WO2019101316A1

WO2019101316A1 - Al-si-mg-zr-sr alloy with particle-free grain refinement and improved heat conductivity

Info

Publication number: WO2019101316A1
Application number: PCT/EP2017/080213
Authority: WO
Inventors: Takeshi Saito; Petter Åsholt
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 2017-11-23
Filing date: 2017-11-23
Publication date: 2019-05-31
Anticipated expiration: 2020-05-23

Abstract

Method for producing an alloy for casting comprising: providing aluminium (Al), providing silicon (Si), providing zirconium (Zr), optionally providing magnesium (Mg), optionally providing copper (Cu), optionally providing strontium (Sr), heating the provided Al, Si, Zr, Cu, Mg and Sr to obtain a melt, wherein the melt is free of solute Ti and B, free of titanium-comprising particles, such as TiB2, Al3Ti, TiC and is free of other boron-comprising particles, such as AlB2, ZrB2, cooling the melt while Zr causes a refined grain structure such as to obtain the alloy having an intended composition comprising Si: 5 to less than 8.5 wt-%, Zr: 0.10 to 0.25 wt-%, Mg: 0.05 to less than 0.4 wt-%, Cu: 0 to 3 wt-%, Sr: 0.01 to less than 0.035 wt-% and balance Al.

Description

Al-Si-Mq-Zr-Sr alloy with particle-free grain refinement and improved heat

conductivity

Background

The present invention generally relates to a casting alloy based on aluminium, silicon, magnesium, strontium and zirconium with particle-free grain refinement, and having excellent mechanical properties and good electrical and heat conductivity. In particular, the present invention relates to a method for producing an alloy for casting comprising 5 or more wt-% silicon (Si) and less than 8.5 wt-% Si and further comprising magnesium (Mg), strontium (Sr) and zirconium (Zr), an alloy comprising more than 5 wt-% Si and less than 8.5 wt-% Si and further comprising Mg, Sr and Zr, and the use of zirconium (Zr) for improving the electrical or heat conductivity of an aluminium (Al) alloy in the T6 temper condition comprising more than 5 wt-% Si and less than 8.5 wt-% Si.

It is generally desirable in casting metals to achieve as small a grain size as possible because small grain size can improve the castability and the mechanical properties of metal castings. For example, a significant grain refinement can be achieved by the addition of titanium (Ti) and/or boron (B) to aluminium alloys.

In aluminium alloys, for example, titanium (Ti) and boron (B) are added in the form of Al-Ti- B master alloys containing T1B2, in the form of Al-Ti master alloys containing AbTi, in the form of Al-B master alloys containing AIB2 or in the form of TiC, which become nucleation sites of grains. It is thought that grain refinement is achieved by nucleation on T1B2 and the TiAb compounds that form by a peritectic reaction and subsequent growth of grains based on the nucleation sites.

It is desirable to have an alternative casting alloy that does not use Ti and/or B and has excellent mechanical and other properties and an alternative process for producing such an alloy with a refined grain structure. It is also desirable to have a casting alloy with improved heat and electrical conductivities.

It is further desirable to have a process for grain refining that yields even smaller grain sizes than obtainable using Ti and/or B for grain refining.

Short Description of the Invention

Throughout this document and in the figures, contents and concentrations are given in weight percent (wt-%) based on the total weight. The balance, or in other words the substance that is required to reach 100 wt-%, is aluminium unless it is described otherwise. When this document refers to an embodiment, it is referred to an embodiment of the invention.

The present inventors have found that the Al-Si-Mg-Sr foundry alloy comprising Zr according to the invention exhibits better grain refining and has a better production efficiency, e.g. compared to alloys comprising Ti and/or B, because the Ti and/or B comprising compounds are insoluble and may settle in the casting furnace resulting in an inhomogeneous particle distribution and an inferior refining effect. On the other hand, alloys according to the invention that do not contain insoluble particles, because Zr contents according to the invention are dissolved in the melt at casting temperatures between 650 - 750°C. The inventors have further found that using Zr in an Al-Si-Mg-Sr alloy according to the invention can significantly increase the electrical conductivity and -as described by the Wiedemann-Franz law- also the thermal conductivity compared to conventional alloys.

