Patent application title: METAL INJECTION MOULDING METHOD
Zhenyun Liu (Sinagapore, SG)
Timothy Barry Sercombe (Western Australia, AU)
Graham Barry Schaffer (Queensland, AU)
THE UNIVERSITY OF QUEENSLAND
IPC8 Class: AB22F310FI
Class name: Powder pretreatment (prior to consolidation or sintering) addition of fugitive material additional material is solid
Publication date: 2010-07-22
Patent application number: 20100183471
Patent application title: METAL INJECTION MOULDING METHOD
Timothy Barry Sercombe
Graham Barry Schaffer
KNOBBE MARTENS OLSON & BEAR LLP
THE UNIVERSITY OF QUEENSLAND
Origin: IRVINE, CA US
IPC8 Class: AB22F310FI
Publication date: 07/22/2010
Patent application number: 20100183471
A method for forming an article by metal injection moudling of aluminium
or an aluminium alloy. The method comprises the steps of forming a
mixture containing an aluminium powder or an aluminum alloy powder or
both and optionally ceramic particles, a binder, and a sintering aid
comprising a low melting point metal. The mixture is injection moulded
and the binder is removed to form a green body. The green body is
sintered. The sintering is conducted in an atmosphere containing nitrogen
and in the presence of an oxygen getter.
1. A method for forming an article by metal injection moulding of
aluminium or an aluminium alloy, the method comprising the steps
of:forming a mixture containing an aluminium powder or an aluminium alloy
powder or both and optionally ceramic particles, a binder, and a
sintering aid comprising a low melting point metal;injection moulding the
mixture;removing the binder; andsintering; wherein sintering is conducted
in an atmosphere containing nitrogen and in the presence of an oxygen
2. A method as claimed in claim 1 wherein the oxygen-getter comprises a metal that has a higher affinity for oxygen than aluminium.
3. A method as claimed in claim 2 wherein the oxygen-getter is selected from the group consisting of the alkali metals, the alkaline earth metals and the rare earth metals.
4. A method as claimed in claim 3 wherein the oxygen-getter is magnesium.
5. A method as claimed in claim 1 wherein blocks of the oxygen-getter are positioned around the article that is being sintered during the sintering step or powder of the oxygen-getter is placed around or on the article that is being sintered during the sintering step or the oxygen-getter is mixed in with the aluminium or aluminium powder alloy, or mixed in with the mixture that is fed to the injection moulding apparatus or the oxygen getter is present as a component of an alloy added to the mixture.
6. A method as claimed in claim 1 wherein the sintering aid is a metal that has a melting point that is lower than the melting point of aluminium and is insoluble in solid aluminium.
7. A method as claimed in claim 6 wherein the sintering aid comprises tin.
8. A method as claimed in claim 1 wherein the sintering aid is present in an amount of up to 10% by weight, based upon the total weight of the metal powder and the sintering aid.
9. A method as claimed in claim 8 wherein the sintering aid is present in an amount of from 0.1% to 10% by weight.
10. A method as claimed in claim 8 wherein the sintering aid is present in an amount of from 0.5% to 3% by weight.
11. A method as claimed in claim 1 wherein the atmosphere in which the sintering step is conducted has a low water content wherein the water vapour partial pressure is less than 0.001 kPa.
12. A method as claimed in claim 1 wherein the binder includes thermoplastic components that enable the binder to melt upon application of heat.
13. A method as claimed in claim 1 wherein the binder is made from two or more materials selected such that they are sequentially removed from the green body.
14. A method as claimed in claim 1 wherein the binder is removed by dissolution in a solvent, by thermal treatment to cause the binder to melt, evaporate or decompose, by catalytic removal of the binder or by wicking.
15. A method as claimed in claim 14 wherein two or more binder removal techniques are used in to remove the binder.
16. A method as claimed in claim 1 wherein the binder comprises stearic acid, palm oil wax and high density polyethylene.
17. A method as claimed in claim 1 wherein the sintering step involves heating the green body to a temperature at which the aluminium or aluminium alloy sinters to form a dense body.
