T.Q. Lei

Crystallization of amorphous Zr65Cu27Al8 alloy with wide supercooled liquid region

The crystallization processes of amorphous Zr75Cu27Al8 alloy, both from amorphous state and supercooled liquid, were investigated. It was mainly controlled by the growth of pre-existing quenched-in three-dimensional nuclei with a constant growth rate. It was observed that the activation energy is much higher in the transition from amorphous state than that from supercooled liquid. The crystallization product from both initial states consists of the b.c.t. Zr2Cu phase and an unknown phase with f.c.c. structure

LASER SURFACE MODIFICATION OF TITANIUM ALLOYS—A REVIEW

Recent developments of laser surface modification of titanium alloys for increasing their corrosion, wear and oxidation resistance are introduced. The effects of laser processing parameters on the resulting surface properties of titanium alloys are reviewed. The problems to be solved and the prospects in the field of laser modification of Ti alloys are discussed. Due to the intrinsic properties, a laser beam can be focused onto the metallic surface to produce a broad range of treatments depending on the input energy. Thus, composite strengthening coatings can be fabricated by the methods of laser alloying, cladding, pulse laser deposition (PLD), etc., which are promising techniques of producing a layer of new materials on the surface of titanium alloys.

MICROSTRUCTURES AND WEAR PROPERTIES OF THE COMPOSITE COATINGS LASER-FABRICATED ON TITANIUM ALLOY Ti-6Al-4V

Composite coatings containing titanium carbides and borides are produced on the surface of Ti-6Al-4V alloy by laser alloying technique. Microstructural analysis shows that coarse bulk-like and dendritic compounds are formed when alloyed with boron and carbon, separately. However, fine and uniformly distributed dendritic compounds are produced when alloyed with boron and carbon mixed powders. With microhardness of 1450–1600 HV0.1, the coatings have excellent wear resistance compared with as-received sample.

WEAR AND OXIDATION RESISTANCE COATINGS FABRICATED ON Ti-6Al-4V BY LASER SURFACE ALLOYING TECHNIQUE

Composite coatings are produced by laser surface alloying of Ti-6Al-4V with B+Ti and B+Ni+Ti mixed powders. Test results show that the coatings have high hardness, excellent wear and oxidation resistance compared with the as-received sample. In addition, the samples alloyed with B+Ni+Ti have better wear and oxidation resistance than that alloyed with B+Ti.

ANALYSIS OF GROWTH MECHANISM OF DENDRITIC TITANIUM CARBIDES PRODUCED BY LASER ALLOYING OF PURE TITANIUM WITH GRAPHITE POWDER

The microstructure of dendritic titanium carbides fabricated by laser surface alloying of pure titanium with graphite powder is investigated using electron probe microanalyzer (EPMA) and high-resolution transmission electron microscope (HRTEM). The growth mechanism of TiC crystals is that crystal grows by the model of continuous growth at the beginning of solidification and then the crystal surfaces that are suited to lateral growth develop fully by the model of spiral growth. So, the final morphology of dendritic TiC looks like it is composed of tiny crystal grains in a linear arrangement.

ANTIFRICTION PROPERTY OF MICROARC OXIDATION COATING ON TITANIUM ALLOY UNDER SOLID LUBRICATING SLIDING CONDITION

Ceramic coatings were fabricated on Ti6Al4V alloy surface by microarc oxidation (MAO) in Na2SiO3–(NaPO3)6–NaAlO2 solution using an AC power supply. Microstructure and phase composition of coating were characterized by SEM and XRD, respectively. The antifriction property of the coating with and without solid lubricant sliding against SAE 52100 steel ball was investigated on a pin-on-disk friction and wear tester. The results show that the microarc oxidation coating is relatively dense and uniform, mainly composed of rutile and anatase. The coating sliding against the steel has friction coefficient as low as 0.2–0.3 at an applied load of 0.5 N and sliding cycle below 2500, which is much smaller than that of uncoated Ti6Al4V against the same counterpart. The transferring of materials from the softer steel ball onto the coating surface is the main wear event, while the microarc oxidation coating is characterized by slight abrasive wear and adhesive wear. Introducing solid graphite lubricant into the porous surface of microarc oxidation coating significantly improves the long-term antifriction property (registering friction efficient of 0.15 in the long-term sliding) under a similar sliding condition. This improvement is attributed to the reduction of materials that are transferred from the softer steel ball onto the coating surface.