In light of this, the present invention provides according to an aspect a method for producing an alloy for casting comprising: providing aluminium (Al), providing silicon (Si), providing zirconium (Zr), providing magnesium (Mg), providing strontium (Sr), optionally providing copper (Cu),, heating the provided Al, Si, Zr, Cu, Mg and Sr to obtain a melt, wherein the melt is at least substantially free (e.g. free) of solute Ti and B, at least substantially free (e.g. free) of titanium-comprising particles, such as T1B2, AtaTi, TiC, and is at least substantially free (e.g. free) of (other) boron-comprising particles, such as AIB2, ZrB2, cooling the melt while Zr causes a refined grain structure such as to obtain the alloy having an intended composition comprising Si: 5 to less than 8.5 wt-%, Zr: 0.10 to 0.25 wt- %, optionally 0.10 to 0.20 wt-% Zr, Mg: 0.05 to less than 0.4 wt-%, optionally Mg: 0.05 to less than 0.3 wt-%, Cu: 0 to 3 wt-%, Sr: 0.01 to less than 0.035 wt-%, optionally Sr: 0.01 to 0.03 wt-%, and balance Al. The such produced alloy may have a hypoeutectic Al-Si microstructure. Accordingly, the alloy may -in addition to optional other phases-, e.g.

comprise a-aluminium and an Al-Si eutectic microstructure. The Al, Si, Zr, Cu, Mg and Sr may be provided in elemental form or in the form of pre-alloys or master alloys, such as an Al-Si alloy, or as a mixture of elements in elemental form and pre-alloys.

The method may further comprise casting the alloy according to the invention into a casted product using low pressure casting or gravitational casting.

The amounts of Al, Si, Zr, Mg, Sr and Cu (if the element is present) may be provided in such amounts taking into account losses, e.g. due to evaporation, that the cooled melt has the intended composition. These losses may e.g. be calculated or may be measured in a calibration test of the production equipment. If there are no losses, the amounts of Al, Si, Zr, Mg, Sr and/or Cu that are provided may be the same as in the alloy that is obtained by cooling the melt.

It is thought that the zirconium (Zr) causes the formation of an intermetallic compounds phase including Al, Si and/or Zr (for example AlSiZr), and optionally different impurity elements that act as nucleation sites for a-aluminium grains. Accordingly, a grain refinement effect is achieved.

The step of heating the provided elements may comprise adding at least the Zr in solid/unmolten form to a liquid melt comprising or consisting of molten aluminium. The added Zr may be in the form of elemental Zr and/or Zr-containing compounds or master alloys. The inventors have found that this results in a more homogeneous distribution of Zr and accordingly more homogeneous formation of AlSiZr intermetallic compounds that is particularly beneficial for grain refining and allows a production with higher efficiency as the alloy e.g. has the same properties throughout the whole casting batch. The Si, Sr, Mg and/or Cu may also be added to the liquid molten Al. However, the elements (all, several or two elements) may also be heated together simultaneously according to a conventional process.

The invention according to a further aspect provides an alloy comprising Si: 5 to less than 8.5 wt-%, Zr: 0.10 to 0.25 wt-%, optionally Zr: 0.10 to 0.20 wt-%, Mg: 0.05 to less than 0.4 wt-%, Cu: 0 to 3 wt-%, Sr: 0.01 to less than 0.035 wt-%, and balance Al, and further comprising incidental impurities (e.g. up to 0.05 wt-% each excluding iron; e.g. up to 0.20 wt-% in total excluding iron; e.g. up to 0.5 wt-% iron), wherein the alloy comprises equal to or less than 0.02 wt-%, optionally equal to or less than 0.01 wt-%, titanium (Ti) and equal to or less than 0.02 wt-%, optionally equal to or less than 0.01 wt-%, boron (B). The alloy may comprise a hypoeutectic Al-Si microstructure. It is thought that no adequate grain refining effect can be achieved with a Zr content lower than 0.10 wt-%. On the other hand, adding more than 0.25 wt-% Zr would result in an inefficient alloy with inferior properties such as a lower ductility.

