## 1 Introduction  
Aluminium alloys are becoming increasingly important in various industrial sectors. These alloys are characterized by a higher ratio of strength to their specific gravity compared to steel. Low specific gravity is an advantage for the automotive industry, where a lower vehicle weight reduces CO2 emissions [1]. Two groups of aluminium alloys are distinguished: cast aluminium alloys [2] and wrought aluminium alloys [3]. The content of the main alloying elements in cast alloys reaches up to 30%, while in wrought alloys up to about 10%. Wrought aluminium alloys usually contain up to 5% of alloying elements and are used in the hardened and heat-treated conditions. The 6000 series aluminium alloys are among the most commonly used grades in the automobile manufacture for body-in-white applications [4]. These alloys contain magnesium (0.2-3%) and silicon (0.2-1.8%) as the main alloying elements. Some of the 6000 series alloys contain manganese (up to 1.4%) and copper (up to 1.2%). The alloys in this group exhibit good formability, good welding performance and good corrosion resistance [5]. The 6000 series alloys, which do not contain copper, have good corrosion resistance and can be anodised. Typical applications of Al-Mg-Si alloys include automotive structural components, interior fittings and profiles for construction [1]. The Al-Mg-Si aluminium alloys gain enhanced mechanical properties through heat treatment processes based on age hardening. The modification of the mechanical properties of aluminium alloys allows for ensuring optimal formability, especially during sheet metal forming processes, which are the fastest method of forming thin-walled components with a complex shape [6,7]. The \(\beta\) phase, consisting of Mg\({}_{2}\)Si dispersed in the aluminium matrix, is the basic mechanism for strengthening 6000 series aluminium alloys [8]. This phase is formed as a result of solutionising and ageing [9,10]. Solution heat treatment consists in heating the charge at a temperature above the solubility limit. Solutionising ends with rapid cooling. During ageing, the solutionizing component is separated from the solution in the form of fine phases with a specific degree of dispersion. The ageing process can occur naturally at room temperature or artificially by heating the material for a specified time at a specified temperature. The precipitates in 6000 series alloys are (i) Mg\({}_{2}\)Si precipitates of angular or spherical shape, depending on the ageing stage; (ii) AlMnSi precipitates, which are spherical or irregularly elongated; and (iii) AlFeSi precipitates of platelet or elongated particle shape. [PERSON] et al. [11] modified the heat treatment conditions of EN AW-6061 aluminium alloy by changing the temperature and duration of the heat treatment in order to improve the formability of the sheets. It was found that the heat treatment affects the precipitate distribution and grain size, thus improving the tensile mechanical properties. [PERSON] et al. [8] studied the effect of the strengthening \(\beta\) phase through solutionizing and artificial ageing heat treatments on the ultimate tensile strength properties of the EN AW-6061 aluminium alloy. The results revealed the formation of equiaxed grains and \(\beta\) precipitates alongside Fe-Al-Si dispersoids. The analysis of the change in material properties is possible by determining the correlation between properties and thermal diffusivity. In [12], the mechanism of how first carburising and quenching of AISI-8620 steel affects the thermal diffusivity depth profiles was investigated. It was found that thermal diffusivity is dominated mainly by changes in microstructure occurring during the quenching of carburised steel. At the same time, the presence of anticorrelation between thermal diffusivity and hardness of the tested material was found. A similar effect was obtained in [13]. Two methods of measuring thermal diffusivity depending on the size of the measurement area were also proposed here: photoreflectance and IR measurements. [PERSON] et al.