The percent elongation defines the ductility of a material and consequently its capability of undergoing deformation without breaking in case of crash. A key parameter in the design of structural aluminium components for automotive applications
by Annalisa Pola, University of Brescia (DIMI – Metal L@bs)*
The percent elongation is a property of the material, which provides a value of its ductility, i.e. of its capability of undergoing plastic deformation when kept under load before breaking.
The greater the deformation reached before breaking, the more the material is considered ductile. This implies a high value of the fracture work, that is, the presence of evident deformations, which provide forewarning as to the break. On the contrary, the less deformation a material undergoes before reaching its breaking point, the more it is considered brittle. Unlike ductile fractures, brittle fractures are characterized by low values of fracture energy and are sudden. Therefore, the higher the ductility, the higher the percent elongation (A% or also El%) of the material.
Why is percent elongation important?
Nowadays the elongation is considered a fundamental parameter in the design of structural components for the automotive sector because it is (or El%) an index of crash resistance. If the material is ductile, therefore with a high A% (or El%), during a crash it will deform plastically. Plastic deformation is the mechanism used to dissipate the kinetic energy of the vehicle in case of accidents, therefore, by absorbing the impact before breaking, it increases passenger safety. Of course, in order to improve crash resistance, when designing an automotive component it is necessary to choose the shape and size of the part adequately. However, fracture behaviour is a property of the material, thus the choice of the alloy and the control of its micro-structure/quality during production play a fundamental role to reach the required performances.
How is it measured?
As we know, A% is measured by means of tensile test.
Tensile test is a static destructive test. It consists in submitting a standard specimen (with specific dimensions) to a gradually increasing tensile load, applied uniaxially along the long axis of a sample, until it breaks. The machine used must be properly calibrated.
Samples may have a circular or rectangular cross section (Figure 1) and we can distinguish in:
• the so-called “parallel length” LC,
• the initial length L0 between the reference marks ( I ), shorter than the previous one and termed “gauge length”;
• the diameter/width of the gauge section that determines an area S0;
• the heads, whose size and shape enable them to be held by the machine’s clamping system;
• the connections between the gauge length and the heads;
• the total length.
It is generally assumed that the total length is greater than the parallel length, particularly LC + 2d0.
There are several standards, which generally prescribe the use of so-called proportional samples, i.e. specimens with a constant ratio between the area of the initial cross section, S0, and the gauge length, L0 :
Regarding parts obtained using foundry techniques, it is not always advisable to check the resistance and ductility using separately cast samples; this is because the castings, having different thicknesses, are characterized by different cooling rate in different points, and therefore different micro-structures. Additionally, as a function of the filling and solidification methods, there may be porosity due to gas or shrinkage or defects localized in various areas, which, evidently, affect the product’s performances.
In the case of sand or shell cast foundry alloys, generally characterized by slow cooling, the standards allow the use of separately cast samples, when they are solidified in conditions comparable to those of the actual part and when the same alloy (meaning, the same metal lot) is used. For die-casing, however, this choice is not recommended (UNI EN 1706) and moreover international standards do not provide detailed instructions (UNI EN 1706, ASTM B 557M). Thus, typically, the samples are derived from the casting itself, in specified positions.
The machining and subsequent finishing of the sample must be carried out with the utmost care, so as not to alter either the geometry or the properties of the material, that is, the sample must not be heated or hardened, nor must there be traces of tools or surface markings which may modify the results of the test.
Even the machining of the clamping heads must be carried out appropriately, so as to prevent a wrong positioning of the specimen in the testing machine, ensuring its perfect centering and alignment (Figure 2, ).
Even the application of the load/deformation rate must be adequately defined; if it is too high, it might lead to the determination of a low A% and a high fracture load, that is, to a brittle behaviour. Standards require:
• different load application rates according to the elastic modulus of the material being tested;
• different deformation rates according to the part of the graph under definition.
During the test, the machine records the load as a function of the elongation, thereby providing as an output an F-∆L curve. Such a curve is converted into a stress-strain graph (Figure 3, ), from which the elastic modulus, the yield strength and ultimate tensile strength may be calculated, once the dimensions of the sample are known.
If the alloy being examined has a ductile behaviour, once the sample breaks, percent elongation is obtained as:
where Lf is the gauge length at the end of the test.
However, it must be underlined that the percent elongation depends on the gauge length L0.
Deformation is uniformly distributed in the sample only until the maximum load is reached (σR in Figure 3); at this point, the phenomenon known as necking occurs, implying the localization of deformation in a restricted area. As shown in Figure 4, the total elongation at breaking point may therefore be seen as the sum of two contributions: a uniform one (εU) and a localized one (εL). The first contribution is given by the ratio ∆LUniform on L0 and, since ∆LUniform is proportional to L0 itself, then the uniform deformation does not depend on the initial length L0. On the contrary, the localized contribution is basically constant, irrespective of L0; therefore, the ratio
∆LLocalized/L0 is larger the smaller the gauge length L0. As a consequence, percent elongation is larger for samples with a shorter L0.
It is now clear that it is necessary to specify the gauge length: for instance, A5% stands for the percent elongation for proportional samples with length L0 equal to five times the diameter.
No less important for the evaluation of the materials ductility is the percent reduction in area (Z%), defined as:
where S0 is the initial cross section of the sample and Sf the final one.
The smaller the value of Sf, the higher the value of Z% and therefore, the more ductile the material.
Typically, the percent reduction in area is measured only on samples with a circular section because the section reduced by necking remains essentially round. On the contrary, with rectangular samples, corners prevent uniform deformation, the final section is no longer rectangular and, as a consequence, measurement becomes more difficult. For this reason, it is preferable to measure A%.