According to embodiments, the alloy may comprise a purposeful addition of copper of equal to or less than 2 wt-% or equal to or less than 1 wt-%. Purposeful addition may mean that there is an intended addition of the element, e.g. copper, to the alloy. This may for example mean equal to or more than 0.2 wt-% Cu or equal to or more than 0.5 wt-% Cu. However, the alloy according to the invention may comprise a maximum of 3 wt-% Cu, as higher Cu contents may have a detrimental effect on grain refining. According to embodiments, the alloy may comprise equal to or more than 0.15 wt-% Zr and/or equal to or less than 0.20 wt-% Zr.

According to embodiments, the alloy may comprise less than 8 wt-% Si, optionally less than 7 wt-% Si.

According to embodiments, the alloy may optionally comprise a phase including Al, Si and/or Zr (for example AlSiZr), and optionally different impurities in the microstructure.

According to embodiments, the alloy comprises no purposeful/intended addition of Ti and/or B.

According to embodiments, the alloy may comprise less than 0.35 wt-% Mg or less than 0.30 wt-% Mg.

According to embodiments, the alloy may comprise equal to or more than 0.10 wt-% Zr and equal to or less than 0.20 wt-% Zr.

According to embodiments, the alloy may comprise less than 0.035 wt-% Sr, e.g. a maximum of 0.0325 wt-% Sr or 0.03 wt-% Sr.

The invention according to a further aspect comprises the use of zirconium for improving the electrical or heat conductivity in an Al-Si alloy comprising 5 wt-% or more Si and less than 8.5 wt-% Si that is free of purposeful additions of titanium and/or boron in the T6 temper condition.

According to the invention, zirconium may be used according to the method as described above. The Al-Si alloy may be any alloy according to the invention described herein.

According to the invention, between 0.10 and 0.25 wt-% zirconium may be used.

The alloys, uses and production methods according to the invention may comprise and/or involve impurities. Depending on the production process and the required specification, impurities may be present up to 0.05 wt-% for each element and 0.20 wt-% in total.

Iron (Fe), which is frequently encountered as an impurity in alloys according to the invention, as it may be included in the raw materials and/or may be released during the electrolysis process during aluminium production and/or may be released from the casting equipment into the melt, may in addition to the other impurities mentioned above be included up to 0.5 wt-% Fe.

When for example materials of higher purity and/or production equipment that allows higher purities (e.g. laboratory equipment instead of production equipment) is used, the impurities may be present e.g. up to up to 0.03 wt-% for each element and up to 0.15 wt-% in total. Iron may for example be included up to 0.15 wt-% Fe in addition to the other impurities when the content of iron is more specifically controlled to be within a narrower limit depending on the intended use.

However, it is an aspect of the present invention that impurities may be present up to the specified levels without significantly changing the grain refining properties of the alloys, as this enables an efficient alloy production process that does not require materials of high purity or complicated and inefficient production equipment.

It is noted that titanium and boron may each only be included up to 0.02 wt-%, optionally up to 0.01 wt-%, as impurities in the alloys and production methods according to the invention. According to the invention, the alloys and methods according to the invention comprise no purposeful additions of Ti and/or B. That is, as titanium and boron serve as grain refiners that use a different mechanism, it may result in more efficient alloys according to the invention when the titanium and boron contents are restricted to lower contents (for example up to 0.02 wt-% for each of Ti and B, or up to 0.01 wt-% for each of Ti and B when the contents are more specifically controlled) than the ones allowed for the other impurities.

Short description of the Figures

The scale bar in the micrographs shown in Figs. 1 to 4 and 6 and 7 corresponds to a length of 2500 pm (2.5 mm) except for the two bottom views in Fig. 6, in which the scale bar corresponds to 500 pm (0.5mm).

