The effect of plastic deformation on structure and properties of chosen 6000 series aluminium alloys

Abstract

In order to determine the effect of increased copper addition on the strengthening behaviour during deformation and subsequent ageing, two types of Al alloys were continuously cast and extruded: 6013 containing 1.15% Mg, 1.08% Si, 0.7% Mn, 0.3% Fe and 1.1% Cu, and 6XXX alloy containing 1.09% Mg, 0.9% Si, 0.1% Fe and 1.6%Cu (in wt.%). The 6XXX alloy aged at 165°C showed a hardness maximum of 150 HV, while alloy 6013 revealed lower precipitation hardening at higher strain hardening. A similar maximum hardness of about 170 HV was observed for both alloys after ageing in a deformed state. Transmission electron microscopy performed after deformation of 60% and 90% by cold rolling showed the presence of large, elongated subgrains. Narrow deformation bands consisting of fine subgrains of large misorientation were observed in the alloys deformed 60% by rolling, and their density increased in the alloys deformed up to 90%. Fine precipitates were observed after an ageing time corresponding to the maximum of hardness for alloy 6XXX deformed in the as quenched state. They formed predominantly on dislocations as confirmed by dark field imaging. In the alloys deformed by 60% the precipitates were identified as belonging to the metastable B2 phase. In the alloy deformed by 90%, larger needle-like precipitates were observed. They seemed to possess a quaternary Q-precipitate structure. A frequent rotational moire pattern observed in this state allowed a misorientation as 2–3° to be determined. The presence of fine subgrains formed after ageing indicated the activation of recovery process at the ageing temperature.

Introduction

The 6000 series alloys have recently found increased application in automotive and construction industry. Therefore, several research works have been undertaken to strengthen the alloys either by small copper additions [1], [2], [3], [4], [5], [13] or by a predeformation treatment [6], [7], [8], [9], [10], [14], [15], [16]. The copper addition increases the peak hardness and yield strength during ageing [1], [2], [3], [4], [5], [13], and it increases even more the peak hardness than that of the solid solution of the as quenched alloys. This indicates that copper concentrates in the precipitates and increases the volume of the precipitates formed [2]. It does not change the electron diffraction pattern from the β″ phase characteristic for Al–Mg–Si alloys [2], [11], but shows very strong streaks due to the needle-like shape. In earlier works it was suggested that the strengthening due to copper addition was caused by the additional S′ and θ′ precipitation [3], [13]. Another type of precipitates reported in Al–Mg–Si–Cu alloys is the quaternary Q phase [12] with a lath morphology formed at later ageing stages.

Two types of precipitates were observed on dislocations in the deformed Al–Mg–Si alloy at an early stage of ageing [14]. Some consisted of small precipitates that looked like string beads on dislocation lines. Others were elongated and resembled end-on of the needle- or rod-shaped precipitates having elongated cross section. There were also random precipitates and the β” phase in the matrix, which formed additionally to those on dislocations. It is concluded that the plastically deformed solution treated alloy has higher wear resistance than the undeformed one [15]. The results of [16] show that the strength of Al–Mg–Si alloys in the underaged condition is greatly increased by pre-stretching immediately after water quench. The β″ precipitates are observed to form directly on dislocations with a larger size than those which form in the dislocation free areas. When the prestretching is less than 5%, the increase in pre-deformation results in the rise of the β″ precipitate density, whereas the pre-stretching by 10% leads to larger sizes on account of density of the β″ precipitates at the same time. In high-resolution transmission microscopy (HRTEM) studies of an Al–Mg–Si alloy [17] artificially aged after mechanical deformation, the observation of precipitates cross sections revealed that the transverse orientation of the precipitate deviated 10 degrees from the [1 0 0] or [0 1 0] orientation of the Al matrix. With longer ageing time, the specific precipitates disappeared and the β′ phase particles became dominant.

The above mentioned results indicate that structure of precipitates formed during ageing of the deformed Al–Mg–Si solid solution can be different from that of aged directly after quenching. Some samples to be aged were deformed in tensile tests [7], [8], [14], [15], [16], [17]. The effect of higher deformation was studied mainly on texture and recrystallization [5], however, then higher annealing temperatures than those for artificial ageing were used; furthermore, no information was found about the effect of deformation on precipitation and mechanical properties of Al–Si–Mg alloys with copper addition. Therefore in the present paper there have been investigated the effect of deformation of the as quenched Al–Mg–Si–Cu alloys with various copper additions on the structure of matrix and precipitates during artificial ageing as well as the effect of increasing deformation by rolling up to 90% on hardness changes during ageing.