Tham khảo tài liệu 'volume 13a - corrosion fundamentals, testing, and protection part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | í l Sweep speed in. s tb Fig. 7 Effects of laser-processing conditions on melt depth a For three materials at one power level b For steel at two power levels Product Characteristics Solidification Effects. The heat flow characteristics of such processing can reasonably be described by dimensional heat flow in which the rate at which energy enters the surface is balanced by the rate at whicl is conducted away by the cool substrate Ref 53 . A number of studies have been devoted to characterizir temperature profiles that result from the laser and electron beam processing of materials Ref 39 41 54 character of the temperature profiles experienced by material at various depths below the surface of the mi shown in Fig. 8. The three reference temperatures indicated represent liqụ dssl dus s and solid- state transformation Xr temperatures. As the laser beam sweeps across the surface of a metal sample the SL begins to heat. When the melting temperature of the material is exceeded a melt front is established and r from the free surface of the sample toward the bulk. Shortly after the beam is turned off or has moved av the melt front slows and then stops. It then moves back toward the free surface as a solidification front unique characteristics of this solidification front are the high cooling rates suctĩ ifâs ầnd the steep temperature gradient such as51 cm . These characteristics promote the formation of metastabk compositions and structures. Time Fig. 8 View of laser-melted surface and temperature profiles experienced at different points on the surface during laser melting. Liquidus Tl solidus Ts and solid-state transformation Tr temperatures are indicated. Because of this steep temperature gradient there is only a thin layer of metal that is cool enough to sol The material above this layer is above the liquidus temperature and the material below this layer has air solidified. There is a strong epitaxial character to the resolidification as a result of the steep tempers .
