Effect of Mg Content and Heat Treatment on the Mechanical Properties of Low Pressure Die-Cast 380 Alloy

Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2016, Article ID 7841380, 12 pages http://dx.doi.org/10.1155/2016/7841380 Research Article Effect of Mg Content and Heat Treatment on the Mechanical Properties of Low Pressure Die-Cast 380 Alloy S. Morin,1 E. M. Elgallad,1 H. W. Doty,2 S. Valtierra,3 and F. H. Samuel1 1 Université du Québec à Chicoutimi, Saguenay, QC, Canada General Motors, Materials Engineering, 823 Joslyn Avenue, Pontiac, MI 48340, USA 3 Corporativo Nemak, S.A. de C.V., P.O. Box 100, 66221 Garza Garcia, NL, Mexico 2 Correspondence should be addressed to F. H. Samuel; Received 5 June 2016; Revised 3 September 2016; Accepted 18 September 2016 Academic Editor: Akihiko Kimura Copyright © 2016 S. Morin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The present study was carried out on a 380 alloy containing 9.13% Si, 3.22% Cu, 1.01% Fe, 0.06% Mg, 0.16% Mn, and 2.28% Zn. The magnesium level was increased to 0.3 and 0.55%, by adding pure Mg to the melt. Tensile and fatigue samples were produced using low pressure die casting. The results show that the average dendrite arm spacing was about 6 𝜇m. Increasing the amount of Mg from 0.06% to 0.55% increased the volume fraction of 𝜋-Al8 Mg3 FeSi6 and Q-Al5 Cu2 Mg8 Si6 phases from 0.8% to 1.7%. Following solutionizing at 490∘ C for 8 h, the maximum ultimate tensile strength was obtained from alloys containing 0.3% Mg. Further increases in Mg content resulted in an increase in the amount of insoluble intermetallics and, hence, low tensile strength. Aging at 155∘ C for times up to 25 h resulted in a linear increase in the alloy strength regardless of the amount of added Mg. Aging at 220∘ C, however, revealed multiple peaks corresponding to the precipitation of various phases. A good relation between the applied force and the number of cycles prior to failure was established. The alloy containing 0.3% Mg produced the best fatigue resistance. The effect of porosity was more pronounced on the fatigue samples than on the tensile bars. 1. Introduction Fatigue is considered the most common mechanism by which engineering components fail and accounts for at least 90% of all service failures due to mechanical causes. Failures occurring under conditions of dynamic loading are termed fatigue failure. Fatigue failure is particularly insidious as it occurs without any obvious warning, resulting in sudden or catastrophic failures [1, 2]. Fatigue is defined as the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point that may culminate in cracks or complete fracture after a sufficient number of fluctuations [3]. Three factors are necessary for fatigue failure: (i) a maximum tensile stress of sufficiently high value, (ii) a large variation or fluctuation in the applied stress, and (iii) a sufficiently large number of cycles of the applied stress. The fatigue process itself can be divided into four stages: (i) cyclic hardening/softening, (ii) crack nucleation, (iii) crack propagation, and (iv) overload (fracture). At low amplitudes, the nucleation stage can occupy the majority of the fatigue life, while, at high amplitudes, nucleation is accomplished within a small fraction of the fatigue life [4]. A fluctuating stress cycle is made up of two components, a mean stress, 𝜎𝑚 , and an alternating stress, 𝜎𝑎 . Taking into consideration the maximum stress, 𝜎max , the minimum stress, 𝜎min , and the stress range 𝜎𝑟 (defined as 𝜎max − 𝜎min ), 𝜎𝑚 and 𝜎𝑎 are defined as follows: Mean stress: 𝜎 + 𝜎min 𝜎𝑚 = max . (1) 2 Alternating stress: 𝜎𝑎 = 𝜎max − 𝜎min . 2 (2) The general factors affecting the fatigue life of cast aluminum alloys include the stress amplitude, mean stress, stress 2 concentration, surface effects (surface roughness, stress raiser at the surface, and surface hardening), size and design of the component, effects of environment (corrosion, oxidation, fretting, and temperature), thermal stresses, and the effect of metallurgical variables [5]. The metallurgical factors that are taken into account, in order to ensure optimum fatigue performance, include heat treatment, alloying additions, stacking fault energy, grain size, and inclusion and porosity content. Ammar et al. [6– 10] studied the effect of porosity on fatigue strength of AlSi alloys. The porosity levels were varied by adding hydrogen gas to the alloy melt. The pores have a negative effect on the fatigue strength in that as the porosity volume fraction in the specimen increases, the fatigue life decreases. Using scanning electron microscopy to examine the fatigue fracture surface, it was found that the fatigue crack is often nucleated at pores. They also observed that the effect of inclusions on porosity formation depends not only on the number, size, and spatial distribution but also on the nature, that is, type and shape of the inclusions. The application of chilling and insulation also affect the solidification time and, consequently, the casting soundness. The appropriate location and sizing of risers are thus very important in the production of high-quality castings. Sigworth and Caceres [11] reported that, in directionally solidified castings, a significant increase in the amount of dispersed microporosity is observed when moving from a location near the chill to a location near the riser. This can create problems in alloys having extremely long freezing range in that the riser section may become mushy and stiff long before the interior of the casting freezes completely. On the macroscopic scale, a fatigue failure can usually be recognized from the appearance of the fracture surface, which consists of two main regions: (i) a smooth region, due to the rubbing action as the crack propagates through the section, and (ii) a rough region, where the component has failed in a ductile manner when the cross-section is no longer able to carry the applied load. The progress of the fracture is indicated by a series of rings, or “beach marks” progressing inward from the point of initiation of the failure [12]. Fracture surfaces are often called typical fatigue failures because they exhibit the following common features [13]: (i) a distinct crack nucleation site (or sites), (ii) beach marks indicative of crack growth, and (iii) a distinct final fracture region. Gundlach et al. [14] studied the effects of microstructural variables such as solidification rate, dendrite arm spacing (DAS), level of porosity, eutectic silicon modification, and Ferich intermetallic phases on the thermal fatigue properties of Al-Si alloys. They found that when the increase in porosity content was accompanied by an increase in DAS, thermal fatigue life dropped by 66%, probabl (...truncated)