The effect of the reactive zone in graphite CFC 222 and TZM molybdenum-alloy brazed joints

摘要

Attempts at brazing CFC 222 composite were started by graphite-wettability tests with active, copper matrix, solders. As a result, the CuCr1 (with 1 percent by weight of active chromium) solder was selected. The task of this component was to form an interface zone on the boundary with graphite. The reactive layer, rich in chromium carbides - predominantly Cr_2C_3 - makes it possible to produce wetting with solder, and to make a joint between CFC 222 graphite and TZM. The chromium, diffusing from the CuCrl solder, increases the capacity of the joint to transmit ductile strains. From the point of view of the joint loading, a narrow reactive zone of a width less than 2 mu m is the most desirable. The optimal brazing gap of about 130 mu m in vacuum is obtained at T_(br) velence 1150 deg C, t_(br) - 5 min and p velence 10~(-4) hPa. An increase in the gap width, and, therefore in the braze thickness, increases the width of the reactive zone. Because of its significant hardness (1631 HV0.015), and accompanying brittlenes, this is undesirable. Also, an increase in the content of the active chromium of up to 5 percent in the soldering paste~produces l mu m wide reactive zone. A zone of such width tends to be susceptible to transverse cracking (see figure 5).