A substantial undercooling up to 250 K was produced in the IN718 superalloy melt by employing the method of molten salt denucleating, and the microstructure evolution with undercooling was investigated. Within the achieved undercooling, 0–250 K, the solidification microstructure of IN718 undergoes two grain refinements: the first grain refinement occurs in a lower range of undercooling, which results from the ripening and remelting of the primary dendrite, and at a larger range of undercooling, grain refinement attributes to solidification shrinkage stress and lattice distortion energy originating from the rapid solidification process. A ‘lamellar eutectic anomalous eutectic’ transition was observed when undercooling exceeds a critical value of ∼250 K. When undercooling is small, owing to niobium enrichment in interdendrite, the remaining liquid solidifies as eutectic ( γ+Laves phase); whereas, if the undercooling achieves 250 K, the interdendrite transforms from eutectic ( γ+Laves phase) to Laves phase, which results from the formation of divorced eutectic arising from the huge variance of the growth velocities of γ and Laves phases.

Functional-surface microstructures are widely used in industrial practice. During the fabrication of microstructures in micro-electrical discharge machining (micro-EDM), the thermal and physical characteristics of both workpieces and electrode materials at room temperature and high temperatures have an important influence on surface quality and distribution of recast layer. In order to study the influence of different electrode material characteristics on the surface integrity of microstructures machined using micro-EDM, red copper, brass, copper-tungsten and tungsten electrode were used to perform micro-EDM on both Ti-6Al-4V alloy and 304 stainless steel. In the experiment, electrode with groove arrays featuring high copying accuracy and surface quality was designed to carry out powder mixed electrical discharge machining (PMEDM) on Ti-6Al-4V alloy, and the machining results were evaluated based on four indicators: microstructure morphology, tool electrode wear (TEW), material removal rate (MRR), and recast layer thickness (RLT). Simultaneously, the surface morphology and recast layer thickness changes of 304 stainless steel workpieces machined using the above four types of electrodes, using both normal polarity and negative polarity micro-EDM were quantitatively analyzed. The results showed that copper-tungsten electrode is recommended to machine Ti-6Al-4V alloy because it has a smaller TEW (139 µm), the highest MRR (255.39 mm3/min), and a thinner recast layer thickness (3.35 µm). This was followed by copper electrode, which featured good machining performance and machinability. When machining 304 stainless steel with negative polarity, the TEW of copper electrode and tungsten electrode was the smallest, and the thickness of recast layer was able to be effectively reduced to about 3 µm.

In order to explore the way to improve the adhesion of the calcium phosphate bioceramic coating to Ti substrate, the CaTiO 3 coating was fabricated on Ti substrate by laser cladding (LC) using powders of CaCO 3 and CaHPO 4 , and then the composition and microstructure of the coatings were investigated. During LC, CaCO 3 can hardly react with Ti, and the coating fabricated using CaCO 3 powder is mainly composed of the process of CaO, the decomposition product of CaCO 3 . Moreover, the coating has a loosened structure and part of it has peeled off from the substrate. CaHPO 4 reacts vigorously with Ti, and the coating fabricated using CaHPO 4 mainly consists of CaTiO 3 which is one of the reaction products between Ti and CaHPO 4 . Chemical bonding is formed at the interface between coating and substrate, which may enhance the adhesion of the CaTiO 3 coating to Ti substrate. Furthermore, CaTiO 3 dendrite and eutectic of CaTiO 3 and Ca 2 P 2 O 7 are found on the surface of the coating, implying that a transition can be formed between CaTiO 3 and some calcium phosphate bioceramic. So CaTiO 3 coating fabricated using CaHPO 4 can be a potential candidate to improve the adhesion between calcium phosphate coating and Ti substrate. However, there are also pores and cracks existing in the coating, which may degrade the mechanical properties of the coating.