Sagging resistance of cold rolled aluminum 4343/3N03/4343 clad sheet

Abstract

Aluminum based condensers have been widely used in air-conditioning systems for automobiles because aluminum has a low density, high thermal conductivity and proper corrosion resistance. Automotive aluminum condensers are generally produced by assembling Al brazing sheets with multiport Al tubes using a brazing method. The brazing sheet used for the condenser is a three-layer composite sheet and typically consists of an Al-Mn core alloy clad on both sides with an Al-Si filler alloy. The whole thickness of the brazing sheet is usually about 100μm. An increased need for enhanced corrosion resistance in the condenser contributes to the development of an advanced material for the brazing sheet. Aluminum 3N03 alloy, Al(1.0–1.5) wt.%Mn(0.05–0.20) wt.%Cu(0.5–2.5) wt.%Zn, is a potential core material for the brazing sheet [1], which can allow improved corrosion resistance of the condenser due to a sacrificial anode effect of Zn [2]. In order to avoid a collapse of the assembly during the brazing cycle, the thin core material should have a high resistance against deformation during the brazing process, generally referred to as sagging resistance. In this study, the effect of the cold rolling condition on the sagging resistance is investigated using a three layer clad sheet composed of Al 4343 (filler, thickness: 10 μm)/Al 3N03 (core, 80 μm)/Al 4343 (filler, 10 μm) alloy. The clad sheets were produced to a 100μm thickness by laboratory fabrication, through casting, hot rolling, and cold rolling. Above all, Al 3N03 core alloys and Al 4343 filler alloys were cast and homogenized. The typical composition range of both alloys and the actual concentrations of cast materials are presented in Table I, showing careful control of the main elements in the cast alloys. Filler alloys were then hot rolled to a thickness of 2 mm (reduction rate: 90%) and clad to both sides of the core alloys having a thickness of 16 mm. The clad materials were hot rolled to a thickness of 2 mm and were then cold rolled to thicknesses in the range of 111–182 μm, followed by intermediate annealing (IA) at 600 or 690 K. After intermediate annealing, the clad materials were finally cold rolled to the thickness of 100 μm, by which the reduction rate of the final cold rolling (CR) changed in the range from 10 to 45%. Fig. 1 shows an optical micrograph exhibiting the cross sectional microstructure of the cold rolled clad sheet with CR to 30% after IA at 690 K, indicating that each cladding thickness is almost equal to 10% of the total sheet thickness which is approximately 100 μm. The sagging resistance of the clad sheet could be evaluated by measuring the sagging distance [3]. Fig. 2 shows the schematic view of a sagging test rig and an illustration of how to measure the sagging distance. The specimens were taken parallel to the rolling direction. One end of the specimen was fixed during the brazing cycle. The free length and the width of the specimen was 35 mm and 22 mm, respectively. The sagging distance is defined by the deflection of the free end of the specimen after the brazing cycle [3]. The sagging test was carried out under a nitrogen atmosphere using a fluoride salt flux. The brazing cycle was as follows: R.T.→ 798 K: 75 K/min, 798→ 850 K: 12 K/min, 850→ 878 K: 6 K/min, 878→ 688 K: cooled in furnace, followed by rapid cooling to R.T.