| Low temperature soldering is a favourable technique for joining heat-sensitive aluminum alloys, in which Sn baesd solder is commonly used. Soldering aluminum alloys with Sn has been well studied and proven techniques have been developed, while the bonding mechanism between Sn and Al remains to be explored. In this study, Sn/Al interfaces obtained under different conditions were analyzed and taken into comparision to study the forming mechanism of the Al2O3 intermediate layer at Sn/Al soldering interface, together with the influence of the amorphous layer to the bonding strength of Sn/Al interface.Pure Sn was used to soldered 1060 Al in air by ultrasonic soldering and scratch soldering methods, there existed an amorphous aluminum oxide intermediate layer in either Sn/Al interface, the conclusion was made that the influence of ultrasonic wave was not the reason for the formation of the Al2O3 intermediate layer. The Al2O3 intermediate layer at Sn/Al ultrasonic soldering interface was uniform in thickness and continuous in distribution, and fully amorphous in structure; while the amorphous layer at Sn/Al scratch soldering interface was not uniform in thickness and not as continuous as that at ultrasonic soldering interface, besides, crystallization was foud and γ-Al2O3 nano-crytalline were dispersed inside it. It was deduced that the influence of ultrasonic wave led to the uniformity and continuity of the Al2O3 intermediate layer, and promoted its structural amorphization.Al grains were found separated out of the oversaturated Sn solder, there also existed an amorphous Al2O3 intermediate layer between the separated-out Al grain and Sn solder, the thickness of which was about 10 nm, far thinner than the 50 ~ 80 nm thickness of the intermediate layer at Sn/Al soldering interface. It could be inferred from this observation that O dissolved in liquid Sn was only part of the O forming the intermediate layer, other O was diffused from air through Sn/Al interface.It’s analyzed that the formation of the Al2O3 intermediate layer was a “liquid-phase depositon” process: Al was dissolved into Sn and then combine with O, and then the amorphous aluminum oxide intermediate layer was depsited.Infliction of 160 MPa stress onto Al base metal at 200 ℃ in vacuum crushed and removed the oxide layer on its surface, rising temperature to 300 ℃ after which led to benign bonding interface between Sn and Al. An amorphous Al2O3 intermediate intermediate layer was also observed at the Sn/Al vaccum soldering interface, with an uniform thickness of about 5 nm. Crushing the oxide layer of Al base metal, followed by a holding stage of 2 ~ 8 h with stress and temperature held, benign diffusion bonding interface of Sn and Al was formed. Direct bonding of Sn and Al was observed, without an intermediate layer between them.A compassion among Sn/Al ultrasonic solering interface, scratch soldering interface, vacuum soldering interface and diffusion bonding interface showed that intermediate layer existed at either of the former three interfaces while at the last interface Sn and Al was directly bonded, implying that dissolution of Al in Sn was the key factor influencing the formation of the amorphous intermediate layer at Sn/Al interface.A comparission was made between the bonding strengths of Sn/Al interfaces with and without Al2O3 intermediate layer, the former was made by ulatrasonic soldering in air, and the latter by diffusion bonding in vacuum, and they had the similar morphorlogy. The shear strengths of the two kinds of interfaces were about 38 MPa(with Al2O3 intermediate layer) and 20 ~ 22 MPa(without Al2O3 intermediate layer), respectively, this result demonstrated that the Al2O3 intermediate layer increased the bonding strength of Sn and Al. |
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