Advanced Materials Research Vols. 189-193

Abstract: 7075 aluminum alloy ingot with the diameter of 100 mm has been produced by horizontal direct chill casting in different casting speed. The effect of casting speed on the sump profile and the ingot surface quality was studied by sump profile observation. It was found that increasing the casting speed results in the deepening of the liquid pool, the adding of segregation knots and the reducing of cold shouts in the ingot surface. It is also found that the depth of the liquid pool is directly proportional to the casting speed and the squared radius of the ingot.

Abstract: The effects of rotating electromagnetic stirring and inoculants on the solidification structures and properties of IN100 superalloy vacuum investment casting was studied with XRD, SEM and optical microscope. The results show that by initiating the 50Hz, 150A rotating electromagnetic stirring as soon as the molten metal was poured into the mould, which was coated with inoculant CoAl₂O₄, the average equiaxed grain size of IN100 superalloy vacuum investment casting can be refined to 95μm and the fraction of equiaxed grains can be increased to 99 %. On condition that the grain size of K417 superalloy vacuum investment casting were refined to 95μm, the tensile properties at room temperature, the morphology of （γ＋γ′）eutectic and the morphology of dendrites can be greatly improved.

Abstract: Surface segregation layer are frequently encountered during aluminium alloy direct chill casting process, and the removal of the surface segregation layer before further processing of the ingot decreases the ingot yield. In this work, the low frequency electromagnetic field was applied to study the effects of low frequency electromagnetic field on segregation layer during the direct chill casting process. The results show that under the effect of the low frequency electromagnetic field, the surface quality of ingot is improved, the structure of the ingot is refined, and the thickness of segregation layer is decreased.

Abstract: In order to investigate the microstructure formation resulted from quasi-peritectic reaction in Pb-Bi-Sn ternary alloy, specimens were directionally solidified with a constant temperature gradient (G=18 K/mm) and the growth velocities ranging from 0.5 μm/s to 10 μm/s. Experimental results indicated that a microstructure, which is composed of the residual primary α phase (Pb-rich solid solution), peritectic β-(Pb7Bi3) phase and Sn-rich solid solution phase was observed. Transition from primary α-cell to primary α-dendrite took place with the increasing growth velocity. The rod-like eutectic was observed in the inter-cellular region under low growth velocity. Detail analyses were adopted to identify the phases and determine the phase compositions, respectively.

Abstract: A research focus on semi-solid metal processing is the preparation of semi-solid slurry with non-dendritic microstructure. During the past several decades, people tended to obtain the non-dendritic structure by stirring melt of alloy which downs to the semi-solid temperature range, such as mechanical stirring and electromagnetic stirring; In recent years, with the technological innovation of semi-solid slurry preparation turned to be more convenient and efficient, most of these processes are based on the control of nucleation and growth process of primary phase during solidification, such as NRC, SSR, SLC, SEED, and CRP. In this paper, a novel process, named as “Self-Inoculation Method (SIM)”, has been proposed for semi-solid slurry preparation. The process involves self-inoculants addition to the melt, and then pouring the melt to mould through a multi-stream mixed cooling channel. The melt was avoided chemical pollution due to the particles of self-inoculants from the same composition as the melt. The semi-solid billets of AM60 alloy with non-dendritic structure were prepared by SIM. The effects of process parameters on the microstructure and the mechanism on refinement of alloy were investigated. The results indicate that pouring temperature, addition amount of self-inoculants and slope angle of the cooling channel are key factors for SIM process. The optimized parameters for the billet preparation of AM60 alloy are obtained: pouring temperature is at 680°C~700°C；addition of self-inoculants are between 5%~7% (mass fraction)；slope angle of the cooling channel is at 30°~45°. The heterogeneous nucleation was enhanced as the addition of self-inoculants; the formation of chill crystal and the fragmentation of dendrites because of cooling and shearing of the cooling channel, resulting in the increase of grains density and a small grain size.

Abstract: Formation of structure of Zn-5wt%Al alloy during the nonequilibrium solidification was investigated. Results show that the primary Al-rich phase(α phase) is precipitated during the nonequilibrium solidification. With increasing of the cooling rate, the primary Al-rich phase(α phase) not only becomes less and less but also transforms from cellular crystal to dendritic crystal gradually. Based on this, the eutectic symbiotic region of Zn-5wt%Al alloy phase diagram was researched, and the qualitative phase diagram of eutectic symbiotic region was lined out. In addition, results show that the hypoeutectic structure which is crystallized during the nonequilibrium solidification hasn’t evident core under high cooling rate, and the lamella of hypoeutectic structure is disordered.

Abstract: A steady-state non-equilibrium dendrite growth model was extended for binary alloy assuming non-linear liquidus and solidus. Satisfactory agreement of the model prediction with the experimental data of Ni-0.7at.%B and Ni30Cu70 alloys was achieved. The velocity plateau as experimentally observed in the velocity versus undercooling is quantitatively analyzed in terms of this model. Accordingly, the initiating point (i.e. corresponding to the critical velocity of absolute solutal stability VC*) and the ending point (i.e. corresponding to the velocity of maximal tip radius VRm) of the plateau are characterized.

Abstract: Ti addition to AZ91 alloy has been investigated with conventional casting. The microstructure and mechanical property were examined. The results show that addition of Ti with an amount of 0.1~0.5%wt resulted in a refinement of the as-cast microstructure. The morphology of β phase is changed from coarse, uneven, semi-continuous skeletal network to small, uniform, short rod-like or granular. When the content of Ti is 0.4wt%, the tensile strength and elongation go up to maximum value of 197 MPa and 6.9% respectively. Small addition of 0.2%wt Ti to AZ91 alloy predominantly forms TiAl₃ phase, lying in β-phases at the grain boundaries. The mechanism of mechanical properties improvement caused by Ti addition is discussed.

Abstract: Mg_72.5Zn₂₆Y_1.5 quasicrystal alloys were treated under different heat treatment process. The microstructures of the alloys were investigated. The quasicrystal and eutectic morphologies and their transformation during heat treatment process were detailedly studied. The results showed that lamellar and dendritic eutectic phases throughout the matrix were evidently formed under heat treatment process 310°C×30min water quenched and 310°C×30min liquid nitrogen deep cooling, respectively. Furthermore, quasicrystals of dragonfly-like and bulk polygon morphology were formed under heat treatment process 575°C×15min water quenched and 700°C×2min cooled with furnace, respectively. Various morphologies’ transformation mechanism during heat treatment process were also studied.

Abstract: Performance and mechanism of furan resin no-bake sand, ester-sodium silicate sand, CO₂-sodium silicate sand and phosphate no-bake sand were compared by test and analyzed by SEM. Results indicate that furan resin sand and ester-sodium silicate sand have higher dry strength and better humidity resistance, because their fracture modes are cohesive fracture and plastic fracture, there is less crack on their bond membrane. Fracture modes of phosphate sand and CO2-sodium silicate sand are cohesive fracture mainly with a few adhesive fracture and brittle fracture, there is more crack on their bond membrane, which is benefit for moisture to invade, so their compressive strength after hardening and moisture absorption are lower, but phosphate sand has higher dry strength and better humidity resistance than CO₂-sodium silicate sand, lower cost and less pollution than furan resin sand, the lowest gas evolution and the best collapsibility , it has a wide application prospect.