Effect of Process Parameters on Morphology and Grain Refinement Efficiency of TiAl3 and TiB2 in Alumimum Casting

This paper presents the effects of different process parameters in producing Al-STi-1B grain refiner,i.e.various sequences and reaction time,on grain refinement efficiency of aluminum castings.It was found that different process parameters resulted in different morphology and size distribution of TiAl-3 and TiB-2 in grain refiner. The experiment was carried out by adding KBF-4 and K-2TiF-6 to molten aluminum.The melting temperature was controlled at 800℃in an electric resistance furnace.Three different sequences of KBF-4 and K-2TiF-6 additions were applied,i.e.,adding KBF-4 before K-2TiF-6,adding K-2TiF-4 before KBF-4 and mixing both KBF-4 and K-2TiF-6 before adding to molten aluminum.Three different holding time at 1 min,30 min and 60 min were applied.The results showed that no significant difference of morphology and size distribution was found by varying three different sequences.Whereas,the different holding time provided major differences in both morphology and size distribution,which are technically expectable from diffusion and agglomeration between particles resulting in larger particle size and wider range of size distribution of TiAI3 and TiB2.If the reaction time was longer than 30 rain,morphology of both TiAl-3 and TiB-2 became too large.If the reaction time was too short,less reaction between TiAl-3 and TiB2 to form would be obtained.For grain refinement efficiency, it was found that mixing KBF-4 and K-2TiF-6 before adding to molten aluminum with a holding time of 30 min resulted in best grain refinement efficiency.     

Eutectic Morphology of Al-7Si-0.3Mg Alloys with Scandium Additions

The mechanisms of Al-Si eutectic refinement due to scandium (Sc) additions have been studied in an Al-7Si-0.3Mg foundry alloy. The evolution of eutectic microstructure is studied by thermal analysis and interrupted solidification, and the distribution of Sc is studied by synchrotron micro-XRF mapping. Sc is shown to cause significant refinement of the eutectic silicon. The results show that Sc additions strongly suppress the nucleation of eutectic silicon due to the formation of ScP instead of AlP. Sc additions change the macroscopic eutectic growth mode to the propagation of a defined eutectic front from the mold walls opposite to the heat flux direction similar to past work with Na, Ca, and Y additions. It is found that Sc segregates to the eutectic aluminum and AlSi₂Sc₂ phases and not to eutectic silicon, suggesting that impurity-induced twinning does not operate. The results suggest that Sc refinement is mostly caused by the significantly reduced silicon nucleation frequency and the resulting increase in mean interface growth rate.     

Effect of Sc on Precipitation Hardening of AlSi6Mg Alloy

The effect of Sc on precipitation hardening of AlSi6Mg was studied.Zr was previously reported that it increased the effectiveness of Sc in wrought aluminum in many areas so Zr was also used together with Sc in this study. Different levels of Sc and Zr additions were added to AlSi6Mg before casting in the permanent mold.The samples were precipitation hardened at different aging temperatures and for various aging time before testing for tensile strength and hardness.It was found that Sc addition into Al6SiMg can change its response to age hardening.Additions of Sc and Sc with Zr increased both yield strength and hardness for both aging temperatures.In addition,Sc was found to modify eutectic Si to obtain fibrous morphology.This effect of Sc on eutectic silicon modification has never been reported before.     

Effect of Alloying Element Antimony on Macrostructural and Microstructural Development in A356 Alloy Directionally Solidified under Unsteady-state Conditions

Microstructure development is well accepted to have a major effect on mechanical properties during its services. One of the most interesting ways to improve mechanical properties is to reduce secondary dendrite arm spacing （SDAS）.SDAS also plays an important role in controlling and providing the well distributed and fine microstructure resulting in better tensile strength and elongation.To reduce SDAS,it is commonly known by increasing cooling rate and increasing interface instability by limited-soluble alloy addition.It is,however, unclear that how both cooling rate and limited-soluble alloy,e.g.Sb,relate to each other.This may be the reason that the limited-soluble alloy may not effectively reduce SDAS.To better understand this phenomenon, influences of Sb on solid/liquid interface instability using columnar to equiaxed transition （CET） were studied in the directionally solidification experiment.From macrographs and micrographs,it was observed that at 0.06-0.18 wt pct Sb the CET min,CET max,CET zone and %CET area gradually increased.The increases of CET max and CET zone in samples with 0.06 to 0.18 wt pct Sb addition results from recalesced zone.On the other hand,the variations of CET max and CET zone in samples with 0.24 to 0.30 wt pct Sb results from equiaxed grain formations that prohibit the growth of columnar grain and latent heat from intermetallic phase solidified.     

Fluidity and Hot Cracking Susceptibility of A356 Alloys with Sc Additions

A356 with scandium (Sc) addition provides interesting results beyond costs. For the practical use of Sc, the effects of Sc on castability must be considered. Fluidity and hot cracking are important factors defining the castability of aluminum casting alloys. In the present work, the influence of Sc addition on the castability of A356 hypoeutectic Al–Si alloy was investigated, which was evaluated through fluidity and hot cracking susceptibility. The fluidity of the alloys was studied by measuring the total volume of solidified aluminum in a multi-channel mold. The hot cracking susceptibility of the alloys was evaluated by using a constrained-rod casting mold test. The results of the fluidity and hot cracking susceptibility test were supported by microstructural analysis. The results indicate that 0.2 wt% Sc addition significantly increases the fluidity of A356 alloy, due to the grain refinement and eutectic Si modification by changing the solidification mode. However, the fluidity slightly decreases when the Sc content increases to 0.4 wt% due to the formation of primary Al₂Si₂Sc intermetallic phase. The hot cracking of A356 alloy was completely diminished when Sc was added to the alloy.