Higher Thermal Rate Research Articles

3D thermal finite element analysis of laser cladding processed Ti-6Al-4V part with microstructural correlations

In this study, a 3D thermal model of laser cladding by powder injection applied to Ti-6Al-4V is developed. The manufactured part is made of a Ti-6Al-4V substrate on which successive layers of laser melted powder are added, leading to a thick deposit. The computed temperature field and its time evolution are compared to experimental measurements. The temperature distribution in the substrate allows the prediction of the depths of the melt pool and the heat affected zone. Correlations between simulated thermal histories and the final microstructure in the thick deposit are established, leading to the enhancement of a dynamic shift of the critical transformation points due to high thermal rates during laser processing. The nature of the phases present within the deposit is discussed.

Phase transformation analysis in titanium at nanosecond time resolution

Polycrystalline and mosaic bicrystalline titanium films were subjected to steep heating/cooling rate of 1011∕108Ks−1 by laser pulsing. The induced phase transformations were followed by imaging and diffraction with a dynamic transmission electron microscope on the time scale of nanoseconds. On heating the film up to near the melting point with a 6-ns laser pulse, the low-temperature hcp phase transformed to the high-temperature bcc phase, with a nucleation rate of 1025m−3s−1 and a crystal-growth velocity of about 1000ms−1. Quenching of molten Ti first produced the bcc phase, which in turn transformed to the hcp phase within a few microseconds. Thus, hcp-bcc transformations occur in Ti, even at the above high thermal rates. They are martensitic and not diffusion limited as claimed for transformations at low thermal rates.

Elimination, via high-rate laser dilatometry, of structural relaxation during thermal expansion measurement of dental porcelains

Objectives: Thermal expansion measurement of glassy materials is complicated by thermal history effects. Excess volume—trapped in quenched dental porcelains after firing—collapses via structural relaxation on first slow heating during conventional dilatometry, making the thermal expansion coefficient ( α) obtained on first heating unreliable. The purpose of this study was to determine whether porcelain thermal expansion measurement at high thermal rates could minimize the influence of thermal history. Methods: Eight thermal expansion specimens each of six body porcelains and the Component No.1 (leucite-containing) frit prepared according to the patent by Weinstein et al. (US Patent No. 3,052,982) were subjected to three heat–cool conventional dilatometry runs at 3°C/min, while eight thermal expansion specimens of each porcelain were reserved as untreated controls. Eight hollow, cylindrical specimens of the same brands were subjected to three heat–cool laser dilatometer thermal expansion runs at 600°C/min, while eight cylindrical specimens of each porcelain were reserved as untreated controls. Thermal expansion data (25–500°C) of all specimens were subjected to repeated measures analysis of variance. Results: The α obtained on first slow heating was significantly lower than values for succeeding slow heat and cool runs in all porcelains ( P<0.001). High-rate α obtained on first heating was not significantly different from values of succeeding heat and cool runs in all porcelains ( P>0.05). Significance: Conventional dilatometer measurements demonstrated occurrence of structural relaxation, as evidenced by the significant difference in the first heating and subsequent runs. High-rate laser dilatometry eliminated structural relaxation, thereby providing a thermal expansion measurement that is free of interference from thermal history effects.