Fig. 1 shows the microstructure and average grain size of Al 5 wt-% Si alloys having different Zr contents according to embodiments of the invention.

Fig. 2 shows the microstructure and average grain size of Al 7 wt-% Si alloys having different Zr contents according to embodiments of the invention.

Fig. 3 shows the microstructure and average grain size of Al 9 wt-% Si alloys having different Zr contents.

Fig. 4 shows the microstructure and average grain size of Al 1 1 wt-% Si alloys having different Zr contents.

Fig. 5 shows a chart that shows the average grain size of alloys according to the invention and according to comparative examples depending on the Si and Zr content of the alloys. Fig. 6 shows the microstructure and average grain size of Al 7 wt-% Si alloys having different Ti contents according to a comparative example.

Fig.7 shows the microstructure and average grain size of Al 7 wt-% Si alloys having different Cu and Zr contents according to embodiments of the invention.

Fig. 8 shows mechanical properties of Al 7 wt-% Si alloys having 0.20 wt-% Zr according to the invention, of comparative Al 7 wt-% Si alloys without grain refiner (0% Zr), and of comparative Al 7 wt-% Si alloys using 0.06 wt-% and 0.10 wt-% Ti, respectively, for grain refining. As further explained below, SHT refers to a solution heat treatment of 8h at 540°C and AA refers to artificial ageing for 7h at 170°C, and Rp0.2 refers to yield strength, Rm refers to ultimate tensile strength and A refers to total elongation.

Detailed Description

The invention achieves a grain refinement effect by the addition of Zr, even without additions of Ti and/or B or any other grain refiner. That is, alloys according to the invention may comprise e.g. equal to or less than 0.01 wt-% Ti and/or equal to or less than 0.01 wt- % B and still exhibit a very well refined grain structure. The invention also achieves an improvement of electrical and thermal conductivities of the alloy, in particular when the alloy is in a T6 temper condition.

As will be shown, the alloys according to the invention show a better grain refinement effect than alloys that use a Ti and/or B based grain refiner.

According to the invention, the alloys may be produced particle-free so that there is no particle settlement of unmolten particles, such as T1B2 particles, in the furnace during casting. Accordingly, the alloys according to the invention may be produced in an efficient and clean process.

The alloys according to the invention show better mechanical properties and better electrical and thermal conductivity than Al-Si-Mg-Sr alloys comprising e.g. Ti and B as grain refiner.

It is thought by the inventors based on metallurgical considerations that better electrical and thermal conductivity are achieved because Zr containing dispersoids form in the T6 temper condition, resulting in less Zr in a-aluminium (that is, in solid solution) in the T6 temper condition. It is also thought by the inventors that the alloy according to the invention has better heat and electrical conductivity because Zr contributes less to a decrease in heat and electrical conductivity than Ti in a-aluminium (that is, in solid solution).

Due to the formation of dispersoids in the alloy during solution heat treatment, it is thought that the alloys according to the invention show better creep properties and better fatigue properties at high temperatures than conventional alloys using Ti and/or B based grain refiners. It is thought that this is because Zr that is dissolved in a-aluminium (solid solution) after casting leads to formation of Zr dispersoids during optional subsequent heat exposure, wherein the so formed Zr dispersoids are stable at high temperatures and hinder movement of dislocations.

It is also thought based on metallurgical considerations that the alloys according to the invention show better hot cracking resistance. It is thought that this is because in general hot cracks nucleate and grow along grain boundaries during solidification. The alloys according to the invention can prevent initiation of such cracks due to the better grain refinement effect resulting in smaller grains.

It is also thought based on metallurgical considerations that the alloys according to the invention show better feedability during casting because the alloys achieve more feeding channels during dendritic growth and eutectic reaction due to the better grain refinement effect resulting in smaller grains.

It has been found that the alloys according to the invention exhibit a similar yield strength in the as-cast condition compared to conventional AlSi alloys having Ti and/or B based grain refiner.

