Mitja Petrič

“In Situ” Measurement of Electrical Resistivity, Dilatometry and Thermal Analysis of Cast Iron

The contribution describes the “in situ” measurement of electrical resistivity, dilatometry and thermal analyses of flake and spheroidal graphite cast iron in liquid state, during solidification and in solid state. The shape of graphite formed during solidification influences electrical properties and variations of dimensions of cast irons. The complex “in situ” measurements were performed using in house developed measuring cell. The samples were systematically quantitatively metalographically investigated. It was found that electrical resistivity of lamellar grey cast iron is greater than the electrical resistivity of spheroidal graphite cast iron since the lamellas of graphite interrupt the iron matrix more than the nodules and the conduction electrons are scattered more on interfaces between graphite and metal matrix. The electrical resistivity of the flake graphite cast iron is increasing during solidification and decreasing after solidification. Based on “in situ” obtained results of electrical resistivity, dilatations and temperatures the materials properties were reconstructed.

Effect of Si and Ni Addition on Graphite Morphology in Heavy-Section Spheroidal Graphite Iron Parts

Metallographic analysis is applied to the study of the chunky graphite morphology in heavy-section castings of spheroidal graphite cast irons. Three castings with different Si and Ni content were prepared. Three positions in casting from the edge to the centre, with different cooling rates, were chosen for microstructure observation. The effect of the Si and Ni content on the graphite morphology and mechanical properties of heavy-section spheroidal graphite cast iron parts were investigated. Cerium containing commercial inoculant was used for in-stream inoculation. Chunky graphite area was estimated in micro-and macrostructure. Mechanical properties were determined on tensile test bars taken from the centre of the casting. Macro-and microstructure examination showed that the castings with high Si-content and Ni addition had chunky graphite present, while the castings produced by use of low Si and Ni containing charge had no chunky graphite. High Si-content is strong chunky graphite promoter, especially in castings with slow cooling rate. Ni addition also promotes chunky graphite formation, but only in thermal centre of the casting (where the cooling rate is the lowest). The elongation is severely lowered when chunky graphite appears in the microstructure.

Thermodynamic characterization of aluminium corner of Al–Cu system with various Nd additions

The effect of various neodymium additions on the characteristic solidification temperatures and on the microstructure development of the laboratory-prepared binary alloy AlCu5.5 was investigated. The aluminium corner of ternary system Al–Cu with various Nd additions was investigated using thermal analysis, differential scanning calorimetry and optical and scanning electron microscopy. The presence of Nd phases was confirmed using X-ray diffractometer. Microhardness of α-Al in AlCu5.5 alloy containing various Nd additions was analysed. The results show that the addition of neodymium raises eutectic (α-Al + Al2Cu) solidification temperature and solidus temperature, which leads to a narrower solidification interval. It was also confirmed that Nd formed two binary eutectics (α-Al + Al2Cu(Nd)) and (α-Al + Al8Cu4Nd). The addition of Nd also caused the increase in microhardness of α-Al phase.

Electrical resistivity measurements of Al–cast alloys during solidification

The aim of this paper was the selection of proper electrodes material regarding four–probe technique for the electrical resistivity measurements of Al–Si cast alloys. The most significant problem for electrodes is oxidation during measurements causing high contact resistance and providing incorrect results. Various materials were tested and aluminium electrodes chosen. The advantage of aluminium electrodes is that they melt within the specimen immediately after being poured and cause no interface resulting with any contact resistance. Pure aluminium and eutectic AlSi12 alloys were tested. Resistivity of Al–Si alloys is increasing with Si content. Grain refinement and modification of βSi were employed. Grain refinement has any effect on electrical resistivity. Modification of βSi phase causes decrease of electrical resistivity.

Die Casting and New Rheocasting

The high-pressure die casting process (HPDC) is a rapid solidification process leading to formation of rapid solidified castings. There are some specifics of the process. The casting of a molten alloy into a mold is complete within several milliseconds. A significant quenching effect and a high production rates are possible. The application of high-pressure enables good contact between molten alloy and die wall that enables: the increase in cooling rate, the increase in heat flow and heat transfer coefficient at the die-melt interface as well as the formation of a net shape casting. Casting defects such as shrink holes which generate by the shrinkage during solidification are reduced. Components with complicated shapes are produced directly from a liquid state even for a molten alloy with high viscosity.