Zhang Jishan

Numerical investigation of the rebounding and the deposition behavior of particles during cold spraying

Particles may rebound from a substrate surface during cold spraying,which affects the quality of the coating.In this paper,the rebound phenomenon and its consequences on deposition behavior have been analyzed using the finite element analysis software ANSYS/LS-DYNA version 970.In a range of particle velocities of 50-1000 m/s,increases of the impact velocity caused a rapid decline of the rebound coefficient R to a low point Rmin.After that,R began to rise slowly.Then the effect of the impact velocities and material properties on the rebound phenomenon were analyzed.Both the material strength and density influenced this rebound phenomenon.Four stages of the impact process and a model of strain distribution were proposed in detail to explain the rebound phenomenon.

A preliminary study of the solidification behaviour of Y-Ba-Cu-O compounds

A preliminary study of the solidification behaviour and vertical sections of the BaCuO2-Y2BaCuO5-CuO pseudo-ternary system has been carried out. Results show that the superconducting phase YBa2Cu3O7-x melts incongruently, i.e. a peritectic reaction takes place at about 1010 degrees c with a wide solidification range which makes it difficult to grow bulk superconducting crystals. The binary eutectic reaction in a CuO-X section may be useful for directional eutectic growth.

EFFECT OF Nb ON THE MICROSTRUCTURE AND PROPERTIES OF SPRAY FORMED M3 HIGH SPEED STEEL

Spray forming with a short process chains has been proven to be a powerful tool for the production of high-alloyed materials.Niobium,as a strong former for the carbide,will mainly form primary MC carbides,such as NbC,which can be formed via the reaction between Nb and C atoms at the beginning of solidification,and it can act as the inoculants and refine the cast structure of steel which can mainly form primary MC carbides.M3 high speed steel with or without Nb addition were prepared via spray forming.The effect of Nb on the microstructure of spray formed M3 high speed steel was investigated by SEM,EDX and XRD methods;the friction performances of these two steels were studied by SRV high temperature tribometer and 3D white-light interfering profilometer.The results show that the amount of primary MC carbides can increase sharply while the reduction of the amount of primary M_2C due to the substitution of 2%Nb for 1%V(mass fraction) in M3 high speed steel.For the high cooling rate during the spray forming,the primary MC carbides can be refined and dispersed.Large number of primary MC carbides can improve the abrasive wear resistance of M3 high speed steel,but cannot enhance its oxidation resistance;M3 high speed steels containing Nb possess high tempering resistance.

HOT DEFORMATION BEHAVIOR AND MICROSTRUCTURAL STABILITY OF SPRAY FORMED Al-22Si-5Fe ALLOY

Hot deformation behavior of spray forming Al-22Si-5Fe alloy at different deformation temperatures with isothermal constant strain rates of 0.005, 0.01, 0.03, and 0.05 s −1 was investigated by using Gleeble-1500 thermo-mechanical simulator, with maximum strain of 30%, the microstruc- tures were studied by using of OM and XRD method. The experimental results showed that the hot deformation behavior of spray formed alloy could also be described by a model containing Z parameter (Zener-Hollomon parameter). The calculated deformation activation energy of the studied alloy was consistent with the self-diffusion activation energy of Al atoms at lower deformation temperature, and much higher than the self-diffusion activation energy of Al atoms at higher temperature because of the influences of large content hard constitutes such as prime Si and Fe-bearing intermetallics. The Si particles changed little when deformed at low temperature, but coarsened obviously when deformed at elevated temperature of 698 K or above. The heating temperature had no obvious effect on the sizes of Fe-bearing intermetallics.