The alloys according to the invention may be generally used for all kinds of aluminium foundry products and may be used in all casting methods, such as low-pressure gravitational casting, sand casting etc.

The better thermal (and electrical) conductivity and better creep properties compared to conventional Ti/B-grain refined alloys make the alloys according to the invention in particular efficient for use in combustion engines, e.g. cylinder heads of combustion engines. The alloys according to the invention may also be used in electrical components.

The better grain refinement properties of the alloys according to the invention make the alloy also in particular efficient for use in vehicles, e.g. for automotive wheels (in particular rims), e.g. for structural components, etc.

Further, the particle free casting process without any settlement or enrichment of unmolten particles in the casting furnace enables a more efficient and clean casting process. Experiments

Various alloy variants having different Zr additions, different Si additions and different Cu additions were cast in a casting mold. The mold is designed to give samples

representative for gravity- or low pressure permanent mold automotive castings such as aluminium wheels, cylinder heads, chassis components etc.

The grain sizes were measured in as-cast condition using the average grain intercept (AGI) method. According to the AG I method, several lines are drawn across a section of the sample and the respective number of grains that are intersected by such a line is counted. By dividing the total sum of lengths of the lines by the total number of grains intersected by the lines, the average grain size of the sample is calculated. In the present case, 100 grains were counted on four rectilinear lines having an arbitrary position and orientation (25 grains per line), and the grain size was calculated by dividing the corresponding sum of the length of each of the lines by 100. Each line extended fully through the two grains at the two ends of each line.

The tensile tests and electrical conductivity measurements were carried out in as-cast condition and in the T6 temper condition. The T6 temper condition is obtained by a solution heat treatment (SHT) at 540°C for 8 hours and subsequent artificial aging (AA) at 170°C for 7 hours. However, depending e.g. on the geometry of a part, the T6 temper condition according to the invention may also be achieved by other temperature treatments.

As-cast condition refers to the state of the alloy after casting and cooling to room temperature and before any temperature treatment is carried out. All samples described in this document were treated and processed in the same manner unless specified otherwise.

The following tables 1 and 2 show the compositions of samples. As is apparent, Si and Zr compositions vary while the content of the other elements (except for Al which is used to balance to 100 wt-%) is held constant. The nominal (“intended”) compositions were confirmed by optical emission spectroscopy (OES) carried out on the samples in the as- cast condition. The nominal composition in table 2 applies for all alloys given in table 1 . The measured values in table 1 as obtained by OES confirm that the samples in the as- cast conditions have the intended nominal composition within the measurement precision of the used OES method. Table 1

Table 2 (nominal composition - all elements in wt-%)

Tables 3 and 4 show the compositions of alloys with varying copper content and varying zirconium content according to the invention and of comparative alloys, wherein balance is aluminium. The nominal composition in table 4 applies for all alloys given in table 3. As in table 1 , the intended nominal compositions in table 3 were confirmed by measured values that were obtained by an OES analysis.

Table 3

Table 4 (nominal composition - all elements in wt-%)

According to the invention, the alloys comprise a minimum of 0.05 wt-% Mg and less than 0.4 wt-% Mg because it is concluded from first principle considerations and from

experience with conventional foundry alloys that up to about 0.4 wt-% Mg, optionally up to 0.35 wt-%, optionally between 0.05 wt-% and 0.35 wt-% Mg, may be contained in an alloy according to the invention without an influence on the grain refining mechanism caused by the Zr addition according to the invention. On the other hand, the minimum of 0.05 wt-% magnesium is added according to the invention in order to improve mechanical properties, in particular strength at room temperature. A reason for this is the formation of metastable precipitates containing Mg, Si and Al in the T6 temper condition. The maximum Mg content according to the invention is limited to less than 0.4 wt-% Mg, because higher Mg contents may have a negative effect on the maximum elongation of the alloy. The reason for this is thought to be the formation of intermetallic compounds which are insoluble during solidification .Therefore castability decreases with Mg contents of 0.4 wt-% or higher.

