Yaohe Zhou

The influences of processing parameters on forming characterizations during laser rapid forming

Abstract Laser rapid forming experiments were carried out with 316L stainless steel and nickel-base alloy to investigate the influences of the processing parameters on forming characterizations systematically. It is found that the height of a single cladding layer was very important for fabrication accuracy and forming stability of components of laser rapid forming because it was affected by almost all processing parameters and was quite difficult to precisely control. For the system with lateral powder feeding pattern, the powder injection point was the most important factor to the height control of single layer. The variation of the width of single clad, which was mainly affected by laser power, spot diameter and scanning velocity, was similar to that in laser surface melting. The surface quality was another important characterization for laser rapid forming and was remarkably affected by oxidation and the powder adhesion. In order to improve the surface quality, the flow flux of shielding gas should be ≮10 l min −1 and the powder stream cannot be injected to the tail part of the melt pool. Based on the investigation, some metal components were fabricated.

Mechanically driven phase separation and corresponding microhardness change in Cu60Zr20Ti20 bulk metallic glass

Rolling deformation of bulk Cu60Zr20Ti20 metallic glass has been performed at cryogenic temperature. The specimens exhibit excellent ductility, and are rolled up to 97% reduction in thickness without fracture. Crystallization is suppressed during the deformation, however, phase separation is observed in the glassy matrix when the thickness reduction exceeds 89%. Once the phase separation occurs, the microhardness of the specimen increases drastically, indicating the existence of work hardening by severe plastic deformation of the metallic glass.

Solidification structure formation in undercooled Fe-Ni alloy

Abstract The Fe alloy melts containing 7.5, 15, 22.5 and 30 at% Ni were bulk undercooled to investigate the structure evolution. When the undercooling of the four melts is lower than the critical value 110, 125, 175 and 325 K, respectively, only the stable face-centered cubic phase crystallizes. In this case a grain refinement caused by solid superheating is observed in all the alloys, but another grain refinement induced by recrystallization can merely occur in the Fe–30 at%Ni alloy undercooled by 190–325 K. Alternate crystallization of the metastable body-centered cubic phase occurs above the critical undercooling. It is indicated that the subsequent heterogeneous nucleation of the stable phase in the metastable solid and remaining liquid coexisting system is influenced not only by the morphology and surface area of the metastable solid, but also by the effective undercooling of the remaining liquid. On the basis of the experimental results and the theoretical analyses, a structure evolution map for bulk Fe–Ni system is constructed.

Free-volume evolution and its temperature dependence during rolling of Cu60Zr20Ti20 bulk metallic glass

The free-volume evolution during rolling Cu60Zr20Ti20 bulk metallic glass at room and cryogenic temperatures has been investigated by differential scanning calorimetry. When the specimen is rolled at cryogenic temperature, the free-volume content increases as the rolling proceeds first, and then saturates accompanied by the occurrence of phase separation as the thickness reduction exceeds 89%. If the rolling is performed at room temperature, although the free-volume content also rises in the earlier stage, it tends to decrease rather than saturate when the thickness reduction exceeds 87%, accompanied by partial crystallization. Phase separation does not change the annihilation rate of free volume, while the appearance of crystal/amorphous boundaries can enhance the annihilation.

History-dependent selection of primary cellular/dendritic spacing during unidirectional solidification in aluminum alloys

History-dependent selection of primary cellular/dendritic spacing is investigated systematically during unidirectional solidification of a series of aluminum alloys. A single crystal is formed in the sample before each experimental run, so that the influence of grain boundary on the primary spacing is avoided. The experimental results are compared with those of the two-dimensional crystal growth in the same alloy system and transparent model alloys. It is found that the primary cellular/dendritic spacing is remarkably history dependent. The average primary spacing is dependent not only on the current growth conditions, but also remarkably on the way those conditions were achieved. There exists a wide allowable range of primary spacings for a given growth condition. Experimental results are also compared with the Hunt-Lu model, which shows excellent fit between them, especially on the selection of cellular spacing. By comparing the three-dimensional experiments with the two-dimensional ones, it is also found that the allowable range of the primary spacing for the three-dimensional growth is wider than that for the two-dimensional growth.

