is present as structures of high aspect ratio clumped together. This is consistent with the ⁇ phase being formed by transformation of ⁇ phase during heat treatment .
• Figure 2(d) shows that prior to heat treatment the as-cast alloy of Figure 2(c) had regions of ⁇ phase. As is evident from Figure 2(c) these ⁇ phase regions were largely transformed to ⁇ phase during heat treatment.
• the drive for alloys with improved mechanical properties stems from the major restraint that mechanical properties place on the design of the casting, or even if a cast alloy can be used to manufacture a certain component.
• the thickness of critical sections needs to be sufficiently large that the cast component can operate without failure. Mechanical properties of the alloys therefore limit the minimum weight of a cast component.
• the thickness of sections of a casting will determine the time required for the casting to solidify. For certain casting methods, such as low pressure die casting, the production rate is often determined by the solidification rate as the casting machine is tied up until the casting has fully solidified.
• the solution treatment, quench rate and ageing treatment of a cast component may be tailored to its design so as not to induce unnecessarily high residual stresses. High residual stresses can cause distortion of the component requiring additional machining.
• the mechanical properties of the base alloy therefore affect all stages of manufacturing from design, to casting the component, heat treatment, machining, final weight and production rate.
• the present invention therefore has the following more specific applications:

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Continuous Casting (AREA)
• Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)
• Forging (AREA)

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• 1997
  • 1997-02-24 AU AUPO5268A patent/AUPO526897A0/en not_active Abandoned
• 1998
  • 1998-02-24 NZ NZ337431A patent/NZ337431A/xx unknown
  • 1998-02-24 JP JP53706898A patent/JP2001513145A/ja active Pending
  • 1998-02-24 KR KR10-1999-7007724A patent/KR100498002B1/ko not_active Expired - Fee Related
  • 1998-02-24 US US09/355,987 patent/US20020155023A1/en not_active Abandoned
  • 1998-02-24 WO PCT/AU1998/000115 patent/WO1998038347A1/en not_active Ceased

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