Strontium is added between 0.01 and less than 0.035 wt-% to the alloys according to the invention in order to improve mechanical properties, especially elongation, due to modification of the Al-Si eutectic microstructure. In order to achieve this effect, a minimum of 0.01 wt-% Sr is added to the alloys according to the invention. Because Sr has no influence on the grain refining mechanism that is used in the alloys according to the present invention, the alloys according to the invention may, depending on the use environment contain up to less than 0.035 wt-% strontium (Sr). Higher Sr concentrations may result in an inefficient product showing lower ductility. It is thought that this is due to the formation of Sr containing intermetallics. Particularly efficient results may be achieved by using between 0.01 to 0.03 wt-% Sr..

Fig. 1 shows the microstructure and average grain size of Al 5 wt-% Si alloys having different Zr contents of 0.10 wt-%, 0.15 wt-% and 0.20 wt-%, respectively, according to embodiments of the invention. The grain sizes were measured in the as-cast condition, i.e. without any additional heat treatment after casting, using the above-described AGI method. It is apparent that the samples have a fine grain structure wherein the grain size decreases with increasing Zr content. Due to the composition, the samples have a hypoeutectic Al-Si microstructure. For reference, samples without any grain refiner (e.g. without Zr according to the invention or Ti and/or B according to conventional grain refining) may have grain sizes generally in the magnitude of 275 miti or higher in the as- cast condition. According to the invention, in order to achieve a desirable grain refinement effect, an upper limit for the Si content of less than 8.5 wt-% is chosen.

Fig. 2 shows the microstructure and average grain size of Al 7 wt-% Si alloys having different Zr contents of 0.10 wt-%, 0.15 wt-% and 0.20 wt-%, respectively, according to embodiments of the invention. The grain sizes were measured in the as-cast condition, i.e. without any additional heat treatment, using the above-described AG I method. It is apparent that the samples have a fine grain structure. In particular, the sample having 0.20 wt-% Zr exhibits an average grain size of less than 100 pm.

Fig. 3 shows the microstructure and average grain size of Al 9 wt-% Si alloys having different Zr contents of 0.10, 0.15 and 0.20 wt-% Zr, respectively. The grain sizes were measured in the as-cast condition, i.e. without any additional heat treatment, using the above-described AG I method. It is apparent that the sample comprising 0.15 wt-% Zr has a larger average grain size than the samples with 0.20 wt-% Zr and 0.10 wt-% Zr.

Fig. 4 shows the microstructure and average grain size of Al 1 1 wt-% Si alloys having different Zr contents. The grain sizes were measured in the as-cast condition, i.e. without any additional heat treatment, using the above-described AGI method. It is apparent that the samples have a grain structure with an increased average grain size, wherein the grain size is lowest for the sample with 0.15 wt-% Zr.

Fig. 5 shows a chart that shows how the average grain size of alloys according to the invention and according to comparative examples changes depending on the Si and Zr content of the alloys. The data shown in Fig. 5 is based on the data shown in Figs. 1 to 4. The Zr content is denoted “high Zr content” (high Zr) corresponding to 0.20 wt-% for all alloys and“low Zr content” (low Zr) corresponding to 0.10 wt-% Zr for the alloys with 5, 7, 9 and 1 1 wt-% Si. It is apparent from the chart that the influence of Zr on the grain size becomes less effective for Si contents between 7 and 9 wt-%. Therefore, the upper limit of the Si content according to the invention is set to less than 8.5 wt-% to enable the efficient and reliable production of alloys in the Si range in which a stable and predictable grain refining effect can be obtained. According to embodiments of the invention, an upper limit of 8 wt-% Si, 7.5 wt-% Si or 7 wt-% Si may be chosen according to the intended use of the alloy to further maximize production efficiency and alloy efficiency.