18. A method as claimed in claim 17 wherein the temperature is within the range of about 550.degree. C. to about 650.degree. C.
19. A method as claimed in claim 1 wherein the mixture includes ceramic particles and the ceramic particles are selected from the group consisting of SiC, Al2O3, AlN, SiO2, BN and TiB.sub.2.
20. A method as claimed in claim 1 wherein the atmosphere comprises nitrogen or a mixture of nitrogen and an inert gas.
21. A method as claimed in claim 1 wherein the atmosphere is essentially free of oxygen or hydrogen.
FIELD OF THE INVENTION
The present invention relates to a metal injection moulding method.
BACKGROUND TO THE INVENTION
Metal injection moulding involves the mixing of powder metal with a binder to form a feedstock. This mixture is then injection moulded using injection moulding equipment that is similar to that used in the plastics industry. This forms a "green body". The green body has sufficient rigidity and strength to enable handling. The green body is then further treated to remove the binder and to sinter the metal powder particles to form the final article.
The binder typically comprises one or more thermoplastic compounds, plasticisers and other organic material. Ideally, the binder is molten or liquid at the injection moulding temperature but solidifies in the mould when the green body is cooled. The feedstock may be converted into solid pellets, for example by granulation. These pellets may be stored and fed into the injection moulding machine at a later time.
Typical injection moulding equipment includes a heated screw or extruder having a nozzle through which the mixture is extruded into the die cavity. The extruder is heated to ensure that the binder is in liquid form and the nozzle temperature is typically carefully controlled to ensure constant conditions. Desirably, the temperature of the die is also controlled so that the temperature is low enough to ensure that the green body is rigid when it is removed from the die.
As the binder can occupy a substantial volume fraction of the green body, the green body will be larger than the final article.
Further processing of the green body involves removing the binder and sintering. The binder may be completely removed before sintering. Alternatively, the binder may be partly removed before the sintering step, with complete removal of the binder being achieved during the sintering step.
Removal of the binder may take place by using a solvent to dissolve the binder or by heating the green body to cause the binder to melt, decompose and/or evaporate. A combination of solvent removal and thermal removal may also be used.
The sintering step involves heating the body to cause the separate metal particles to metallurgically bond together. Sintering in the production of metal injection moulded parts is generally similar to sintering used in the production of traditional powder metal parts. Non-oxidising atmospheres are typically used during the sintering step in order to avoid oxidation of the metal. During sintering in metal injection moulding methods, the very porous body remaining after removal of the binder densifies and shrinks. The sintering temperature and temperature distribution will typically be closely controlled in order to retain the shape of the article during sintering and to prevent distortion of the article. In this fashion, net shape articles can be recovered from the sintering step:
Metal injection moulding is suitable for producing articles from almost any metal that can be produced in a suitable powder form. However, aluminium is difficult to use in metal injection moulding because the adherent aluminium oxide film that is always present on the surface of particles of aluminium or aluminium alloy inhibits sintering.
U.S. Pat. No. 6,761,852, assigned to Advanced Materials Technologies Pte Ltd, describes a metal injection moulding process for forming objects from aluminium and its alloys. In this process, a powder of aluminium or an aluminium alloy is mixed with a powder containing a material that is said to form a eutectic with aluminium oxide, such as silicon carbide or a metallic fluoride. This mixed powder is then mixed with a binder, injection moulded, subject to removal of the binder, and sintered.
In the process of U.S. Pat. No. 6,761,852, the silicon carbide or metal fluoride is said to form a eutectic mixture with aluminium oxide which supposedly dissolves the aluminium oxide, thereby allowing intimate contact between aluminium surfaces during the sintering step.
The applicant does not concede that the prior art discussed in this specification forms part of the common general knowledge in Australia or elsewhere.
Throughout this specification, the term "comprising" and its grammatical equivalents are taken to have an inclusive meaning unless the context indicates otherwise.
