Abstract
During the extrusion of aluminum alloys profiles using porthole dies, the temperature of the material in the welding chamber is one of crucial parameters determining the quality of longitudinal welds. In order to extend the permissible temperature range, the billets intended for this process should be characterized by the maximum attainable solidus temperature. Within the present work, the homogenization of AlZnMgCu alloys DC-cast (Direct Chill-cast) billets was investigated, with the aim of solidus temperature maximization. Conditions of soaking and cooling stages were analyzed. The materials were homogenized in laboratory conditions, and the microstructural effects were evaluated on the basis of DSC (Differential Scanning Calorimetry) tests and SEM/EDS (Scanning Electron Microscopy/Energy-Dispersive Spectroscopy) investigations. For all examined alloys, the unequilibrium low-melting microstructure components were dissolved during soaking, which led to the significant solidus temperature increase, in comparison to the as-cast state. The values within the range of 525–548 °C were obtained. In the case of alloy with highest Cu concentration, the application of two-step soaking was necessary. In order to take advantage of the high solidus temperature obtained after soaking, the cooling rate from homogenization must be controlled, and the effective cooling manner is strongly dependent on alloy composition. For high-Cu alloy, the solidus decreased, despite the fast cooling and the careful billets preheating being necessary.
Highlights
In current industrial practice, the extrusion with the use of porthole dies is commonly applied for aluminum as well as its low- and medium-strength alloys
The hard-deformable Al alloys, in particular those of 7xxx series with Cu, are characterized by low solidus temperature [7], which becomes a factor significantly limiting the permissible temperature of material in the welding chamber
The DC-cast billets of AlZnMgCu alloys are characterized by dendritic microstructure with numerous eutectics and particles
Summary
The extrusion with the use of porthole dies is commonly applied for aluminum as well as its low- and medium-strength alloys. Among parameters determining the quality of longitudinal welds, the temperature in the welding chamber is one of great importance [1,3,4,5,6]. The hard-deformable Al alloys, in particular those of 7xxx series with Cu, are characterized by low solidus temperature [7], which becomes a factor significantly limiting the permissible temperature of material in the welding chamber. The careful selection of alloys’ chemical composition as well as suitable billets preparation for extrusion are important steps in the elaboration of hard-deformable alloys extrusion technology. The nonequilibrium solidification during DC casting results in low billets solidus temperature, most often values in the range 470–480 ◦C are noted [9,14,15,16], but lower can be found in the literature [17,18]. The solidus temperature can be increased by homogenization annealing; obtained effects are strongly dependent on process parameters
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