EFFECT OF GRAIN REFINEMENT AND MODIFICATION OF EUTECTIC PHASE ON SHRINKAGE OF AlSi9Cu3 ALLOY

All metals and alloys with exception of Bi are shrinking during cooling, mostly during the solidification process. In order to avoid shrinking porosity, casting process, and gating and feeding system must be properly designed. Solidification front must move towards the feeding system where solidification is then completed. No separate liquid pockets where solidification ended without feeding should exist. In order to meet these demands the feeders and feeder necks must have suitable modules to keep metal liquid during transport to the solidification front 1 . There exist two types of solidification, exogenous and endogenous. Exogenous solidification is typical for pure and eutectic alloys that solidify at constant temperatures. Solidification starts there on interface with the mould where solid layer of metal is formed, and solidification front is then moving towards the center of casting or towards the area of the highest temperature. Exogenous solidification type is further divided into three subgroups, i.e. solidification with smooth shell, dendritic solidification, and spongy-layer solidification. Endogenous solidification is typical for alloys with hypo and hypereutectic compositions that are solidifying in a temperature range. Solidification starts throughout the whole volume of the casting, and solid shell on interface with the mould is formed more slowly. There are two subtypes of endogenous solidification. One is mushy, and the other one is shell-forming type 2 . Solidification types are mixed in all the real cases. Solidification starts with one prevailing type which is changed into another one after some time. This mechanism depends on cooling rate since this is changed during the solidification process 2, 3 . Type of solidification has also influence on feeding ability of alloy. Alloys with smooth-wall type of solidification have the best feeding abilities which result that feeder neck remains liquid during the whole solidification process and thus transport of liquid metal to the solidification front is enabled. Feeding ability of alloys is lower with the endogenous dendritic type and it becomes even worse with the mushy-type solidification where flow of liquid metal is disturbed by floating crystals that are already present in liquid metal. Feeding abilities of alloys dependent on solidification type are presented in Figure 1. Nucleation potential has big influence on shrinkage porosity, as well. When alloy is properly grain-refined it has higher nucleation potential than the basic alloy and