BRIEF DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide a metal injection moulding method that allows for the production of articles from aluminium, aluminium alloys or aluminium matrix composites.
In a first aspect, the present invention provides a method for forming an article by metal injection moulding of aluminium or an aluminium alloy, the method comprising the steps of: forming a mixture containing an aluminium powder or an aluminium alloy powder or both and optionally ceramic particles, a binder, and a sintering aid comprising a low melting point metal; injection moulding the mixture; removing the binder; and sintering; wherein sintering is conducted in an atmosphere containing nitrogen and in the presence of an oxygen getter.
The oxygen-getter may comprise any metal that has a higher affinity for oxygen than aluminium. Some examples of suitable metals for use as the oxygen-getter include the alkali metals, the alkaline earth metals and the rare earth metals. Where one or more rare earth metals are used as the oxygen-getter, it is preferred that rare earth metals from the lanthanide series are used.
Magnesium is the preferred metal for use as the oxygen-getter because it is has a high vapour pressure, it is readily available and it is relatively inexpensive.
In some embodiments, blocks of the oxygen-getter may be positioned around the article that is being sintered during the sintering step. In other embodiments, powder of the oxygen-getter may be placed around or on the article that is being sintered during the sintering step. As a further alternative, the oxygen-getter may be mixed in with the aluminium or aluminium powder alloy, or mixed in with the mixture that is fed to the injection moulding apparatus.
In a further embodiment, the oxygen getter is present as a component of an alloy added to the mixture, such as being present in an alloy powder added to the mixture. For example, powder of an alloy containing aluminium and magnesium (and possibly other components) may be added to the mixture or incorporated into the mixture. Examples of some alloys that can be incorporated into the mixture include Al-7.9 wt % Mg and Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si.
Without wishing to be bound by theory, the present inventors have postulated that the oxygen-getter removes any oxygen that may be present in the atmosphere surrounding the part during the sintering step. The oxygen-getter may also act to reduce the aluminium oxide that surrounds the aluminium or aluminium alloy particles. This assists in disrupting the aluminium oxide layer around the particles, exposing fresh metal, thereby allowing sintering of the aluminium or aluminium alloy particles to take place.
As mentioned above, magnesium is a suitable oxygen-getter. In addition to being relatively inexpensive, magnesium also has a high vapour pressure. Consequently, during the sintering step (which takes place at elevated temperature), magnesium vapour may surround the article that is being sintered.
A sintering aid is added to the mixture prior to injection moulding of the mixture. The sintering aid is a low melting point metal. For example, the sintering aid may be a metal that has a melting point that is lower than the melting point of aluminium. Preferably, the sintering aid comprises a low melting point metal that is insoluble in solid aluminium. Some examples of suitable sintering aids include tin, lead, indium, bismuth and antimony. It has been found that tin is especially suitable in assisting in sintering of aluminium and aluminium alloys. Therefore, tin is a preferred sintering aid.
Tin is a preferred sintering aid for use in the present invention because it has been found that tin suppresses the formation of aluminium nitride during sintering (thereby avoiding formation of excessive aluminium nitride, which might have a detrimental effect on the properties of the final article) and also changes the surface tension of molten aluminium, thereby promoting good distribution of liquid aluminium phase during sintering.
The sintering aid may be added in an amount of up to 10% by weight, based upon the total weight of the metal powder and the sintering aid. Preferably, the sintering aid is present in an amount of from 0.1% to 10% by weight, more preferably 0.5% to 3% by weight, even more preferably about 2% by weight.
Where tin is used as the sintering aid, it may be added in an amount of from 0.1% to 10%, more suitably from 0.5% to 4%, even more suitably from 0.5% to 2.0% by weight of the mixture.
Tin melts at 232° C., which is considerably lower than that of aluminium (660° C.) and there are no intermetallic phrases. Tin is sparingly soluble in solid aluminium: the maximum solid solubility is less than 0.15%. Aluminium is completely miscible in liquid tin and no immiscible liquids form. Further, the surface tension of liquid tin is significantly less than that of aluminium and trace amounts of tin have been shown by the present inventors to improve the wetting characteristics and sintering behaviour of aluminium. For these reasons, tin is an especially preferred sintering aid.