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF SPRAY FORMED H13 TOOL STEEL

H13 steel is well-established for hot-work applications in different parts of the world, combining good hot strength, plasticity, toughness, oxidation resistance and thermal fatigue resistance due to its reasonable design of alloying elements. However, the alloying elements segregation and coarse primary carbides are serious and unavoidable in traditional cast H13 steel, which develops into band structure after the subsequent hot-working process, causing a poor isotropy, and finally reduces the service life of tool. Spray forming, as one of the rapid solidification technologies, fills the gap between casting and powder metallurgy, combining advantages of rapid solidification, and gaining a fine grained microstructure without macro-segregations. H13 steel was prepared by traditional casting and spray forming respectively, subsequently forged and conventional heat treated. The microstructures of as-cast and spray-formed H13 steels at various stages of processing were studied by OM, SEM and XRD. Mechanical properties were tested at ambient and elevated temperatures for both steels under the same heat treatment processes. The microstructure of as-cast H13 steel is characterized by coarse dendritic structure and primary carbides,while spray forming resulted in a much refined equiaxed grain structure without segregation of carbides. Meanwhile, the as-deposited H13 steel contains higher volume of austenite than that of the as-cast H13 steel, induced by the high cooling rate during the atomization process in spray forming. The density of the spray-formed H13 billet before and after hot-forging process are 98.2% and 99.7% respectively, indicating pores are completely eliminated.Compared to the as-cast H13, the spray-formed H13 has better temper resistance, higher room temperature tensile strength and hot strength. Moreover, the spray-formed H13 increases the room temperature impact toughness two times than that of the as-cast H13. The band structure which is obvious in the cast H13 steel can hardly be seen in the spray-formed H13 steel, thereby better isotropy can be expected in the spray-formed H13 steel. The improvement of mechanical properties of the spray-formed H13 can be attributed, to a large extent, to the refined structures without coarse primary carbides and macro-segregation, leading more alloying elements dissolved into the matrix,distributed more uniformly. Consequently, the secondary precipitates during tempering are more uniform and the grains are finer.

Alloying Effect on the Microstructure and Property of Single Crystal Ni3Al Alloys

The alloying effects of Cr, Zr, Ta, Nb and B on the microstructure and properties of single crystal Ni 3 Al alloys have been studied. The strength of the alloys has been improved markedly with promising plasticity. The 0.2% yield stress of some of these alloys is higher than that of IN713C, a typical cast nickel-base superalloy, in the temperature range from 25 to 950°C and some of them reach the strength level of advanced cast nickel-base alloys such as IN100 in the high temperature range. The creep strength of these alloys is also improved markedly. The single crystal Ni 3 Al alloy with a high Cr content ( 7 at-%) obviously shows an intermediate temperature low plasticity behavior. Although heat treatment can only slightly improve the strength, it can double the elongation of the alloy from 25% to 50%. In the Ni 3 Al monocrystal containing 1.2at%Zr there exist great amounts of the low melting point Ni 5 Zr-γ eutectics around the ordinary γ/γ′ eutectics. This makes it difficult to homogenize the coarse casting structures completely.

STUDY ON THE KINETICS OF PHASE TRANSFORMATION β→α IN THE HOMOGENEOUS HEAT TREATMENT OF Al－Mg－Si SERIES ALLOYS

According to the present experiment results about the transformation fromβ-AlFeSi toα-Al(FeMn)Si phase in Al-Mg-Si series alloys,the model and kinetics of phase transformation, and microstructure evolution at 555℃were systematically investigated.The results show that, the homogeneous temperature T can give a significant effect on the transformation kinetics,and with increasing T,the transformation rate increases fast.The nucleus radius ofαphase only affects the breakthrough time along the thickness direction ofβphase.The distance / between twoαphases mainly has an influence on the dissolving of rim forβphase.The thickness ofβphase normally has a greatest influence on the phase transformationβ→α,especially for the breakthrough stage ofβphase,with increasing the thickness ofβphase,the main transformation with longer time in the whole phase transformation process can changes from rim dissolving ofβphase to breakthrough along the thickness direction ofβphase.The breakthrough time ofβphases with the thickness of 0.1,0.2 and 0.3μm is above 5,10 and 15 h,respectively,which corresponds with the real experiment results.In addition,if the thickness ofβphase is smaller than 0.3μm,the transformation rate f(α) easily reaches above 0.9 after homogeneous heat treatment at 555℃for 16—24 h.Therefore, the importance of optimizing casting process to make sure the thickness ofβphase below 0.3μm, is the same as the homogeneous heat treatment for the better controlling the phase transformationβ→α