Fig. 6 shows the microstructure and average grain size of Al 7 wt-% Si alloys having different Ti contents according to comparative examples. The stated titanium content of the comparative examples also includes 1 .0 kg/ton (1 kg per metric ton of the total weight of the alloy corresponding to 0.003 wt-%Ti and 0.001 wt-% B in the comparative examples) of AITΪ3B1 added as Al-Ti-B master alloy to increase the number of Ti-B particles. As mentioned, using Ti and/or B for grain refining is a conventional refining technique. While adding Ti and B to the alloys results in a grain refinement, the use of Zr according to the invention allows to obtain even smaller grains as is apparent e.g. from the sample with 0.20 wt-% Zr in Fig. 2 that has an average grain size of 99 pm, whereas the conventional Ti and B containing alloys only reach a larger average grain size of 165 pm.

Fig.7 shows the microstructure and average grain size of Al 7 wt-% Si alloys having different Cu and Zr contents according to the invention. Copper may be added to the alloys according to embodiments to improve the properties at high temperatures, in particular the mechanical properties. Copper may form metastable precipitates with aluminium or other elements included in the alloy (e.g. Mg, Si) that improve properties at high temperatures needed e.g. for automotive internal combustion engines. As is apparent from Fig. 7, acceptable grain sizes are obtained for copper concentrations up to 3 wt-% for an alloy containing 7% Si. It is expected from metallurgical considerations that for alloys containing up to 3 wt-% Cu, the grain refining effect of Zr is also present for other silicon

concentrations according to the invention.

The electrical conductivity of alloys according to the invention and of conventional alloys was measured in the as-cast condition and in the T6 temper condition. The T6 temper conditions is obtained by a solution heat treatment (SHT) at 540°C for 8 hours and subsequent artificial aging (AA) at 170°C for 7 hours.

The electrical conductivity and the thermal conductivity of metals are related to each other. In general, a better electrical conductivity also implies a better thermal conductivity and vice versa as is e.g. also described by the Wiedemann-Franz law. In the following, the electrical conductivity is given in“megasiemens per meter” (MS/m).

In the as-cast condition, the electrical conductivities as shown in Table 5 were measured for alloys according to the invention comprising 7 wt-% Si and using different amounts of Zr (from top to bottom in Table 5: 0.10, 0.15 and 0.20 wt-% Zr, respectively) with balance Al. Table 5

For comparison, the electrical conductivities as shown in Table 6 were measured for conventional Al alloys in as-cast condition comprising 7 wt-% Si and using 0.06 wt-%, 0.10 wt-% and 0.14 wt-% Ti (balance Al) respectively for grain refining.

Table 6 Comparative Alloys

In the T6 temper condition, the electrical conductivities as shown in Table 7 were measured for alloys according to the invention comprising 7 wt-% Si and using different amounts of Zr (0.10, 0.15 and 0.20 wt-% Zr respectively as shown in Table 7) with balance Al.

Table 7

For comparison, the electrical conductivities as shown in Table 8 were measured for conventional Al alloys in T6 temper condition comprising 7 wt-% Si and using 0.06 wt-%, 0.10 wt-% and 0.14 wt-% Ti (balance Al) respectively for grain refining.

Table 8

It is apparent from tables 5 and 6 that in the as-cast condition, the electrical conductivities are generally similar for the alloys according to the invention using Zr and the conventional alloys using Ti and B additions. It is further apparent from tables 7 and 8 that in the T6 temper condition, the electrical conductivities of the alloys according to the invention are better than the electrical conductivities of the comparative conventional alloys using Ti and B for grain refining. As mentioned, a better electrical conductivity also implies a better thermal conductivity. Accordingly, the alloys according to the invention are in particular suitable in high temperature applications such as in combustion engines, e.g. the cylinder heads thereof.

Fig. 8 shows mechanical properties of an Al 7 wt-% Si alloys having 0.20 wt-% Zr according to the invention, of comparative Al 7 wt-% Si alloys without grain refiner (0% Zr), and of comparative Al 7 wt-% Si alloys using 0.06 wt-%and 0.10 wt-% Ti respectively (balance is Al for all alloys) for grain refining.

From Fig. 8 it is apparent that the maximum strength is generally not influenced by the Zr addition according to the invention.

