LAUSR

The emissivity models of aluminum 7075 were studied during the growth of oxide layer over the temperature range from 800 to 910 K at a wavelength of 1.5 μm. In the experiment, the samples were heated to a certain temperature and kept at that temperature for approximately 6 h. To determine the accurate emissivity, the surface temperature of specimens was measured by the two thermocouples, which were symmetrically welded onto the front surface of samples. The average of their readings was regarded as the true temperature. Eleven models were employed to evaluate the variation of emissivity with temperature or growth of oxide film on the specimen surface at a certain condition. The effect of the number of parameters used in the models on the fitting quality was studied at a certain temperature. The conclusion was obtained as more the number of parameters used in the models, the better the fitting quality became on the whole. The variation of emissivity with temperature was investigated at a certain thickness of oxide layer. The three approximate models of variation in emissivity with temperature and thickness of oxide film were proposed. The strong oscillations of emissivity were observed during the initial heating period, which were affirmed to arise from the interference effect between the two radiations stemming from the oxide layer and coming from the substrate.