Acta Metallurgica Slovaca

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Year 2004

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Year 2004 No 2

Petrík J., Szarvasy P., Špeťuch V.

THE PROPERTIES OF IRON, NICKEL AND MANGANESE CONTAINING Al-Si ALLOY

Abstract
Iron is in practice present in Al-Si cast alloys as a result of iron contaminated mill scrap use. The iron contents above 0,5% are undesirable, because they effect segregation of brittle and hard intermetallic phases. Extrahazardous are long needles of ? phase, transversing aluminium matrix and eutectic cells. This phase effects the premature failure of the castings by notch effect. A reliable and economic method of iron elimination from alluminium alloys has not been well-known yet in metallurgical practice. The negative effect of iron can be eliminated by additon of suitable corrector, whose compounds with aluminium, silicium and iron segregate in less harmfull shape than needles, the most often in the form of skeleton shaped „chinese script“. Co, used as iron corrector in former times is practicaly replaced by cheaper manganese at present. Cr, V, Mo, Ni or Be are used ad idem.
The aim of submitted work is to evaluate simultaneous influence of Mn and Ni on the microstructure, fluidity and mechanical properties (hardness, ultimate tensile strength, plastic properties) of the Al-Si alloy with increasing iron content (up to 2,1%).
The materials used in experiments besides Al contained: 9,75% Si, 0,2% Mg and iron in six levels up to 2,1%. The iron correctors were 0,7% Mn, 0,7% Ni and 0,4%Ni + 0,4%Mn. The chemical analysis of the castings was realized by atomic absorption method on the spectrometer Perkin Elmer 306A. AlSiSr10 master alloy containing 8,35% Sr was used as Si modifier. Final Sr content - 0,015% was sufficient for real modification of eutectic Si. The pouring temperature was 760°C. The melt was poured into:
1) the chill mould for mechanical properties tests, mould initial temperature was 18°C.
2) the steel chill mould "lyre" with six rising pipes with graduated diameter for fluidity test (Yz), mould initial temperature was between 80°C and 130°C, the cooling rate of the castings was 18°Cs-1 - 13°Cs –1
The tensile tests were made according to STN 42 0310, two specimens were used for every temperature of mould and iron level. The hardness HV10 was evaluated according to STN 42 0374 (ISO 6507-1), 5 impressions were done for every temperature of mould and iron level. The microstructure was evaluated at the point of fracture and at both ends of the tensile-test samples. 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 microstructure of aluminium matrix and eutectic silicium gives evidence to complete modification with used strontium concentration. Used iron modifiers, especially Ni, have positive influence because shifts start of long overshooting needles (? - phase) segregation towards higher iron levels. Increasing of cooling rate of castings moderately decreases the length of intermetalic needles. Manganese, in contrast to nikel induces segregation of skeleton shaped particles.
Nikel has more positive effect on ultimate tensile strength, plastic properties and fluidity and manganese on the hardness on the other hand. Simultaneous using of Ni and Mn suppresses typical growth of fluidity about 1% of iron.

Keywords: Al-Si alloy|iron,|microstructure|mechanical properties|

No 2 (2004), p. 73-79