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| Petrík J., Szarvasy P., Špeťuch V. |
| THE INFLUENCE OF COOLING RATE AND MN AND FE CONTENT ON THE STRUCTURE AND MECHANICAL PROPERTIES OF AL-SI ALLOY |
Abstract
Increasing of iron content in row materials and in metallurgical semiproducts in consequence of using of bad mill scrap is very significant problem of alluminium casting alloys. The importance of this effect is accented by the fact, that the negative influence of iron on the mechanical and technological properties is more significant than it of other secondary elements, frequently existing in alluminium alloys. A reliable and economic method of iron elimination from alluminium alloys isn't well - known yet.
Iron forms brittle intermetallic compounds with aluminium and silicium which seriously impair mechanical properties of Al - Si alloys. Extrahazardous are long needles of FeSiAl5 phase (sometimes marked as ? phase). This hard and brittle intermetallic compound effects the premature failure of the matrix by notch effect at tensile stress. The negative effect of iron can be eliminated by superheating of the melt, increasing of cooling rate or by additon of suitable metal, for example manganese, chromium, cobalt or beryllium. Manganese is the first of all prectically used for this purpose. The addition of manganese changes the morphology of the iron containing phases from needles to less harmful skeleton shaped "chinese script" particles
The influence of iron (0.3 and 1.4%), manganese (up to 0.8%) and cooling rate (18, 2 and 0.03°Cs-1) on microstructure and mechanical properties of the Al alloy with 9.75% Si and 0.2% Mg were evaluated. AlSiSr10 master alloy containing 8.35% Sr was used as modifier, the final concentration of Sr was 0.015% in the castings. The pouring temperature was 760°C. The cooling rate was regulated by initial temperature of the moulds, or in the extreme case by cooling in the furnace. The hardness HV10 was evaluated according to STN 42 0374 (ISO 6507-1). The tensile tests were made according STN 42 0310, six specimens were used for every content of iron and manganese. Polished metalographic samples were etched with 25% H2SO4 at 75°C and consequently with 0.5% HF. The morphology of eutectic Si (? phase) according to STN 42 0491, its interparticle spacing ??, dimensions and shape of intermetallic iron containing phases were evaluated.
The eutectic silicium - ? phase has perfect oval shape if the cooling rate is 18 or 2°Cs-1, but cooling rate 0.03°Cs-1 (in the furnace) results in practicaly non-modified needle shaped eutectic silicium.
The needle shaped particles don't arise if the iron content is less than 0.3%. The number of skeleton shaped particles of "chinese script" increases with increasing of manganese content. Their dimensions are more sensitive on manganese content if iron content is low.
The creation of long needles of intermetallic phase FeSiAl5 in alloy with 1.4% iron, overshooting eutectic cells and dendrits of aluminium matrix, was practicaly repressed if the cooling rate was 18°Cs-1 and the content of manganese was more than 0.4%. But, on the other hand, creation of "chinese script" was observed in this case. The particles of "chinese script" are less deleterious than needles of intermetallic phase FeSiAl5 indeed, but in regard to frequent occurrence of cracks their presence is not wellcome as well as the presence of needles of FeSiAl5 phase. The influence of manganese decreases with decreasing cooling rate. Even maximal content of manganese isn't able to avoid the creation of FeSiAl5 phase needles completely, the influence of manganese on their shortening is repressed if the cooling rate is 0.03°Cs-1.
Because of the risk of results distortion by porosity of the castings cooled with cooling rate 0.03°Cs-1, the tensile tests were not done in this case.
As for mechanical properties the hardness increases as a result of increasing cooling rate and iron content, the influence of manganese is less significant. The cooling rate 0.03°Cs-1 results in significant low hardness without influence of iron content. Ultimate tensile strength increases as a result of decreasing of iron content and increasing of cooling rate. Manganese moderately increases ultimate tensile strength with maximal values about 0.4%. The decreasing at higher manganese content is connected with precipitation of "chinese script" particles. Both elongation and reduction of the area increase with increasing manganese concentration and decreasing of cooling rate and iron concentration. The low values of plastic properties at manganese concentration between 0.3 and 0.7% is probably result of star (petal) shape particles segregation.
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| Keywords: AlSi alloy|cooling rate|structure|mechanical properties| |
| No 1 (2002), p. 50-58 |
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