Recently, some brazed parts of mechanical products are used under a conditiom receiving fatigue stress in the mechanical industry.
The purpose of this study is to clarify the effect of surface roughness of base metal on the fatigue strength of the brazed joint. The materials used in this ezxperiment are mild steel (SS41) and BAg-8 (Ag 72%, Cu 28%). In this experiment, each brazing surface of base metal was polished with various grade emery papers. (No 60, 120, 240, 400, and 600 mesh) And also, the brazing surface of each specimen was machined by the following processes :
(1) Maching by plaim milling cutter
(2) Grinding by 60 mesh grinding wheel
(3) Grinding by 80 mesh grinding wheel
(4) Electrolytic polishing
(5) Super fimishing by 240 mesh grinding stone
In these specimens, the roughness of base metal machined by milling cutter was very high, its value being 14μ Rmax, and the others were lower, their values being 1.6-0.6μ Rmax. As carried out hithrto, these specimens were brazed with BAg-8 filler metal in the fixture as shown in Photo 1 using the 35 KW resistance furnace in hyrogen atomsphere. Then, these brazed spcimens were machined and ground according to design of Fig I.
Fatigue test of these specimens was carried out by a rotary bending machine, and it gave results as follows :
(1) In the specimens polished with emery papers, the fatigue strength of specimen polished with 120 mesh emery paper was very high, its value being 17.5 Kg/mm
2 and that of one polished with 600 mesh emery paper was lowest its value being 10.6 Kg/mm
2.
(2) In the case of the machined specimens, the fatigue strength of specimens machined by milling cutter was high, its value being 17.4 Kg/mm
2, and those of other specimens machined by various processes were lower, their values being 12.0 Kg/mm
2-10. Kg/mm
2.
(3) From these results, it is clear that the fatigue strength of coarse surface specimens is higher than that of smooth surface specimens. This comes from the difference in the actual brazed surface area of each specimn and other physical properties. And such a tendency of fatigue strength in these specimen; is much the same as tendency of tensile strength in mild steel joints brazed with BAg-8.
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