The invention refers to a process for preparation of aluminium-based alloys, in particular for casting of corrosion-resistant highly stressed thin-walled motor vehicle parts like e.g. a casing of a turbo charger.

Such inventions belong to chemistry, namely to non-ferrous metallic alloys, and in particular to processes intended to changing the physical structure of an aluminium-based alloy, in which silicon is the next major constituent.

The purpose of the invention is to produce, merely of secondary aluminium, an alloy AlSi5CulMg with physical properties, which would enable manufacturing of corrosion-resistant highly stressed thin-walled cast motor vehicle parts.

A process for manufacturing an aluminium-based alloy suitable for using in automotive industry is described in EP 2 698 216 Al . In accordance with the proposed invention, the previously defined technical problem is solved by means of a process, in which the alloy AlSi5CulMg with the content of iron per weight

Fe≥ _> 6%,

is obtained from the secondary i.e. recycled aluminium, upon which by adding a pure primary aluminium the content of iron is reduced to

Fe≥ 0,2%,

wherein at least one of master alloys is added, which are selected from the group comprising AISU2, AlCu50, AlMg8, AlSrlO, ΑΙΏ5Β1,

and optionally at least one electrolytic pure element, which is selected from the group comprising Ni, Mn, Cu and Mg, which is then followed by casting and solidifying of each obtained motor vehicle part, and upon that by thermal treatment of such obtained casting.

Casting of such manufactured alloy is optionally performed into a steel mould, wherein the temperature of molten alloy is between 720C° and 730°C, and the temperature of said steel mould is between 380°C and 420°C. Casting can also be performed into a mixture of sand and furane, wherein the temperature of molten alloy is between 730C° and 740°C.

Said thermal treatment of cast parts consists of homogenizing annealing, which is followed by artificial aging. The step of homogenizing annealing comprises gradually heating to temperature between 525C ⁰ and 530°C in duration of at least 2h, which is followed by maintaining at said temperature in duration of 8 - lOh, which is then followed by quenching in water with temperature approximately 20°C for 10 min. Said step of artificial aging is performed by gradually heating to temperature between 150C° and 155°C in duration of at least approximately 0,5h, which is followed by maintaining at said temperature for 8-1 Oh, which is followed by cooling in the atmosphere.

The invention also refers to use of the alloy, which is manufactured by means of the previously described process, for manufacturing of a turbo charger casing for an internal combustion engine of a motor vehicle.

Reduction of content of Fe below 0,2% prevents formation of ferrous intermetallic phases within the aluminium-based alloy, which results in excellent mechanical properties and corrosion-resistance, while adding of Ti and Sr enables achieving of fine-grained structure.

Example

The raw-material for preparation of Al-alloy by means of the process according to the invention was an alloy on the basis of secondary aluminium, namely selectively recycled aluminium waste with the following constitution:

Orientational constitution of primary Al is the following:

The required or expected constitution of the AlSi5CulMg alloy is the following:

or the purpose of preparing 350 kg of molten alloy, 200 kg of secondary Al and steel moulds were used, upon which the content of Fe in the molten alloy was reduced below 0,2% by adding 135 kg of primary Al.

Casting of molten alloy was performed on the one hand into a mixture of sand and furane, and on the other hand into a steel mould, wherein the cores were in both cases manufactured according to Croning-process. The molten alloy was then refined with AlTiSBl and Sr, wherein 7 kg of AlTi5Bl and 0,6 kg of Sr were added per 350 kg of the molten alloy.

Chemical constitution of the charge in the furnace was the following:

Technologic properties of the molten alloy were the following:

KF was between 9 and 15, content of hydrogen Di < 4, undercooling between 4°C and 8°C. Concentration of inoculant for reduction of primary a was both by casting into a steel mould as well as into a sand mould between 0,05 and 0,3% Ti, and for minimizing the growth of eutectic grains between 0,004% and 0,05% Sr. By concentration of Fe approximately 0,15%, and in particular 0,12%, the probability of formation of intermetallic phases FeAl ₃ , FeMnAl ₆ and aAlFeSi is essentially reduced, which results in improvement of specific extension. Microstructure of thermally treated alloy comprises spheroidal eutectic Sz ^' -particles, which are distributed within the interdendritic area of primary a-grains.

Production of the alloy according to the invention including the thermal treatment, namely homogenization annealing followed by artificial aging resulted in tension strength 275 Mpa and yield stress 174 Mpa by casting into a sand mould, and tension strength 275 Mpa and yield stress 174 Mpa by casting into a steel mould. Simultaneously, the so-called trial bars with diameter 18 mm pursuant to Standard NF A 57 - 702 have been for the purposes of providing species for performing of tensioning test. The tension strength of the material of said trial bars sticks was 360 Mpa, and yield stress 310 Mpa.

Metallographic presentations of microstructure with a-dendrites, Al-Si eutectic and intermetallic phases is presented in Figs 1 - 4, while the microstructure upon the thermal treatment is presented in Figs 5 - 8.