aluminium in commercial vehicles - European Aluminium Association

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52 EUROPEAN ALUMINIUM ASSOCIATION ALUMINIUM IN COMMERCIAL VEHICLES CHAPTER V 8. Influence of fabrication on the properties of the alloys 8.1. Work hardening of non-heat treatable alloys Hardening is achieved by cold deformation, known as work hardening, that improves the physical properties and the hardness of the metal. It also reduces the metal’s capacity for deformation and its ductility (Figure V.2). The greater the deformation or higher the work hardening rate, the more pronounced is the effect. It is also governed by the composition of the material. The 5083 alloy, for example, which contains between 4 and 4.9% of magnesium, acquires a great hardness but its capacity for deformation is less than that of the 5754 alloy which contains between 2.6 and 3.6% Mg. Work hardening is a general phenomenon that takes place whatever the method of deformation used: rolling, deep drawing, folding, hammering, bending, pressing, etc. This means that it will also occur during fabrication in the workshop. 8.2. Softening by annealing and recovery It is possible to restore the ductility of the work hardened metal by heat treatment known as “annealing” (partial or full annealing). In this process, which takes place at temperatures between 150°C and 350°C, the hardness and mechanical characteristics of the metal slowly begin to decrease: this is the recovery phase [A-B] (Figure V.3). At lower annealing temperatures this leads to medium-strength material properties. They then fall away more rapidly at high temperatures above 280 °C during recrystallization [B-C] and eventually attain a minimum value that corresponds to the mechanical characteristics of the fully annealed metal [C-D]. Restoration and annealing are accompanied by a change in the texture and size of the grains of metal observed under a microscope with X50 magnification. The texture can change from a fibrous structure to a fully recrystallized structure (Figure V.3). The grain can grow in size during recrystallization and annealing. This growth is revealed during subsequent working, e.g. folding, by the rough “orange peel” effect on the surface of the metal. Grain growth above around 100 microns reduces the deformation capacity of work hardening aluminium alloys. The following conditions are essential if a fine-grained annealed structure is to be achieved: • The metal must have undergone a sufficient rate of deformation corresponding to a rela- WORK HARDENING CURVE OF ALLOY 5083 MPa A% 500 400 300 200 ‰ FIGURE V.2 R m R p0,2 100 ‰ Temper 0 H12 H14 H16 H18 Work hardening % 10 20 30 40 50 60 70 80 A 30 20 10
EUROPEAN ALUMINIUM ASSOCIATION ALUMINIUM IN COMMERCIAL VEHICLES CHAPTER V 53 tive reduction in section of at least 15%. This is “critical work hardening”. If this condition is not met, then heat treatment must be restricted to restoration without recrystallization, • A rapid temperature gradient of 20 to 60°C per hour, • Temperatures over 350 to 380°C must be avoided, • Holding times must be limited to 2 hours maximum. For 5000 series alloys, annealing is usually performed between 320°C and 380°C for 30 to 120 minutes. Note: Non-heat treatable alloys in the annealed temper (O, H111) can only be brought to higher strength by work hardening. 8.3. Heat treatable alloys If some plastic deformation must be done on products of heat treatable alloys, it should be carried out in the T4 temper; first the allowable degree of deformation is bigger than for the T6 temper and second there is a moderate effect of work hardening. If for the final product e.g. a bent extrusion in a 6XXX alloy T6 temper is needed, age hardening can be carried out. Table V.8 gives an indication how to proceed, using typically a hot air furnace. Micrographic views Hardness O A HARDNESS CURVE DURING ANNEALING Recovery B Recrystallization FIGURE V.3 C TABLE V.8 (typical) Annealing 6000 SERIES ARTIFICIAL AGEING D Time Alloy Initial temper Artificial ageing Final Temper* 6060 T1 - T4 6hat 185°C or 8 h at 175°C T5 - T6 6005 T1 - T4 8hat 175°C T5 - T6 6106 T1 - T4 8hat 175°C T5 - T6 6061 T4 8hat 175°C T6 6082 T1 - T4 16 h at 165°C or 10 h at 170°C or 8 h at 175°C T5 - T6 * T5, for an initial temper of T1, T6 for an initial temper of T4.
Page 1 and 2: ALUMINIUM IN COMMERCIAL VEHICLES
Page 3 and 4: Aluminium in Commercial Vehicles ha
Page 5 and 6: VII. FABRICATION . . . . . . . . .
Page 7 and 8: CHAPTER II ALUMINIUM IN TRANSPORT 1
Page 9 and 10: EUROPEAN ALUMINIUM ASSOCIATION ALUM
Page 15 and 16: CHAPTER III WHY USING ALUMINIUM 1.
Page 29 and 30: CHAPTER IV FREQUENTLY ASKED QUESTIO
Page 39 and 40: CHAPTER V ALUMINIUM ALLOYS FOR COMM
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Page 57 and 58: CHAPTER VI DESIGN AND CALCULATION 1
Page 91 and 92: CHAPTER VII FABRICATION 1. INTRODUC
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EUROPEAN ALUMINIUM ASSOCIATION ALUM
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CHAPTER VIII WELDING 1. FOREWORD .
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CHAPTER IX OTHER JOINING TECHNIQUES
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CHAPTER X DECORATION AND FINISHING
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Cell wall EUROPEAN ALUMINIUM ASSOCI
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CHAPTER XI CORROSION RESISTANCE 1.
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CHAPTER XII CLEANING OF ALUMINIUM C
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CHAPTER XIII REPAIR OF ALUMINIUM CO
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