INFLUENCE OF HEAT TREATMENT ON PROPERTIES OF EN AW 6082 ALUMINIUM ALLOY

The main aim of this work is to point out on possibility of properties improving of the aluminium alloy EN AW 6082 (AlSi1MgMn) with an appropriate combination of pre-ECAP solution annealing, the application of the severe plastic deformation by ECAP technology (equal channel angular pressing) and post-ECAP artificial aging. The effect of the severe plastic deformation and artificial aging on the alloy structure was evaluated by metallographic analysis, and alloy mechanical properties by uniaxial tensile test at room temperature, the Vickers hardness and by tribology test of resistance to abrasive wear. As a result of strain hardening by severe plastic deformation it reaches the improvement in hardness (by 56%), strength characteristics (yield strength by 92%, tensile strength by 29%) and abrasion wear resistance is 28 %.


Introduction
The aluminium alloys belonging to 6XXX groups, which are widely used in structural applications, the construction industry, the automotive industry and architectural section as extrusions products [1].AlSi1MgMn alloy is characterized by good mechanical properties, resistance to tribology (abrasion wear) and corrosion degradation, low density.Due to good tribological properties of alloy can be used in excellent application.These properties can be increased by strengthening mechanisms: alloying additives respectively heat treatment and the severe plastic deformation [2][3].The AlSi1MgMn alloy is alloyed from the main elements namely magnesium and silicon.At higher magnesium content tends to increase strength properties of hardened alloy formation of Mg 2 Si phase.Silicon improves mechanical properties by changing the shape of the grain.Except of the main alloying elements, there are added the alloy chromium and manganese which form dispersion particles.These are larger than the other precipitates that can act as nucleation sites for strengthening precipitations, and have good thermal stability, which influences the recovery and recrystallization [4][5][6] Another way of improving the properties of the alloy is heat treatment (consisting of solution treatment, quenching in water, and a natural or artificial ageing treatment).From the  [11][12].The formation of the equilibrium β phase (Mg 2 Si) there is significant strengthening of the alloy.With the increasing ratio of Mg:Si, the strength of the alloy is higher [13].Addition of copper in the alloy formed transition Q´ phase (Al 4 CuMg 6 Si 6 ) and stable Q phase (Al 4 Cu 2 Mg 8 Si 7 ).Due to the low diffusion of copper into α-Al and subsequent coarsening Q'phase, alloys are thermally stable [14][15][16].On the other hand, the alloys with copper have low corrosion resistance [17][18].The significant improvement in properties of the alloy can be achieved by a suitable combination of mentioned heat treatment followed by application the severe plastic deformation.Influence of the severe plastic deformation is to achieve ultra-fine grained structure optionally nanostructure [19][20][21][22][23]. ECAP technique (Equal channel angular pressing) is the most used technique from the severe plastic deformation methods.The principle of the extruded material through a die consists of two perpendicular channels.The material is in the transition of shear without changing of it is cross-sectional [24][25][26].
The goals of this present work are to determine the optimal temperature pre-ECAP solution annealing and post-ECAP artificial aging evaluation of changes hardness and examine the change characteristics of aluminium alloy AlSi1MgMn from the initial state and the state after heat treatment (pre-ECAP solution annealing + water + cooling extrusion through the ECAP die + post-ECAP artificial aging).

Experimental material and methodology of experiments
As experimental material has been used commercially produced the aluminium alloy AlSi1MgMn with the chemical composition shown in Table 1.The samples were delivered in a form of bars with heat treatment T3 (solution annealing and natural aging).The experiments for pre-ECAP and post-ECAP treatments were performed on the samples with optimal conditions.The samples for metallographic analysis were prepared by standard procedures: grinding, polishing on the diamond paste and emulsion OP-S Suspension followed by etching with a solution of Kroll (92 ml distilled water, 6 ml HNO 3 , 2 ml HF).The microstructures were observed and documented using the light microscope OLYMPUS Vanox-T.The thin foils were prepared by double jet electro polishing in TenuPol -5.The microstructures were observed using transmission electron microscopy Jeol JEM 200 FX in the bright field images(operated at 200 kV).Strength and plastic characteristics were measured by uniaxial tensile test at room temperature on a Zwick 1387 machine at a speed of 0.5 mm / min.The Vickers hardness was measured at a load of 95 N. The abrasive wear was carried out on the SVUM-AB-1 machine with a load of 5 N with the track 40 m using sandpaper with aluminium particles and roughness120.The prepared samples were weighed before and after the experiment to within ± 0.0001 g.The fracture surfaces and the surfaces of the abrasive wear were studied using scanning electron microscope JEOL JSM 7000F.The particles identification was carried out using the EDX analysis with the analyser INCA-sight.

Results and discussion
The surfaces of samples in the initial state and after post-ECAP artificial aging are documented in Fig. 4 and Fig. 5.The microstructure of AlSi1MgMn alloy is characterized intermetallic phasesMg 2 Si, AlFe, AlFeSi, Al 15 (FeMn) 3 Si 2 , Al 5 Cu 2 Mg 8 Si 6 [27].The EDX analysis in both treatments showed presence of phases based on MgSi, AlFeMnSi, AlSiMg (Fig. 6 and Fig. 7).The microstructure after post-ECAPartificial aging is characterized by deformation bands that characterize the uneven deformation after the sample cross-section.The microstructures observed by TEM are documented in the Fig. 8 and Fig. 9.The average size of grains was 4 µm in initial state and under 0.3 µm after post-ECAP state.After post-ECAP artificial aging the microstructure was not recrystallized.The recovery and changes of dislocation density were not visible in the microstructures after post-ECAP artificial aging.The fracture surfaces after static tension tests are documented in Fig. 11 and Fig. 12.In both cases, we are talking about transgranular ductile fracture with pit morphology.The identified particles found in the dimple are based on: AlFeMnSi, AlSiMgMn and AlSiMgMnFe.The surfaces of the samples after abrasive wear in mode backscattered electrons are shown in Fig. 14 and Fig. 15.In both cases, the observed degradation of the surface grooves.This is caused due to chipping of the particles of the abrasive paper which are pressed into the surface of the material.The detailed mechanical damage, cracking and breaking of the particles is documented in Fig. 16 and Fig.

Fig. 3
Fig. 3 Influence of artificial aging temperature on the hardness HV10

Table 2
Mechanical properties of AlSi1MgMn

Table 3
Weight losses of samples after abrasion wear