Chen Ruirun

Effects of ultrasonic vibration on the microstructure and mechanical properties of high alloying TiAl

To modify the microstructure and enhance performances, the ultrasonic vibration is applied in the mould casting of TiAl alloy. The effects and mechanism of ultrasonic vibration on the solidifying microstructure and mechanical properties are investigated and the model for predicting lamellar colony size is established. After ultrasonic vibration, the coarse microstructure is well modified and lamellar colony is refined from 534 μm to 56 μm. Most of precipitated phases are dissolved into the lamellar colony leading to a homogenous element distribution. The phase ratio of α2-Ti3Al and γ-TiAl is increased, and the chemical composition is promoted to more close to equilibrium level by weakening the influence of β-alloying elements. The microhardness and yield strength are gradually improved by 23.72% and 181.88% due to the fine grain strengthening, while the compressive strength is enhanced by 24.47% through solution strengthening. The critical ultrasonic intensity (Ib) for TiAl alloy is estimated at 220 W cm−2 and the model for average lamellar colony size is established as . The ultrasonic refinement efficiency exponentially increases as the ultrasonic vibration time with a theoretic limit maximum value of Elim = 88% and the dominating refinement mechanism by ultrasonic vibration is the cavitation-enhanced nucleation rather than cavitation-induced dendrite fragmentation.

Characteristics of array holes with large aspect ratio in aluminum-based cast alloy

ABSTRACT Array holes were obtained by machining methods or nontraditional machining methods, and casting process was rarely used in the preparation of array holes. In this experiment, stainless steel thin rods coated with alcohol group graphite paint were chosen as cores to prepare array holes on aluminum-based cast alloys, and the roughness and roundness of holes were analyzed. The results show that array holes cast with 2 mm pitch of holes, 0.54 mm diameters, and large aspect ratio of 100 were obtained. The roundness and roughness of holes were influenced by consumption of carbon element from surface of hole core and wettability between molten metal and hole core surface; the lower roughness and the better roundness could be acquired under these experimental conditions. And roughness of holes (Ra) was about 6.3 µm, which is close to that obtained by machining, and the value of hole shape factor (K, characterizing the roundness of the hole) was above 0.7; the shape of the hole approached a circular shape.

EFFECT OF PROCESSING PARAMETERS ON SOLID-LIQUID INTERFACE OF Nb-Si BASE ALLOY FABRICATED BY ELECTROMAGNETIC COLD CRUCIBLE DIRECTIONAL SOLIDIFICATION

Nb-Si base alloys have attracted considerable attentions as the potential high temperature structural materials working in the service temperature range of 1200~1400 ℃ because of their high melting points(1750 ℃),moderate densities(6.6~7.2 g/cm3) and excellent high temperature strength.However,the mismatching between room temperature fracture toughness and high temperature strength has limited their practical applications.Directional solidification(DS) and alloying have been proved to be the effective methods to overcome this critical issue.The DS processes used to prepare Nb-Si base alloys included Czochralski directional solidification in a copper crucible,electron beam directional solidification,optical floating zone melting,integrally directional solidification and electromagnetic cold crucible directional solidification(ECCDS).The previous studies focused on the effect of process parameters on microstructure and mechanical properties in the steady-state growth region(SSGR).However,the microstructure in the SSGR was controlled by the solid-liquid interface,and the solid-liquid interface was controlled by process parameters.Therefore,the study about the effect of process parameters on solidliquid interface was very important.In this work,the master alloy with the nominal composition of Nb-22Ti-16Si-3Cr-3Al-2Hf(atomic fraction,%) was prepared by vaccum non-consumable arc-melting first,and then induction skull melting.The DS experiments were performed in the ECCDS device equipped with a square water cooled copper crucible(internal dimension:26 mm×26 mm×120 mm) and a Ga-In alloy pool.There were three processing parameters in ECCDS including heating power of power supply,withdrawal rate and holding time.The DS ingots were prepared according to the orthogonal test(L9(33)).Instability degree was defined as the ratio of the height of solid-liquid interface to the width of the DS ingot.The results showed that there were three macroscopic morphologies of solid-liquid interfaces;the increase of holding time,decrease of withdrawal rate and elevation of heating power were conducive to keeping the solid-liquid interface macroscopic morphology planar.With the increase of withdrawal rate,primary dendrite arm spacing(d1) and secondary dendrite arm spacing(d2) decreased gradually;with the increase of heating power,d1 and d2increased gradually;with the increase of holding time,d1 and d2increased first and then decreased.The higher withdrawal rate,lower heating power and less holding time were beneficial to refining the d1 and d2.