Die castings with high percent elongation
Structural castings must ensure high performances (UTS up to 350-380 MPa) but also a high A% (considerably above the values found in traditional die casting).
In order to achieve such performances it is necessary to operate in a way that:
• the part is practically free from gas porosity/defects;
• the proper heat treatment are chosen;
• the appropriate alloys are used.
1) Eliminating/reducing porosity and defects in general
Porosity, caused by gas or shrinkage, just like other defects in castings, may be particularly dangerous because, when a load is applied, in correspondence of these discontinuities in the matrix there is a concentration of the stress, with a consequent localized plastic deformation and the development of micro-cracks, which may lead to rupture. As a result, the ultimate tensile strength and A% are reduced (as shown in Figure 5 in the case of an AlSi9Cu3 alloy) besides the impossibility of carrying out heat treatments.
In order to avoid or at least to reduce as much as possible porosity and other defects that may occur during filling (or solidification) it is necessary to use vacuum, to design adequately the casting runners and gating systems and to thermoregulate the die accordingly. In this respect, the use of process simulation is essential.
2) Defining the correct heat treatment
The correct combination between temperature and treatment time determine the achievement of better performances and these parameters must be defined purposely for die casting alloys for structural parts (as shown by the speech by Prof. A. Morri during the same Study day).
3) Choosing the correct alloy
Die casting alloys belong mainly to the Al-Si + Cu and/or Mg family (Table 1).
For some specific applications, Al-Mg or Al-Cu alloys may also be used, but this rarely occurs.
The reason that leads to preferring such compositions is the need to ensure an excellent fluidity/castability to fill in lower thicknesses.
From the analysis of the most widely used alloys (Table 1) it is possible to note the presence of a remarkably high Iron content, needed to reduce the risk of die soldering (a well-known issue in die casting). This represents an advantage on one hand, since it allows the use of secondary (recycled) alloys, but at the same time it provides the disadvantages of the presence of a high number of intermetallic compounds, which reduce the casting’s performances, as shown by several studies [5-6] (Figure 6).
Fe generates intermetallics (e. g., Al5FeSi o Al15(Mn,Fe)3Si2) which make the alloy more brittle and, in presence of a load, break more easily than the Al matrix, besides representing a preferential path for cracks propagation. Generally, Mn (or Cr) is added to change their morphology (Figure 7), making them less dangerous but increasing their number, besides increasing the risk of forming sludge in the furnace (Sludge factor).
Some studies also show the existence of a critical Fe value, function of the percentage of Si%, above which intermetallics form more easily. Additionally, they have a negative effect on gas porosity and, above all, on shrinkage micro-porosity because they prevent the alloy from flowing into the inter-dendritic spaces .
It is therefore evident that, in order to improve the performances of die cast parts for structural applications, it is necessary to use an alloy characterized by very high purity, and above all by a very low Fe content.
But, in this case, how can die soldering be prevented if Fe is low? Recent studies show the validity of Sr as anti-soldering, besides that it seems to change the morphology of Fe intermetallics, and not just that of eutectic Si, thereby reducing the risk of formation of shrinkage micro-porosity (Table 2).
However, not only the iron is responsible for the reduction in the properties of castings; even eutectic Silicon affects performances, representing a preferential path for the cracks propagation; fractures, in fact, propagates through the eutectic phase, to the a-Al/eutectic-Si interface or along the particles of Si itself. In these points the fracture is not ductile but by cleavage.
Smaller lamellae ensure better performances, but do not solve the problem. What can be done? Recent studies are showing interesting results obtained by a modifying treatment of die casting alloys.
Percent elongation is a fundamental parameter for the design of structural castings. Its measurement must be carried out carefully (preparation of the sample, machining, test methods).
The quality of the casting is of course essential for high levels of A%.
The micro-structure, i.e. the way the casting solidifies, affects performances; therefore, the choice of suitable alloys (in terms of chemical composition and treatment of the liquid metal) as well as the heat treatment parameters represent fundamental aspects in order to achieve the properties that structural castings must guarantee.
1] W. Nicodemi, Metallurgia principi generali, Zanichelli, Bologna, 2000.
2] J.R. Davis, Tensile Testing, Second Edition, ASM International, 2004.
3] W. Callister e D. Rethwisch, Materials Science and Engineering, Wiley, 2010.
4] https://www.giessereilexikon.com/en/foundry-lexicon/Encyclopedia/show/gas-porosity-3790/?cHash=2101492df6a9cf38707fd1fa8938d60f.pdf April 2019].
5] R. Lumley, Fundamentals of Aluminium Metallurgy: Recent Advances, CRC Press, 2010.
6] S. Midson e J. Brennan, Effect of iron on the mechanical properties of T5 heat treated 360 alloy die casting, in NADCA Die casting Congress & Tabletop, 2014.
7] M. Tocci, A. Pola, G. La Vecchia e M. Modigell, Characterization of a New Aluminium Alloy for the Production of Wheels by Hybrid Aluminium Forging, in Procedia Engineering – XXIII Italian Group of Fracture Meeting, IGFXXIII, 2015.
8] J. Taylor, Iron-Containing Intermetallic Phases in Al-Si Based Casting Alloys, in Procedia Materials Science – 11th International Congress on Metallurgy & Materials SAM/CONAMET 2011.
9] < www.diecasting.org/docs/information/State%20of%20the%20IndustryPPT-nov2015.pdf> [April 2019].