| With the development of science and technology, magnesium alloy is widely used in automobiles, home appliances, aerospace, electronic industries and so on. Due to their low resistances of corrosion and abrasion, they were restricted used. The optimized and controllable process parameters were obtained by studying the electroless Ni-B plating on AZ31 magnesium alloy. Moreover, these process parameters can meet the requirements of production. Based on getting a good Ni-B film, the interfacial reactions between electroless Ni-B and lead-free solders, Sn-3.5Ag, Sn-0.7Cu and Sn-3.0Ag-0.5Cu were carried out by performing thermal aging time at 150℃up to 400h.Firstly by changing the four parameters, the molar concentration ratio of Ni2+ to BH4-, the molar concentration ratio of ethylenediamine to Ni2+, NaOH concentration and temperature, effect on plating speed of electroless Ni-B on AZ31 magnesium substrate, surface morphology and stability of solution were studied. The research results showed that if the molar concentration ratio of Ni2+ to BH4- was below 3:1, the electroless Ni-B bath would decompose; the Ni-B plating rate reached the maximum when the molar concentration ratio was 4:1 and then decreased with increasing molar concentration ratio up to 7:1. If the molar concentration ratio of ethylenediamine to Ni2+ was around 7, the plating rate would reach the maximum; if the ratio was below 5, nickel was not completely complexed with ethylenediamine. When the sodium hydroxide concentration was below 70 g/L, the Ni-B plating bath would decompose due to the low alkaline; and then the Ni-B plating rate increased with increasing sodium hydroxide concentration and reached the maximum at 110 g/L; further increase in sodium hydroxide concentration led to the decrease in the plating rate. With increasing the temperature, the plating rate was increased. When the temperature was below 80℃, the plating rate was very slow; when the temperature was above 90℃, the reaction was very active and lots of air bubble was formed on the surface.Then, four factors in three levels orthogonal test was studied in the present work. The results showed that at the same level temperature had the biggest influence on deposit rate. The other three factors in order with the influence on deposit rate are the content of NaOH, the molarities ratio of Ni2+ to BH4-, and the molarities ratio of ethylenediamine to Ni2+.According to the determined parameters, we could obtain a Ni-B film. In this Ni-B film, B content was between 2.88 to 4.29(wt%). The hardness of Ni-B film could achieve 676HV. The adhesion was about 6N.Finally, the solid/liquid and the solid/solid interfacial reactions between electroless Ni-B and three lead-free solders, Sn-0.7Cu, Sn-3.5Ag and Sn-3.0Ag-0.5Cu, were investigated. The transformation of IMC type during the aging was studied. The results showed that, Sn-0.7Cu/Ni-B, Sn-3.5Ag/Ni-B and Sn-3.OAg-0.5Cu/Ni-B, can wet with Ni-B layer after reflow soldering for 60s. The wetting areas, which to be inferior to solders with Ni-P, were 1.14,1.2768 and 1.045 mm2, respectively. After reflow soldering for 60s, Ni3Sn4 formed at the Sn-3.5Ag/electroless Ni-B interface; while (Cu,Ni)6Sn5 formed at the Sn-0.7Cu/electroless Ni-B interface; (Cu,Ni)6Sn5 formed at the Sn-3.0Ag-0.5Cu/electroless Ni-B interface, respectively. The type of IMC after aging time at 150℃up to 400h remained. Compared with the as-soldered state, the thickness of IMCs and the size of grain both increased. The morphology of the IMCs was not changed. |
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