The sintering step is conducted in a nitrogen atmosphere. Without wishing to be bound by theory, the present inventors have postulated that conducting the sintering step in a nitrogen atmosphere may promote the formation of aluminium nitride. The present inventors have postulated that forming aluminium nitride in the sintering step may assist in disrupting or breaking down the aluminium oxide film that normally surrounds the particles of aluminium or aluminium alloy. The use of tin as a sintering aid may also assist in controlling the formation of AlN as formation of excessive amounts of AlN during sintering may cause detriment to the properties of the final article.
If high purity aluminium is being used as a feed powder, the present inventors have found that conducting sintering of aluminium powder in a nitrogen atmosphere can result in the rapid conversion of the aluminium to aluminium nitride. Due to the rapid rate at which the aluminium can be converted to aluminium nitride in these circumstances, there is a risk that the entire article may be converted to aluminium nitride. Using tin as a sintering aid acts to limit the formation of excess AlN in such circumstances.
Without wishing to be bound by theory, the present inventors have postulated that the nitrogen atmosphere disrupts the aluminium oxide film on the surface of the aluminium or aluminium alloy particles by forming aluminium nitride. It is further postulated that this disruption of the aluminium oxide film enables sintering of the aluminium or aluminium alloy particles to occur.
The atmosphere in which the sintering step is conducted may have a low water content, for example, it may have a water vapour partial pressure of less than 0.001 kPa. The atmosphere used on the sintering step may have a dew point of less than -60° C., more preferably, less than -70° C. Magnesium, when used as an oxygen getter, reacts with oxygen and water, thereby further lowering the water content of the atmosphere. It is believed that water vapour is extremely detrimental to the sintering of aluminium.
The atmosphere is an atmosphere containing nitrogen. The atmosphere may be predominantly nitrogen. The atmosphere may be 100% nitrogen. The atmosphere may also include an inert gas. The inert gas may comprise a minor part of the atmosphere. The atmosphere may be essentially free of oxygen and hydrogen. In this regard, the gas that is supplied as the atmosphere during sintering suitably contains no oxygen or hydrogen.
The binder used in the present invention may be any binder or binder composition known to be suitable for use as a binder in metal injection moulding. As will be known to persons skilled in the art, the binder is typically an organic component or a mixture of two or more organic components.
The binder desirably includes thermoplastic components that enable the binder to melt upon application of heat. The binder should also impart sufficient strength to the green body following injection moulding to enable the green body to be handled. Desirably, the binder is able to be removed from the green body in a manner that retains integrity of the body during the binder removal process. It is preferable that the binder leaves no residue following removal.
The binder may be made from two or more materials. The two or more materials that comprise the binder may be selected such that they may be sequentially removed from the green body. In this fashion, a controlled removal of the binder is more easily achieved, thereby facilitating retention of shape integrity of the body during binder removal. In this regard, it will be appreciated that if the binder is removed too rapidly, the risk of the body losing its shape integrity is increased.
The binder may be removed by one or more of the known techniques used in metal injection moulding for removing the binder. For example, the binder may be removed by dissolution in a solvent, by thermal treatment to cause the binder to melt, evaporate or decompose, by catalytic removal of the binder or by wicking.
Two or more binder removal techniques may be used in the binder removal stage. For example, a first step in the binder removal may involve solvent extraction, followed by thermal removal of the remainder of the binder.
The person skilled in the art will appreciate that a large range of binder materials may be used. Some examples include organic polymers such as stearic acid, waxes, paraffin and polyethylene.
Without wishing to be limited in any way, the present inventors have used a binder comprising stearic acid, palm oil wax and high density polyethylene in experimental work relating to the present invention.
