Parameters controlling the performance of AA319-type alloys: Part II. Impact properties and fractography

doi:10.1016/j.msea.2003.09.096

Materials Science and Engineering: A

Volume 367, Issues 1–2, 25 February 2004, Pages 111-122

https://doi.org/10.1016/j.msea.2003.09.096 Get rights and content

Abstract

The Charpy impact energy of Al–Si–Cu AA319-type alloys was measured in terms of the total absorbed energy. The Charpy specimens were machined from end-chilled castings to incorporate the effect of cooling rate on the impact properties. Unnotched specimens were used to increase the accuracy of the measurements, and to emphasize the effect of microstructure. The influence of the microconstituents on the impact strength was investigated by adding various alloying elements (i.e. Sr, Fe, and P) to the AA319 base alloy, and applying two different heat treatments (T5, and T6). The results show that strontium-modification enhances the impact properties, so that the Sr-modified AA319 alloy exhibits the highest impact properties compared to the base, and other alloys at any given dendrite arm spacing (DAS). The impact energy increases with increase in cooling rate, while iron, and phosphorus additions have a detrimental influence due, respectively, to the formation of β-Al₅FeSi, and phosphorus oxide particles during solidification. T6 treatment assists in the even distribution, and dissolution of the microconstituents (including the block-like CuAl₂ particles) into the aluminum matrix. With more Cu available for strengthening during aging, the impact toughness is greatly enhanced. In the unmodified AA319 base alloy, crack initiation, and propagation occur mainly through Si-particle fracture, and the mechanism of void coalescence. In the Sr-modified, 1.2% Fe-containing 319 alloys, however, crack initiation takes place through fragmentation of β-Al₅FeSi, Si, and CuAl₂ or Cu₂FeAl₇ particles. Crack propagation occurs through cleavage of the β-Fe platelets, and fracture of the Cu-intermetallics, and brittle Si particles. Such samples exhibit very low impact energies.

Introduction

The mechanical properties of AA319 alloy castings are essentially controlled by the secondary dendrite arm spacing (SDAS), and silicon-particle morphology. Only recently have the users of these alloy castings realized the importance of fracture, fatigue, and impact property data in optimizing design parameters, where impact strength can provide a useful estimation of the ductility of an alloy under conditions of rapid loading. It is well known that various heat-treatment procedures will provide a wide range of mechanical, and physical properties. Recent studies have indicated that strontium-modification can substantially lower heat treatment times [1], which can lead to a significant decrease in the overall cost of the finished AA319 alloy component. In addition, impact values depend strongly on the testing technique used, and in particular, on the size, and shape of the test specimens [2].

In the present work, impact specimens subjected to different heat treatments (i.e. T5, and T6) were tested unnotched in order to increase the accuracy of the measurements, and to emphasize the effect of the microstructure. In the case of brittle materials which possess low impact strengths, the presence of a notch may lower the impact values even further, by up to 50%. Also, if a notch is present, the absorbed energy may be more dependent on the notch geometry than on the microstructure [3].

The use of instrumented impact testing equipment in our work allowed the fracture response of the impact specimen to be studied in terms of the total absorbed fracture energy. The results are discussed in terms of the influence of the microstructure on the impact strength of the alloy. At the same time, in order to obtain a better understanding of the relationship between impact strength and tensile strength, total absorbed energy versus percent elongation and ultimate tensile strength plots have also been presented. (The tensile properties were discussed in Part I of this article).

In addition to metallography, fractography is generally used in upgrading material specifications, improving product design, and analyzing failures for improving product reliability [4]. In the present case, the fracture behavior of AW (base) alloy and DW (modified base alloy with 1.2% Fe addition) alloy samples obtained after T5, and T6 heat treatments have been presented, to emphasize the role of various parameters and microconstituents with respect to crack initiation and crack propagation in these alloys. The fractographs presented in this part are back-scattered (BS) electron images, which were deemed suitable to bring out the various features observed on the fracture surfaces of the alloys studied.

Section snippets

Experimental procedure

The melt treatment and casting procedures used to obtain the six alloys studied, and the heat treatments applied were the same as those described in Part I of this article (Parameters controlling the performance of AA319-type alloys. Part I. Tensile properties).

Results and discussion

In general, impact energy means the total energy absorbed by a specimen that undergoes fracture when tested under high strain rate or rapid loading conditions. This total absorbed energy E_t of the sample subjected to impact testing is the sum of the energy required for crack initiation, E_i, and the energy required for crack propagation, E_p, and can therefore be used as a parameter to describe the impact toughness of the sample material. The most common laboratory measurement of impact energy is

Impact properties

1.
An increase in the DAS value leads to a decrease in the total absorbed energy (E_t) for all the alloys studied.
2.
Strontium addition results in changing the morphology of the Si particles from acicular to fibrous nature. This lessens the stress concentration during impact testing, and effectively increases the volume fraction of the aluminum matrix, so that the Sr-modified CW alloy displays a higher impact energy than the AW base alloy at any given DAS.
3.
Iron addition leads to an increased

Acknowledgements

The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fondation de l’Université du Québec à Chicoutimi (FUQAC), the Centre québécois de recherche et de développement de l’aluminium (CQRDA), General Motors Powertrain Group (USA), and Corporativo Nemak (Mexico) for financial and in-kind support.