Effects of melt thermal treatment on hypoeutectic Al-Si alloys

Abstract A new refining method—melt thermal treatment without additive to melt is studied in this paper. Microstructure analysis and property evaluation of hypoeutectic Al–Si alloys treated with this method show that the solidification microstructure can be refined significantly with a considerable increase in elongation ratio and strength. Effects such as cooling rate, holding time and alloy composition on the solidification microstructure and mechanical properties have been evaluated. It is shown that the strengthening and toughening effects on the treated samples vary with alloy composition. The property increment of the alloy rich of iron is relatively more remarkable than those rich of Cu or Mg element. Specifically, the structure of the low temperature melt is identified as a primary factor, on which the solidification structure of the treated melt is dependent.

On primary dendritic spacing during unidirectional solidification

Abstract In-situ observations of directionally solidifying interface of typical transparent model alloys, succinonitrile-ethanol and succinonitrile-acetone, have been performed on a Hunt-Jackson type temperature gradient stage and a Bridgman apparatus, respectively. Experimental results show that there exists a wide allowable range of primary spacings for both the two-dimensional and the three-dimensional dendritic arrays under a given growth condition. The upper and lower limits of the allowable range are very sharp at the same current growth condition, while the average primary spacing is remarkably related to the history of variation of both temperature gradient and growth velocity. The upper limit, λmax, and the lower one, λmin, of the allowable range as a function of growth velocity, V, obtained from the experiments of step-increment and step-decrement in growth velocity, can be generally expressed as λmax = AV−b, λmin = cV−d, where a, b, c and d are constants for given alloy and temperature gradient. The lower limit obtained experimentally by us is compared with that calculated theoretically with the Hunt-Lu model and the Warren-Langer model; it shows an excellent fit between experimental results and the Hunt-Lu model, and good agreement between experimental results and the Warren-Langer model at the high growth velocity.

Mode of dendrite growth in undercooled alloy melts

Abstract The mode of dendrite growth in the undercooled Ni–50at%Cu alloy was investigated. At lower undercoolings, the dendrite growth is mainly controlled by solute diffusion, and the formed dendritic morphologies are similar to those of the conventional as-cast equiaxed crystals, except that here the branches are much denser. At higher undercoolings, however, the severe solutal trapping that results from high dendrite growth velocity weakens the effect of solute diffusion on the dendrite growth. In this case, the dendrites branch in the bunching form. The dendrite spacings were measured, and the results were interpreted with the current dendrite growth theories.

Prediction of average spacing for constrained cellular/dendritic growth

Abstract Since 1990, it has been gradually understood that there exists an allowable range of cellular/dendritic spacings during unidirectional solidification. The lower and upper limits of the allowable range are very sharp at the same current growth condition, while the average spacing for cellular/dendritic arrays depends on the history of variation of solidification parameters. The present work puts forward a novel numerical model to predict the ultimate average spacing for constrained cellular and dendritic growth, according to the characteristics of unsteady-state evolution of cellular and dendritic arrays. Our analysis considering the history-dependent selection of the average spacing for cellular/dendritic arrays, we find — with no adjustable parameters — good agreement with experimental results obtained in different alloys and in different velocity histories.

Microstructure evolution and metastable phase formation in undercooled Fe-30 at.% Co melt

Abstract The microstructure evolution and phase selections of Fe–30 at.% Co alloy at various undercoolings were investigated in this paper. The metastable body-centered cubic (b.c.c.) phase was detected in a highly undercooled Fe–30 at.% Co alloy. Applying the regular solution model, the equilibrium Fe–Co alloy phase diagram was evaluated and its metastable extension was taken into account to explain the formation of the metastable b.c.c. phase. Based on the classical nucleation theory, the activation energy ΔG* and the steady-state nucleation rate Ihs have been calculated in terms of stable face-centered cubic (f.c.c.) (γ) and metastable b.c.c. (δ) phases. The crystal growth velocities of b.c.c. and f.c.c. phases as a function of undercoolings ΔT and tip radius R were also given according to Bottinger, Coriell and Trivedi (BCT model). The critical undercooling for the formation of metastable b.c.c. (δ) phase in Fe–30 at.% Co alloy was 204 K. The transmission electron microscopy results indicated that the microstructure of the primarily nucleated b.c.c. phase exhibited fine dendritic structure. The composition analysis showed that the b.c.c. dendrite structure was enriched in cobalt.