Issue 35

A. Kowalski et alii, Frattura ed Integrità Strutturale, 35 (2015) 449-455; DOI: 10.3221/IGF-ESIS.35.51 450 ways of achieving a good resistance and plasticity is their thermomechanical treatment [2]. Their mechanical properties and resistance to cracking are considerably attained by the jointly acting mechanisms of strain hardening and precipitation. A crucial feature affecting the widely understood properties of aluminium alloys is their microstructure, mainly the size and morphology of the particles of the intermetallic phases arising due to alloying additions, usually soluble phases remaining in a thermodynamic equilibrium with the matrix. The microstructure of aluminium alloys is also an essential factor of their fatigue strength. Their resistance to cracking due to fatigue depends both on the mechanism of the nucleation of cracking and the aptitude to dissemination in the material. The microstructural factors restricting an easy nucleation of cracking due to fatigue are mainly hardening precipitations with a considerable dispersion, not sheared by mobile dislocations and the reduction of the relative volume of big particles of intermetallic phases, as well as the formation of fine recrystallized grains in the structure of the aluminium alloys. The destruction caused by fatigue often displays the character of brittle cracking, even in ductile alloys, due to accompanying slight plastic deformation. The fatigue of the material caused by a sustained development of cracking due to cyclically changing loads is, therefore, a dangerous form of degradation of many load-bearing structures and mobile elements of machines. This is why further investigations are still required concerning this problem. Thus, the aim of the undertaken experimental investigations was to determine the influence of differentiated conditions of loading in the course of low- and high-cyclical oscillatory bending on the fatigue strength and resistance to fatigue of cracking of aluminium alloys subjected to plastic working of the AlZn6Mg0.8Zr type of the 7000 series after their low-temperature thermomechanical treatment [3, 4]. E XPERIMENTAL PROCEDURE he investigated material was an industrial aluminium alloy of the Al-Zn-Mg type belonging to the grade 7003 according to PN-EN [5] in the shape of a sheet with the dimension 400x200x20 mm. The chemical composition of this alloy is to be seen in Tab. 1, and its mechanical properties in the delivery state and after the thermomechanical treatment in Tab. 2. Al-Zn-Mg alloys of the 7000 series display the highest potential of strength among the alloys subjected to hardening precipitation. Some of them contain copper in order to improve their resistance to stress corrosion. The overall content of Z+Mg < 6% warrants a satisfactory resistance to cracking. Denotation of the alloy and the type of analysis Chemical composition (mass %) Zn Mg Mn Fe Cr Si Zr Cu Al EN AW- 7003 EN AW-Al Zn6Mg0.8Zr 5.0 6.5 0.5 1.0 0.3 0.35 0.2 0.3 0.05 0.25 0.2 bal. Analysis of smelting 6.13 0.74 0.29 0.20 0.17 0.12 0.08 0.04 bal. Table 1 : Chemical composition of investigated alloy. Mechanical properties State of materials 0.2 p R [MPa] m R [MPa] A [%] Z [%] Delivered 347.0 400.4 14.1 35.5 Low-temperature thermomechanical treatment 255.8 321.2 10.2 49.5 Table 2 : Mechanical properties of investigated alloy. The investigated alloy was subjected to a low-temperature thermomechanical treatment as shown in Fig. 1:  preheated up to 500 o C and soaked for one hour,  cooled down in water, T