Abstract
In automotive air conditioning system, the latest technologies of aluminium heat exchangers are:
- the tubes and fins brazed heat exchangers
- the stamped plates and fins brazed heat exchangers
Tubes and stamped plates have the same function: the refrigerant or the coolant liquid passes through them. In both cases, the convective thermal exchange is done by the fins. In the latest technology, the fins are made of thin aluminium rolled strips which are cold shaped and cut to length in special machines. The strips have thickness of less than 100 microns with various tempers. To reach such low thicknesses two technologies are available. Some suppliers produce these strips by DC casting: hot rolling them in a first step and decreasing further the thickness by cold rolling them in several steps. A final step of thermal heat treatment could be done on material to diminish the residual stresses. A high number of cold rolling steps are necessary. This induces a discrepancy of mechanical properties which doesn't match with a single set up of fin stocks shaping machine. The other one, the TRC casting (1) is based on a continuous casting and rolling technology.
Another issue is the continuous decrease of fin stock thickness as each new generation of heat exchanger to lighten cars and reduce fuel consumption (1).
The fin forming process is very complex, composed of several steps including bending, cutting, slitting so as to give final fin pitch, louver opening and louver angle. The main parameter for fin forming process is the coil strain that must be set for avoiding material break down or louver tearing at production start. In another way for a fin forming process set up, the main issue is to be able to determine the quality of the material strip using an industrial testing method.
The aim of this work is to propose a "quick" test for fin manufacturing lines to validate the material choice and to set the right parameter for fin forming.
In this paper, a method based on hardness testing to evaluate the strip quality is presented. A relationship between the mechanical properties of 3xxx aluminium alloy coil and fin shaping process is discussed. In order to establish such a relationship, several tests have been performed on raw and brazed materials of 3xxx aluminium alloy serials such as tensile tests and measurements of Vickers hardness.
Keywords: aluminium alloys, Vickers hardness, tensile test, fin forming, heat exchangers