The sintering step used in the present invention involves heating the green body to a temperature at which the aluminium or aluminium alloy sinters to form a dense body. The sintering step suitably involves heating to a temperature within the range of about 550° C. to about 650° C., more suitably 590° C. to 640° C., most suitably between 610° C. to 630° C. The sintering time may vary. Typically, a shorter sintering time may be used for a higher sintering temperature. Essentially, the sintering time should be long enough to ensure that maximum densification of the article has occurred. Sintering at temperatures of from 620° C. to 630° C. for up to two hours has been found to provide satisfactory results. However, both longer sintering times and shorter sintering times are encompassed by the present invention.
The heating rates and thermal profile used in the sintering step are normally closely controlled in metal injection moulding methods to obtain optimum properties in the final article. The person skilled in the art will readily understand how to determine suitable heating rates and temperature profiles for use in the sintering step.
The method of the present invention is suitable for use with aluminium metal and aluminium alloys. Any aluminium alloy can be used in the present invention, including aluminium alloys from the 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, 7000 series and 8000 series.
Ceramic particles can be mixed with the aluminium or aluminium alloy powders to create an aluminium metal matrix composite. The ceramic particles are used to improve or control the properties of the sintered article. Such properties can include, but are not limited to, wear resistance, stiffness or coefficient of thermal expansion. A non-exhaustive list of typical ceramic materials include SiC, Al2O3, AlN, SiO2, BN and TiB2.
The method of the present invention may be carried out in known metal injection moulding apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a photomicrograph of the fracture surface of a test bar made in accordance with an embodiment of the present invention following debinding;
FIG. 2 shows photographs of a green body and a sintered body for test bars made in accordance with an embodiment of the present invention;
FIG. 3 is a graph of density and hardness of test pieces made in accordance with embodiments of the present invention;
FIG. 4 shows graphs of tensile curves of test bars after sintering at various conditions;
FIG. 5 shows microstructures of sintered products made in accordance with an embodiment of the present invention;
FIG. 6 shows a graph of the effect of elemental Mg additions on sintered density;
FIG. 7 shows the liquid content as a function of temperature for the alloys listed in FIG. 7;
FIG. 8 shows a graph of the sintered density of AA6061+X % Sn loose powders as a function of temperature; and
FIG. 9 shows a graph of sintered density as a function of temperature for the feedstock mixtures listed in that Figure.
A variety of alloy and powder compositions, particle sizes and particle shapes were tested. Spherical AA6061 powders with a D50 of 10 μm and spherical Sn with a particle size <45 82 m were preferred. The metal injection moulding feedstock consisted of 6061 powder with 2 wt % Sn and a binder system of 3 wt % stearic acid, 52wt % palm oil wax and 45 wt % high density polyethylene. The raw materials were mixed at 165° C. for 180 minutes. After granulating, the feedstock was injection moulded into standard tensile bars using an Arburg moulding machine. Solvent debinding was conducted in hexane at 40° C. for 24 hours. The removal of the remaining binder and sintering were combined and conducted in a sealed tube furnace. The preferred atmosphere is high purity nitrogen gas flowing at 1 litre/min. The thermal profile used in the experimental work is shown in Table 1. Magnesium bars were placed around the article during sintering.
Tensile testing was conducted on as-sintered material. The extensometer gauge length was 25 mm and the crosshead speed was 0.6 mm/min.
Rockwell hardness (HRH) was measured on both top and bottom surfaces with a 1/8 inch steel ball and 60 kg load.
TABLE-US-00001 TABLE 1 Heating profile for debinding and sintering. Step 1 2 3 4 5 6 7 Rate 3 0.5 0.5 0.5 0.8 0.5 10 (° C./min) Temp 150 250 375 450 620 550 25 (° C.) Hold 0 120 120 60 120 0 End Time (minutes)
FIG. 1 shows a fracture surface of a debound part. The powder morphology has not changed from the original.
FIG. 2 shows the injection moulded (green) body and the sintered part. The sintered part is free of defects such as blisters, cracks and warpage. It also has a good surface finish.
