H.B. Tang

Microstructure and wear resistance of laser clad TiB–TiC/TiNi–Ti2Ni intermetallic coating on titanium alloy

Abstract A wear resistant TiB–TiC reinforced TiNi–Ti 2 Ni intermetallic matrix composite coating (TiB–TiC/TiNi–Ti2Ni) was prepared on Ti–6.5Al–2Zr–1Mo–1V titanium alloy by the laser cladding process using Ti+Ni+B 4 C powder blends as the precursor materials. Microstructure and worn surface morphologies of the coating were characterized by optical microscopy (OM), scan electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS) and atomic force microscopy (AFM). Wear resistance of the coating was evaluated under dry sliding wear test condition at room temperature. The results indicate that the laser clad coating has a unique microstructure composed of flower-like TiB–TiC eutectic ceramics uniformly distributed in the TiNi–Ti 2 Ni dual-phase intermetallic matrix. The coating exhibits an excellent wear resistance because of combined action of hard TiB–TiC eutectic ceramic reinforcements and ductile TiNi–Ti 2 Ni dual-phase intermetallic matrix.

Low cycle fatigue behavior of laser melting deposited TC18 titanium alloy

Abstract Low cycle fatigue (LCF) behavior of laser melting deposited (LMD) TC18 titanium alloy was studied at room temperature. Microstructure consisting of fine lamella-like primary α phase and transformed β matrix was obtained by double annealed treatment, and inhomogeneous grain boundary α phase was detected. Fatigue fracture surfaces and longitudinal sections of LCF specimens were examined by optical microscopy and scanning electron microscopy. Results indicate that more than one crack initiation site can be detected on the LCF fracture surface. The fracture morphology of the secondary crack initiation site is different from that of the primary crack initiation site. When the crack grows along the grain boundary α phase, continuous grain boundary α phase leads to a straight propagating manner while discontinuous grain boundary α phase gives rise to flexural propagating mode.

Effect of Hot Isostatic Pressing on Fatigue Properties of Laser Melting Deposited AerMet100 Steel

The ultra-high strength steel AerMet100 was fabricated by laser melting deposition (LMD) process. The effect of hot isostatic pressing (HIP) on high-cycle fatigue properties of the LMD AerMet100 steel was investigated, and the influence of defects on fatigue behavior was discussed. Results showed that the LMD AerMet100 steel had fine directionally solidified cellular-dendrite structure and coarse columnar prior austenite grains. Metallurgical defects such as gas pore and lack-of-fusion porosity were produced during the laser deposition process. After HIP treatment, the number and size of metallurgical ddects had remarkably decreased. Moreover, high-cycle fatigue properties of the alloys after HIP treatment were superior to the as-deposited alloys.

Microstructure evolution of laser deposited Ti60A titanium alloy during cyclic thermal exposure

Cyclic thermal exposure tests of infrared heating to 800 °C in 120 s followed by compressed air cooling to 150 °C in 60 s were performed for the laser deposited Ti60A (Ti5.54Al3.38Sn3.34Zr0.37Mo0.46Si) alloy. The effects of thermal exposure cycles on length of β phase, area fraction of α phase and microhardness of alloy were examined by OM, SEM and EDS. The results indicate that thermal exposure cycles have significant effects on length of β phase, area fraction of α phase and microhardness of the alloy. The original fine basket-weave β and 78.5% α transform to transient wedge-like β, finally leaving granular β and 97.6% coarsened α with the increased thermal exposure cycles. The formation mechanism of coarsened α and broken-up β microstructure is discussed. The alloy after 750 thermal exposure cycles has the maximum microhardness, 33.3% higher than that of the as-deposited alloy.

Microstructure and Mechanical Properties of Plasma Arc Welding Joint for Laser Melting-Deposited AerMet100 Ultrahigh-Strength Steel

The repair of laser melting-deposited AerMet100 ultrahigh strength steel (UHSS) heat-treated samples with groove machined was conducted by low-cost plasma arc welding (PAW). And the microstructure and mechanical properties of welding joint were examined by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-hardness test and the tensile mechanical test. The experimental results indicated that the welding zone with low hardness values mainly consisted of columnar grains with about 200μm width which epitaxial growth from substrate grains, and in which the cellular morphology character appearing at the bottom in comparison with dendrite with lateral branching appearing at the top. Three zones, i.e., sufficient quenched zone, insufficient quenched zone and high-temperature tempered zone, were divided by heating affected temperature and microstructure characteristic in heat affected zone (HAZ), and there was a lowest hardness value region distributed in high-temperature tempered zone. Compared to that of undamaged heat treated forged one, the tensile mechanical property of the repaired laser melting deposited sample got a few decrease but was still well, in which the tensile strength σb, yield strength σs, elongation δ5 and reduction of area Ψ was 1627Mpa, 1285Mpa, 10.5% and 45% respectively. In addition, the isothermal thermal simulation test surveyed that the tensile fracture position locating in high-temperature tempered zone with the lowest hardness value could ascribe to the growth of alloy carbide and increase of reverted austenite in over-aged temperature.