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The sagging mechanisms of a three-layer aluminum clad foils (AA4343/AA3003Mod/AA4343) with final rolling reduction of 20-50% during brazing were studied. The results show that the sagging behavior of the aluminum brazed foils during the brazing cycle can be divided into four stages: Stage I is the slight sagging stage, during which sagging deformation can almost be ignored; Stage II is referred to as the accelerative sagging stage, and the sagging behavior is mainly ascribed to recovery. In stage II, the greater the reduction ratio is, the earlier the recovery ends, and the smaller the value of the sagging distance is. Stage III is the slow sagging stage, and sliding deformation along the grain boundaries is responsible for the slow sagging rate. Stage IV is the erosion stage. During stage IV, the coarsely recrystallized grains can inhibit erosion and are helpful in obtaining excellent sagging resistance, while the foils with finely recrystallized grains or non-recrystallized structure show severe erosion and limited sagging resistance.

The formation and growth of intermetallic compounds (IMC) were investigated using the Al-St clad materials with additional different Si content to aluminum and steel substrate by differential scanning calorimetry, scanning electron microscopy and energy-dispersive x-ray spectroscopy, x-ray diffraction and transmission electron microscopy. The addition of Si to Al (Al-0.8Si/St) can drastically delay the formation of IMC and accelerate the growth of IMC layer. The Si (Al-0.8Si/St) segregates at the interface and inhibits the diffusion of Al atom to steel before the IMC forms, which further decreases the diffusion depth of Al in the steel layer. By contrast, the addition of Si to St (Al/St-0.8Si) has limited impact on delaying the formation and decelerating the growth of IMC layer. The Si (Al/St-0.8Si) hardly affects the diffusion of Fe atom to the aluminum layer. Among the four clad materials, the major IMC layer (η-Fe2Al5) always exhibits the parabolic growth phenomenon. The addition of Si cannot influence the phase compositions of IMC at the interface between the η-Fe2Al5 layer and the aluminum. However, we observe an extra Fe-rich phase β1-Fe3Al at the interface between the Fe2Al5 layer and steel substrate in the Al-0.8Si/St clad material.

Effect of microstructural evolution on sagging behavior of aluminum foil was investigated during the brazing thermal cycle. During the brazing thermal cycle, the sagging behavior consists of three stages: slight sagging stage, accelerative sagging stage and slow sagging stage. The sagging of cold-rolled aluminum foil mainly occurs in the accelerative sagging stage, which is governed by recovery under external stress in the sagging test. The coarse recrystallized grain is responsible for the slow sagging stage by reducing grain boundary sliding. Increasing cold-rolled reduction and addition of final annealing heat treatment can improve sagging resistance by shortening recovery process during the brazing thermal cycle. Dissolution of dispersoids has few effects on sagging deformation.

The present work studies the effect of eutectic silicon particle morphology on the fluidity of 4045 aluminum alloy filler through dynamic spreading experiment, filling test and welding seam experiment. The experimental results indicate that the morphology of eutectic silicon particle has an excellent influence on the melting speed at the initial melting stage of brazing process: the alloy loaded with smaller eutectic silicon particles exhibits a faster melting speed and better fluidity. When the alloy completely melts along with those internal eutectic silicon particles in a variant morphology, particles sizes have limited impact on the fluidity. In addition, it is discovered that alloy introduced with smaller particles possesses a much lower surface activation energy; furthermore, the proportional relationship the ratio of length to diameter (S) and surface activation energy (Q) is verified in this article, and is consistent with the formula: Q=2.35S+94.47.

The deformation behaviors of 4A60 aluminum alloy/08Al steel clad materials with different thickness of each layer were investigated in this study. The amounts of residual aluminum and cracks on the steel surface of deformation zone after peeling were measured, and the morphologies of steel surfaces after peeling and caustic wash were observed by scanning electron microscope (SEM), respectively. In addition, the mechanical of aluminum/steel roll bonding was discussed. The results showed that the deformation of aluminum alloy has precedence over steel layer, and the stable thickness reduction of collaborative deformation of aluminum layer and steel layer decreased along with the thickness of steel layer increase, and increased with the thickness of aluminum layer. The effect of ditches generated by steel brush on bonding strength of the aluminum/steel composite materials of CRB was larger than transverse cracks growth along rolling direction.

The effect of interface microstructure by wire brush treatment on the bond strength of Al-St clad strips by cold roll bonding was investigated. For the purpose, wire brush surface preparations on steel strip surface have been done before the cold roll bonding of the Al-St clad strips, and evaluate the sample bonding strength by peeling test of the rolling deformation zone. The results show that bond strength of Al-St strip by wire brush treatment increases with the total reduction increases and the threshold reduction of Al-St strip is 15%. Meanwhile the total crack area increase with the reduction. The oxide film on the surface of brittle layer hinds the metal bond and the crack between the scratch brush layer and substrate metal is the weak point and decrease the bond strength. A major feature of the bonding mechanism described in this paper is that extruded aluminum squeezed into the cracks of steel surface by normal roll pressure action, and metallic bond with underlying fresh steel metal.

The brazing behavior of 3003 fin assemble with 4343(filler layer)/4A60(transition layer)/08Al(steel layer) brazing sheet (the ratio of filler layer thickness to transition thickness were constant 1:2) were investigated. This study elucidates the brazing results vary with the layer thickness and brazing temperatures. Results showed that keeping the holding time at brazing temperature constant (the holding time of this study was 10 minutes in all brazing progress), under the condition of brazing temperature less than or equal to 580 °C, thinner filler layer led to well formed joint, while thicker filler layer led to defects at brazing joints because of poor mobility. When brazing temperature was up to 600 °C, a thinner transition layer trended to lead to the formation a thick intermetallic compounds layer on the Aluminum-Steel surface. It was inferred that Si diffused from molten filler metal to the interface of transition layer and steel layer and promoted the formation of intermetallic compounds. Furthermore, it was found that in the surface away from brazing joints, with lower Si content, the intermetallic compounds were mainly η (Fe 2Al5 ), while higher Si content region nearby brazing joints trended to form τ 5 (Al 8 Fe 2 Si) and τ 6 (Al 4.5 FeSi). Meanwhile, a transition layer with thickness above 134μm could avoid the formation of intermetallic compound. The optimum brazing process was that temperature range was 590 °C to 595 °C with 10-minutes holding time and the transition layer thickness was suggested to be equal or more than 134μm.