FIG. 3 shows the density and hardness of the test bars under various sintering conditions. For parts sintered for 1 hour at 620° C. in nitrogen, the sintered density was 90.0±0.6% and the hardness was 39.1±12.3. The big variation of hardness is possibly due to the high porosity level. When the sintering time was increased to 2 hours, the density and hardness increased to 94.9±0.3% and 66.9±2.9, respectively. However, further increasing the sintering temperature to 630° C. did not significantly increase the density and hardness. The density in this condition was 95.3±0.3% and the hardness was 69.0±0.9.
Typical stress/strain curves of parts sintered at various conditions are plotted in FIG. 4. The part sintered at 620° C. for 2 hours has the best mechanical properties and recorded a 0.2% yield strength of 58 MPa, a tensile strength of 156 MPa and elongation to failure of 8.9%. The tensile properties of the parts sintered at 630° C. were slightly lower than this, despite the higher density. This was possibly due to microstructural coarsening at the higher sintering temperature.
For the parts sintered at 620° C. for 1 hour, the low density produced poor mechanical properties. The tensile strength was 98 MPa and strain was 1.7%.
FIG. 5 shows the microstructure of a sample after sintering at 620° C. for 2 hours. The optical micrograph shows that the grain size remains at about the original particle size and is less than 20 μm. The backscattered electron image shows the distribution and size of the Sn rich phase (white contrast in electron image, black contrast in optical image). No obvious pores were visible.
Various percentages of -325 mesh elemental Mg powders or Mg rich pre-alloy powders were formulated and mixed into the feedstock. The feedstock was then compacted into 25.4 mm diameter discs using a hot moulding machine. The discs were sintered in nitrogen without Mg blocks being present in the furnace. Before sintering disks with pre-alloyed powders in them, the furnace was run empty at 680° C. for four hours in vacuum to remove any Mg residue in the furnace. The parts were loaded into a steel crucible with a loose lid to minimise the effect of gas flow.
The effect of elemental Mg additions on sintered density is shown in FIG. 6. It was found that 1.0 wt % Mg gave the highest sintered density of ˜94%. With 0.5 wt % Mg, there is insufficient gettering of oxygen and the part was distorted due to a porous surface layer. The addition of 2.0 wt % elemental Mg powder into the feedstock results in low sintered density (˜80%) due to nitriding. Due to safety concerns, adding elemental Mg powder into the feedstock is not preferred. However, adding Mg into the feedstock in the form of pre-alloy to powders will overcome some of the disadvantages of elemental power.
Examples--Adding AIMg Powder to Feedstock
Pre-alloyed powders of composition Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si and Al-7.9 wt % Mg were obtained from the Aluminium Powder Company. The Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powders have an average particle size of about 25 μm whilst the Al-7.9 wt % Mg powders have an average particle size of about 40 μm. Both have a regular particle shape. The Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si has a solidus temperature around 540° C. and it is fully liquefied at 600° C. The Al-7.9 wt % Mg has a solidus temperature around 540° C. and it is fully liquefied at 620° C. FIG. 7 illustrates the liquid content as a function of temperature for these alloys, as well as alloy AA6061 and for a mixture of AA6061+7.5 wt % Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si.
It has been found that sintering a mixture of AA6061+7.5% Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si+2 wt % Sn feedstock at 610° C. for two hours in nitrogen produces distortion free parts which have a density of ˜97% of theoretical.
Examples--Use of Tin as Sintering Aid
Sn has been used as an effective sintering additive for the pressed or un-compacted aluminium alloys and compacts prepared by rapid prototyping processes. The present inventors have demonstrated that Sn plays an important role in the sintering of tapped loose powders and powder injection moulded aluminium compacts. However, Sn will remain at the grain boundaries after sintering since tin is almost insoluble in solid aluminium. Excess amounts of tin will deteriorate the mechanical properties, especially the ductility, which is extremely desirable for aluminium alloys prepared from powders.
