The paper presents an investigation of the effect of process variables and material condition on the forgeability of magnesium wrought alloys of the Mg-Al-Zn group. The experimental work included the studies of forging capabilities of the alloys in open-die forging at hot-and warm-working temperatures. Forging tests were performed for the material in both the as-cast and as-worked conditions, for two variants of the work-piece geometry. Different variants of the work piece indicated fracturerelated problems in forging magnesium alloys in the warm-working temperature mode, which involved an interaction between the material composition and process variables, and the state of stress. By means of numerical calculations it was concluded that, in addition to the material condition, the favourable state of stress, provided by a closed die, could greatly improve the formability of magnesium alloys in the warm-working range. Keywords: forging, magnesium alloys, automotive applicationŝ lanek predstavlja preiskavo vplivov procesnih spremenljivk in materiala na kovnost magnezijevih zlitin iz skupine Mg-Al-Zn. Eksperimentalno delo je vklju~evalo {tudij sposobnosti za kovanje zlitin v odprtem orodju pri temperaturah vro~ega in toplega preoblikovanja. Preizkusi kovanja so bili izvr{eni za material v litem stanju in za`e predelan material za dve vrsti izkovkov z razli~no geometrijo. Razli~ne variacije izkovkov so pokazale pri kovanju magnezijevih zlitin te`avo z razpokami pri toplem preoblikovanju ter interakcijo med sestavo materiala, procesnimi spremenljivkami in stanjem napetosti. Z numeri~nimi izra~uni je bilo ugotovljeno, da dodatno k razmeram materiala lahko ugodno stanje napetosti, ki se ga dose`e z zaprtim orodjem, mo~no izbolj{a preoblikovalnost magnezijevih zlitin v obmo~ju toplega preoblikovanja. Klju~ne besede: kovanje, magnezijeve zlitine, uporaba v avtomobilski industriji

The results of an experimental study on influence of cerium addition on structural characteristics of 42CrMo4 steel are presented. Alloying with cerium was carried out using profile filled with powdered mixture of mischmetal. The samples were taken from two ingots cast in the VHM's steelworks with standard time of casting of about 14 minutes. Three steel bars from one of the produced ingots were prepared by forging. Chemical composition, macro-and microstructure, X-ray EDX chemical microanalysis, hardness of the all steel samples were obtained. Cerium addition resulted in the formation of micrometer size inclusions which can be utilized for controlling the grain size structure of steel castings. The majority of the particles have settled at the bottom part of the casting, indicating that the convection flow during solidification was very weak. The cerium addition slightly diminished hardness of the steel. A segregation phenomenon causing inhomogeneous distribution of cerium over entire volume of as-cast samples after relatively rapid crystallization process of the steel was revealed.

Intensity of structural changes during forging is usually determined in dependence on the forging ratio. Forging ratio at elongation is most often calculated with use of simple relations, which are based on the change of cross section of the forged pieces. The intensity of structure development is influenced not only by the simple change of cross sections, but also by relative length of the stroke, magnitude of deformation in individual passes, manner of edging, temperature, friction and shape of anvils. The paper gives an analysis of influence of the relative length of the stroke on the forging ratio.

Effects of Cerium (Ce) addition on solidification structure of a low-carbon 42CrMo4 steel was investigated. The addition of up to 0.067 wt.% of Ce in the steel produced greatly improved solidification structure with a suppressed columnar grain zone, finer grain size in an equiaxed grain zone and zero area fraction of casting shrinkage cavity. The added Ce occurred in the steel both in the form of Ce oxy-sulfide inclusions and as dissolved atomic Ce segregated together with other elements at prior austenite grain boundaries and at interdendritic spacing. The Ce oxy-sulfide inclusions were found to play a major role in the observed improved grain structure meanwhile dissolved Ce had pronounced effects on morphology of dendritic networks. The fraction of Ce dissolved in the melt appeared to bring about increase in fluidity of the molten steel, leading to total elimination of interdendritic shrinkage porosity in solidification structure of the steel with added Ce. Ce addition can be considered as a potential solution for grain structure refinement in heavy-weight castings of 42CrMo4 steel grade.