Search

Bulk and thin samples of Cu75Ni25 (at. pct) alloy were solidified over a range of undercoolings. Unlike the bulk samples, in which grains were significantly refined at undercoolings between 45 K and 75 K, the thin samples solidified consistently into developed dendrites till the experimentally achieved largest undercooling of 150 K. The thin samples were subsequently rapidly reheated at a rate of 300 K/s to different target temperatures between the solidus and liquidus temperature interval. Letting the target temperature of heating equal to the highest recalescence temperature in the bulk sample, grain-refined microstructures were obtained in the thin sample that initially solidified at an undercooling between 45 K and 75 K. For the other thin samples, however, the dendritic morphology was still maintained. Raising the target temperature of heating to an enough high level, grain refinement also occurred in these thin samples. It is demonstrated grains are refined only when large-scale remelting occurs in the original dendrite stems with a critical remelted fraction of about 0.82 at the stem center. [ABSTRACT FROM AUTHOR]

Ni75B25 alloy was solidified at various undercooling. The formation and subsequent transformation of metastable Ni23B6 phase were clearly identified. If undercooling prior to nucleation is less than a critical value of 240 K (240 °C), the alloy solidifies completely into Ni3B phase. At larger undercooling, metastable Ni23B6 phase primarily forms in the melt but then is decomposed into α-Ni and Ni3B through a eutectoid reaction. The decomposition simultaneously triggers the rapid solidification of residual liquid, due to which a second temperature recalescence occurs. The α-Ni/Ni3B eutectoid is partially remelted if temperature exceeds the eutectic temperature during the second recalescence. Then, residual Ni3B grows into coarse round grains while the remaining liquid re-solidifies into α-Ni/Ni3B eutectic structure in the remelted region. In the case that the eutectic temperature is not reached, the eutectoid product with dot α-Ni distributing in Ni3B matrix is retained in the solidification structure. A longer delay time between the two temperature recalescence events means less residual liquid, lower recalescence temperature and thus depressed remelting. The formation competition between Ni3B and Ni23B6 phases in the alloy melt is nucleation controlled. The heterogeneous site in Ni75B25 alloy melt is a better nucleation substrate for Ni23B6 phase than for Ni3B phase. [ABSTRACT FROM AUTHOR]

Ti-6Al-4V alloy is an attractive material in many industries due to its unique and excellent combination of strength to weight ratio and their resistance to corrosion. However, because of its low thermal conductivity and high chemical reactivity, Ti-6Al-4V alloy is generally classified as a difficult-to-cut material that can be characterized by low productivity and rapid tool wear rate even at conventional cutting speeds. It is well known that tool wear has a strong relationship with the cutting forces and a sound knowledge about correlation between cutting forces variation and tool wear propagation is vital to monitor and optimize the automatic manufacturing process. In the present study, high-speed end-milling of Ti-6Al-4V alloy with uncoated cemented tungsten carbide tools under dry cutting conditions is experimentally investigated. The main objective of this work is to analyze the tool wear and the cutting forces variation during high-speed end-milling Ti-6Al-4V alloy. The experimental results show that the major tool wear mechanisms in high-speed end-milling Ti-6Al-4V alloy with uncoated cemented tungsten carbide tools are adhesion and diffusion at the crater wear along with adhesion and abrasion at the flank wear. The cutting force component in the negative y-direction is more dominant of the three components and displays significantly higher magnitudes than that of the other two components in x- and z-directions. The variation of cutting force component Fy has a positive correlation with the tool wear propagation, which can be used as a tool wear indicator during automatic manufacturing process. [ABSTRACT FROM AUTHOR]

By adopting an equivalent geometry model of chip, a finite element model was developed to study the mechanism of chip formation during high-speed milling of alloy cast iron. Several key technologies such as material constitutive model, friction model, chip separation criteria, chip damage criteria, heat dissipation, and transfer were implemented to improve the accuracy of finite element simulation. Saw-tooth chip of alloy cast iron was observed. The chip shape and cutting force agreed well with experimental results. The simulation results show that the maximum cutting temperature produced with appearance of saw-tooth chip crack, and it is located on the chip-tool contact surface. The saw-tooth chip is caused by double actions of thermoplastic instability and plastic instability. The chip saw-tooth degree decreases when increasing the rotating speed, while it increases when increasing the feed speed. This work provides a useful understanding for chip formation process and helps to optimize machining parameters and process of high-speed milling of alloy cast iron. [ABSTRACT FROM AUTHOR]

Ni-18.7 at. pct Sn eutectic melt was undercooled and spontaneously solidified in the encasement of a glass flux. Structure morphologies of the sample surface as well as inside the sample were systematically examined, and a critical undercooling of 130 K was clearly revealed for the alloy. Below the critical undercooling, coupled lamellar eutectics (α-Ni and β-Ni[sub3]Sn) dendritically grow in the undercooled melt. Beyond the critical undercooling, α-Ni dendrites first form during the early recalescence. β-Ni[sub3]Sn nucleates uniformly in the remaining liquid and then separately grows with α-Ni. Solidification of the remaining liquid into lamellar eutectics only occurs at the places in the sample surface layer where the space between the α-Ni dendrite arms is large enough. The finding that all the solidification structures at undercooling above 20 K comprise anomalous eutectics indicates that both coupled eutectic growth and decoupled dendritic growth in the rapid solidification can result in the anomalous eutectic formation. The results also indicate that it is feasible to measure the crystal growth velocity by observation of the recalescence front when undercooling exceeds 50 K for this alloy. [ABSTRACT FROM AUTHOR]

Discusses the relation between the mother ingot microstructure and the glass forming ability of a bulk glass forming alloy. Amorphous alloys are formed when the liquid atoms are frozen into a non-crystalline arrangement; Glasses can be produced at cooling rates as low as about 102 K/s in some multicomponent systems; Liquid state will determine the size of the short-ranged orders in the glassy phase.

Anomalous eutectics in the solidification structure of undercooled Ni-18.7 at.% Sn eutectic alloy were examined by optical metallography and electron backscattered diffraction. It was revealed that a-Ni particulates are, in principle, randomly distributed in the anomalous eutectics in the undercooling range investigated. Another eutectic phase, β-Ni3Sn, is well orientated at low undercoolings but gradually becomes inconsistent in orientation as undercooling increases, accompanied by an increasing number of grain boundaries in it. As the solidification structure changes from a mixture of anomalous eutectics plus lamellar eutectics to full anomalous eutectics beyond a critical undercooling of 130 K, however, misorientation in the β-Ni3Sn phase disappears completely from the measurement area. Partial remelting of the primary solid was proposed to be the origin of the anomalous eutectic formation, and quantitative analyses were performed. [ABSTRACT FROM AUTHOR]