The debound part (brown part) of powder injection moulded aluminium compact only has a relative density about ˜85%. After the polymer binders are removed, there are open channels connecting to the part surface in the porous debound part. Tapped loose powders only have a relative density about 40-60%, connected pores may form open channels to the surface. Significant amount of liquid is needed to seal these channels. In previous examples, we found that 4% Sn helps the sintering of loose compacted pure aluminium powders; addition of 2% Sn enhanced the sintering of powder injection moulded AA6061 compact. In the present examples, we try to minimize the amount of Sn additions while maintaining the liquid volume by adding some pre-alloyed aluminium powders. The addition of the heavily pre-alloyed powders will also help increase alloy content in the sintered part and improve its strength. The decrease of Sn content may help to improve ductility. By such means, the mechanical properties of the alloy system could be further improved.
Elemental Sn (<43 μm) is used as a sintering aid to enhance the liquid phase sintering of fine AA6061 powders (<20 μm) mixed with pre-alloyed Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powder (<30 μm). The powders were mixed in a Turbula mixer for 30 minutes according to the formulation of AA6061+Xwt % Sn+Ywt % Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si. The mixed powders were poured into alumina crucibles, tapped and enclosed by aluminium foils. Then they were sintered in a steel tube furnace for 2 hours at various temperatures under nitrogen gas flow of 0.5 litre/min. The sintered density was obtained by Archimedes' method and was converted into percentage of the theoretical density (TD %) of each alloy. Polished samples were used for both optical and scanning electron microscopy (SEM).
FIG. 8 shows that the sintered density of AA6061+Xwt % Sn loose powders increased with higher sintering temperature. There is a density increase for 2 wt % Sn alloy system at 580° C. and 1 wt % Sn system at 590° C. The addition of Sn obviously enhanced the sintering and a much higher sintered density was obtained for alloys with Sn. The sintered density was ˜95% or higher for the alloys with 1.0 or 2.0 wt % Sn in the sintering temperature window of 600-630° C. On the contrary, AA6061 loose powders without Sn only achieved 83%, 88% and 93% at 610° C., 620° C. and 630° C., respectively.
For liquid phase sintering, the liquid volume is one of the most critical factors for the densification and part shape retention. The liquid volume in the Al--Sn alloy system is controlled by temperature, aluminium alloy composition and the Sn content. FIG. 7 shows the effect of temperature on the liquid volume fraction for the tested alloys. This data was calculated using ThermoCalc. No Sn addition is considered. For AA6061+xwt % Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si alloys, the calculations were based on the final total alloy content. The pre-alloyed Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powder has a solidus of 582° C. and it is fully liquefied at 604° C. So, this alloy will be very difficult in process control with such a narrow melting window if it is sintered alone. However, this early formed liquid with high Mg content may scavenge oxygen in the sintering furnace and help to seal the open channels in the loose powders before serious oxidation occurs, which usually starts at about 580-600° C.
FIG. 9 shows the sintered density of AA6061+0.5 wt % Sn loose powders with addition of 0%, 2.5% and 7.5% pre-alloyed Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powders after sintering at various temperatures for 2 hours in nitrogen. The sintered density of AA6061+0.5 wt % Sn increases steadily as function of temperature till 630° C. as liquid volume increase. The liquid from the melting of Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powders sharply increased the density at sintering temperature of 600° C. for 2.5 wt % addition and 590° C. for 7.5 wt % addition. However, excess liquid soon resulted density decrease at 620° C. after it peaked at 610° C. for the AA6061+0.5 wt % Sn+7.5 wt % Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si alloy system. The density decrease is possibly due to early formed liquid entrapped gas inside the part. The 2.5 wt % addition of pre-alloyed Al-2 wt % Cu-9.3 wt % Mg-5.4 wt % Si powders helped to maintain a density plateau of ˜97% in temperature range of 600620° C. The density started to decrease at 630° C.
Those skilled in the art will appreciate that the present invention may be subject to variations and modifications other than those specifically described. It will be understood that the present invention encompasses all variations and modifications that fall within its spirit and scope.
Patent applications by THE UNIVERSITY OF QUEENSLAND