References (8)

Q.G Wang et al.
The fracture mode in Al–Si–Mg casting alloys
Mater. Sci. Eng. A
(1998)
S Shivkumar et al.
Effects of solution parameters and simplified supersaturation treatments on tensile properties of cast Al–Si–Mg alloys
AFS Trans.
(1990)
M Tsukuda et al.
The heat treatment of Al–7% Si–0.3% Mg alloy
J. Jpn Inst. Light Met.
(1978)
F Paray et al.
Impact properties of Al–Si foundry alloys
Int. J. Cast Met. Res.
(2000)

There are more references available in the full text version of this article.

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Effect of Fe content on the fracture behaviour of Al-Si-Cu cast alloys
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Castings were prepared from 319.2 alloy melts, containing Fe levels of 0.2–1.0 wt%. Sr-modified (∼200 ppm) melts were also prepared for each alloy Fe level. The end-chilled refractory mold used provided directional solidification and a range of cooling rates (or dendrite arm spacings, DAS) within the same casting. Impact test samples were machined from specimen blanks sectioned from the castings at various heights above the chill end provided DASs of 23–85μm. All samples were T6-heat-treated before testing keeping with Aluminum Association recommendations. The results show that at low Fe levels and high cooling rates (0.4% Fe, 23 μm DAS), crack initiation and propagation in unmodified 319 alloys occurs through the cleavage of β-Al₅FeSi platelets (rather than by their decohesion from the matrix). The morphology and the size of the platelets (individual or branched) are important in determining the direction of crack propagation. Increasing the DAS to 83μm leads to cleavage fracture. In this case, the fracture path follows a transgranular plane that is usually a well-defined crystallographic plane as judged by the relatively large smooth surfaces of the β-Al₅FeSi phase platelets. Cracks also propagate through the fracture of undissolved CuAl₂ or other Cu-intermetallics, as well as through fragmented Si particles. In Sr-modified 319 alloys, cracks are mostly initiated by the fragmentation or cleavage of perforated β-phase platelets, in addition to that of coarse Si particles and undissolved Cu-intermetallics.The amount of undissolved Cu- intermetallics is directly related to the applied cooling rate. Slow cooling rate (DAS ≈83µm) results in the precipitation of Cu- containing phases on the β-platelets, amplifying the likehood for crack propagation through these loacations.
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Variability in the impact properties of A356 aluminum alloy on microporosity variation
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However, the existence of micro-voids influences the overall deformation behavior of a material through the variation in the load-bearing capacity inside a material, which induces the degradation of fatigue and impact properties [14,15] as well as tensile properties [1–13]. Li et al. reported that the impact resistance of the A319 alloy is markedly decreased by the existence of micro-voids as well as by the variation of DAS and intermetallic compounds [14]. In terms of the load-bearing capacity, the decrease in the cross-sectional area due to the existence of micro-voids will have a practical influence on the impact response to an external load.
This study aimed to investigate the effects of variations in the microporosity of a low-pressure die-cast A356 alloy on the impact properties of the alloy and the maximum impact value achievable in the defect-free condition. The study also aimed to determine the optimal T6-treatment conditions to minimize the effect of micro-voids on impact properties. The impact properties of as-cast and T6-treated specimens were evaluated using a conventional Charpy test, and the microporosity was described with a fractographic porosity term measured through SEM observation of the fractured surface. The effect of variation in microporosity on the absorbed impact energy can be described as an inverse power relationship in terms of the exponent of defect susceptibility and the maximum impact energy that can be achieved in the defect-free condition. The defect susceptibility of the absorbed impact energy to microporosity variation was remarkably improved by T6 treatment compared to the as-cast condition. Although the defect susceptibility has little influence on the solutionizing time over a range of conventional solutionizing temperatures, the maximum impact energy achievable in the defect-free condition depends strongly on the holding time. The defect susceptibility gradually decreases with the decreasing aging time from solution treatment, and the maximum value of the impact energy decreases abruptly after 16 h of aging.
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Parameters controlling the performance of AA319-type alloys: Part II. Impact properties and fractography

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Impact properties

Acknowledgements

Mater. Sci. Eng. A

Effects of solution parameters and simplified supersaturation treatments on tensile properties of cast Al–Si–Mg alloys

AFS Trans.

The heat treatment of Al–7% Si–0.3% Mg alloy

J. Jpn Inst. Light Met.

Impact properties of Al–Si foundry alloys

Int. J. Cast Met. Res.