Research On Behavior Of Solids/Liquids Interface Wetting And Filler Metal Flowing Through The Gap

During the process of brazing, many properties of the joint, such as the extent ofscatter, homogeneity, strength of the joint and so on are affected by the wettingbehavior of filler metal on base metal. If the wettability between filler metal and basemetal isn’t good, the brazed joint is not reliable. The process of filler metal filling gapis affected by temperature, clearance and wettability between filler metal and basemetal. Some defects such as filling gap deficiently, pin hole, slag inclusion, porositycan easily emerge in the brazed joint. To study the interface wettability and thefluidity of filling metal, and improve the quality of the brazed gap, Bi/Cu、Sn/Al、Sn-Zn/Al systems had been studied in this research. Further more, the effects of grainboundary, roughness of base metal, addition of alloying element in the filler metal onwettability was investigated. The effect of ultrasonic time and clearance on fluidity offiller metal was investigated in Mg-Al-Zn filler metal and AZ31B magnesium alloy.Besides, the mechanism of ultrasound forced filling gap was studied by meas ofphysical simulation. Further more, the capillary forced filling gap and ultrasonicforced filling gap were distinguished.It was indicated that the wettabilty was affected considerably by grain boundaryin Bi/Cu system. The wettability of Bi/poly crystal Cu is better than Bi/single crystal(the equilibrium contact angle of Bi/poly crystal Cu and Bi/single crystal are73°and84°respectively). Surface roughness of base metal also has a great effect onwettability under ultrasonic vibration. The spreading area decreases and wettability isworse with the roughness of Al substrate increases when ultrasonic time is not long(1~2s). Then the spreading area of the three substrates approach, and the spreadingarea of rougher substrate becomes larger than the smoother substrates gradually alongwith ultrasonic time extension (2~4s). The barrier effect is remarkable with shorterultrasonic time, which give rise to a worse wettability. Then the induction effect playsa leading role when ultrasonic time becomes longer, which result in that thewettability of rougher substrate approach and then surpass the wettability of smoothersubstrate gradually. The effect of alloying addition in filler metal on wettability underultrasonic vibration was remarkable. Addition of Zn in filler metal made interfaceaction intensify, and Sn-Zn-Al solid solution emerged at the interface. Besides, thewettability of Sn/Al system is much better than that of Sn-Zn/Al.Molten Mg-Al-Zn filler metal flowing through the parallel gap with differentclearance and through non parallel gap under different ultrasonic time wereinvestigated by analog technique and OM. The results showed that the velocity offiller metal flowing through the parallel gap decreases as the clearance increases. Compared with traditional sessile drop method and destructive test, this test method offluidity of brazing filler metal is more realistic. For the non parallel gap, it is foundthat the displacement of filling gap decreases as ultrasonic time increases whenputting the filler metal at the part of maximum, and the compactness of the joint isfine except the initial part. Moreover defects were observed in all over the joint Whenputting the filler metal at the part of minimum clearance, and with increasing ofultrasonic time, the unjointed segments decrease and the compactness of the jointimproves.It has been found that ultrasonic vibration has effect on liquid filling gapvertically (the velocity of filling gap under wetting only condition and ultrasonicinduction conditionare13.3mm/s,19.2mm/s respectively whenthe clearance is50μm).Under wetting only condition, the rate of climbing increases as clearance increases(50~200μm). Nonetheless, clearance has little effect on the velocityof climbing underultrasonic vibration. The result shows that capillary action during ultrasonic action isnot the main reason to fill the gap through the test of brazing flux action andultrasonic induction. There are more important influencing factors working, whichgive rise to that the velocity of filling gap are faster under ultrasonic vibration and thevelocity increases as clearance decreases.