It is further apparent that compared to conventional aluminium alloys having 7 wt-% Si and using Ti and B for grain refinement, similar levels of yield strength and maximum strength can be achieved according to the invention. However, it appears that the elongation is slightly lower in the Zr-comprising alloy according to the invention. It is thought that is caused by primary Zr-containing intermetallic particles and Zr in solid solution.

In summary, it is apparent from Fig. 8 that the alloys according to the invention have similar or better mechanical properties than the conventional alloys using Ti and B for grain refining.

It is noted that all embodiments described herein may be combined with each other. Features that relate to the method also relate to the alloy produced by said method and vice versa. In particular, preferred composition ranges that are described herein for the alloy also apply for the method for producing the alloys.

Claims

1. Method for producing an alloy for casting comprising:

providing aluminium (Al),

providing silicon (Si),

providing zirconium (Zr),

providing magnesium (Mg),

providing strontium (Sr),

optionally providing copper (Cu),

heating the provided Al, Si, Zr, Cu, Mg and Sr to obtain a melt, wherein the melt is at least substantially free of solute Ti and B, at least substantially free of titanium-comprising particles, such as T1B2, AbTi, TiC and is at least substantially free of other boron-comprising particles, such as AIB2, ZrB2.

cooling the melt while Zr causes a refined grain structure such as to obtain the alloy having an intended composition comprising

Si: 5 to less than 8.5 wt-%

Zr: 0.10 to 0.25 wt-%

Mg: 0.05 to less than 0.4 wt-%

Cu: 0 to 3 wt-%,

Sr: 0.01 to less than 0.035 wt-%

and balance Al.

2. Method according to claim 1 , wherein the heating the Al, Si, Zr, Mg, Sr and optionally Cu to obtain a melt comprises adding solid Zr to the liquid melt comprising aluminium.

3. Alloy comprising

Si: 5 to less than 8.5 wt-% Zr: 0.10 to 0.25 wt-%

Mg: 0.05 to less than 0.4 wt-%,

Sr: 0.01 to less than 0.035 wt-%,

Cu: 0 to 3 wt-%,

and balance Al, and further comprising incidental impurities, wherein the alloy comprises equal to or less than 0.02 wt-%, optionally equal to or less than 0.01 wt-%, titanium (Ti) and equal to or less than 0.02 wt-%, optionally equal to or less than 0.01 wt-%, boron (B).

4. Alloy according to claim 3, wherein the alloy comprises a purposeful addition of copper of equal to or less than 2 wt-%.

5. Alloy according to claim 4, wherein the alloy comprises a purposeful addition of copper of equal to or less than 1 wt-%.

6. Alloy according to any of claims 3 to 5, wherein the alloy comprises less than 8 wt- % Si and/or comprises less than 0.35 wt-% Mg and/or comprises equal to or less than 0.03 wt-% Sr.

7. Alloy according to any of claims 3 to 6, wherein the alloy comprises equal to or more than 0.10 wt-% Zr and equal to or less than 0.20 wt-% Zr.

8. Alloy according to any of claims 3 to 7, wherein the alloy comprises less than 7 wt- % Si and/or less than 0.30 wt-% Mg.

9. Alloy according to any of claims 3 to 8, wherein the alloy is in the T6 temper condition.

10. Use of zirconium for improving the electrical or heat conductivity in an Al-Si alloy comprising 5 wt-% or more Si and less than 8.5 wt-% Si that is free of purposeful additions of titanium and/or boron in the T6 temper condition.

11. Use according to claim 10, wherein zirconium is used according to claim 1 or claim 2.

12. Use according to claim 10 or 11 , wherein between 0.10 and 0.25 wt-% zirconium are used.

13. Use according to any of claims 10 to 12, wherein the Al-Si alloy is an alloy as described in any of claim 3 to 9.

14. Use according to any of claims 10 to 13, wherein 0.10 to 0.25 wt-% zirconium is present in the alloy to form particles that act as nucleation sites for grain refining.