Mechanical Properties How to Exploit the Maximum Mechanical Properties from an Aluminum Casting Alloy
In the multidisciplinary field of foundry, much can be done for good mechanical properties, but much can also be spoiled. This article addresses the question of how to achieve the right casting properties.
For high mechanical properties, good metallurgy, proper gating and feeding system including the good position of the casting in the mold (depend from casting technology: gravity casting, tilt-casting, Rotacast, roll-over process, LPDC, counter pressure casting, HPDC) are assumed. We choose the material of the mold, we work with different coating systems, we can cool the mold and heat treatment has a great influence. Would you take a guess how the solidification rate affects mechanical properties compared to heat treatment? The answer is ambiguous. It also depends on the alloy! Therefore, it can be said that the effect can be 50/50, or the solidification rate represents only 10 % and 90 % will be contributed by the heat treatment. We may have state-of-the-art equipment, but when technology fails, we do not understand the context, effort is futile. This article deals with the topic.
How Far Does the Solidification Rate of the Casting Affect its Mechanical Properties?
The first decisive criterion is mold material when the cooling effect is comprehensively characterized by the heat accumulation coefficient of the mold and physical quantities thermal conductivity, specific heat capacity and density are applied here. Therefore, the mechanical properties of sand and mold castings can be easily found in the standards for individual alloys . For example, for alloy A356 (AlSi7Mg0.3), the difference in tensile strength values for sand and mold castings is from 10 to 20 %, for ductility the difference between sand and mold casting is more significant when it rises by 30 to 50 %. If we need to choose a material that reliably removes heat from the casting, and also easily withstands the effects of shot blasting or the chemical-thermal effect of the Al-alloy melt, then Densimet 185 with a thermal conductivity of 85 Wm-1K-1 is suitable (brass has 110-130, bronze ~70, hot work steel < 35, cast iron up to 63). With its conductivity ~390 Wm-1K-1, the copper internal chill is ideal, but it requires demanding maintenance. Of course, in the place of local cooling, the protective coating, common for the steel mold, is not used or thickness of layer is really small (to minimize warm insulation effect). The choice of technology does not only depend on properties of the cast material. Economic factors are taken into account, where sand casting can be highly productive and tools (model plates) incomparably cheaper than steel molds, dimensional accuracy of production, surface quality, etc. are taken into account as well.
Nevertheless, the situation has significantly changed by heat treatment. Tensile strength 340 MPa for alloy A356 (AlSi7Mg0.3) can be reliably achieved by heat treatment. However, it has certain preconditions. Heat treatment increases the tensile strength against the casted condition by 40 % and more. When we help with heat treatment, a T6 heat-treated casting from a permanent mold can have twice the ductility than a heat-treated sand mold casting, but the difference in strength is definitely not so significant. It would seem that high-quality heat treatment of sand or shells castings will ensure "high mechanical properties". Nonetheless, a distinction must be made between strength and ductility (indicating that the material is tough). At the same time, the difference of 10 % strength can be 30 MPa and this is a big difference.
You can work with the speed of solidification - the molds can be cooled with pressure air and water, where a different story is written. What can be expected from water cooling of the mold? Water has a thermal conductivity of 0.604 Wm-1K-1, while air only 0.026; the specific heat capacity of water is 4,2 kJkg-1K-1, while the air has only approx. 1 kJkg-1K-1 and the heat transfer coefficient is also at least an order of magnitude higher in the case of water than in the case of air (Wm-2K-1). These are the reasons for at least twice as intensive heat transfer by water as by compressed air. An important factor is economical when compressed air is the most expensive medium in a foundry. The effect of solidification rate can be elegantly measured by the size of the S-DAS (distance of secondary arms of alpha-phase dendrites), when the solidification time has a strong influence on the microstructure and thus, mechanical properties (S-DAS value of 10 µm is unattainable in normal operating practice; it is possible to calculate in real practice with MIN 14 µm and still close to the casting crust).
In addition to the fact that the mechanical properties increase with a fine-grained structure, the influence on the size of material defects. For simplicity, let us now not consider the influence of dissolved hydrogen, and especially, oxide inclusions - let's assume an ideally purified melt and a technologically mastered filling of the mold cavity. When taking a sample for the tensile test, we come across an oxidic macro-inclusion in the material (I perceive the boundary between micro and macro 0.40 mm), then we can hardly expect a higher elongation than 1 %. The effect on fatigue strength is also fatal if internal defects occur in the area of macro-size. Inclusions are simply a disaster, and those who cannot cope in the metallurgy foundry, let them try no longer.
Let's focus on the quality of heat treatment. Large differences of heat treatment impact on mechanical properties can be read from the literature. Holding time parameters are sometimes absurd long at both dissolution and aging temperatures - no common customer would buy such castings. Therefore, foundries can be recommended performing internal tests and finding out the real hardening curves, i.e. where the actual maximum strength lies, typical of the T6 mode depending from aging temperature. Overaging induces microstructural changes in the material (changes can be documented by STEM) and that S-DAS alone has no longer an effect on strength. Completely uniform heating up of the charge in the furnace, temperature homogeneity of the furnace volume throughout the holding time of solution annealing and cooling rate (determined by CQI-9 and NADCAP standards) are required for heat treatment. The perfect microstructure of the material from the solidification of the casting will not help us, if during dissolution the undissolved intermetallic phases with hardening elements remain in the structure. At the same time, the charge is not well (rapidly) cooled so, the atoms of the hardening elements form again intermetallic phases instead of strengthening material in the Guinier-Preston zones.
However, it will always be necessary to feed volume shrinkage during the alloy solidification. The more complex the casting, the rather simple feeding is not enough. No heat treatment will save any negative effect of dendritic porosity formed during the solidification. Nevertheless, a technologist has the opportunity to work with more directed solidification thanks to the water cooling of suited mold areas. Not only are the pores absolutely smaller with the solidification rate, but the solidification front can be controlled by water cooling. In doing so, it is necessary to avoid interrupting the temperature gradient towards the riser (feeding system), if the design allows it slightly at least. Another task is the care of cooling circuits. Significant differences in heat transfer cannot be accepted due to gradually increasing sediments such as calcium and magnesium oxides on the walls of the cooling channels. Due to this, the position of temperature axis of the solidifying casting can even be changed in the order of centimeters.
For simplicity in this paper, I did not take into account the very negative influence of inappropriate morphology of intermetallic phases, e.g. based on iron, on mechanical properties. Incidentally, the intermetallic phase as such in the final structure of casting material never positively contributes to mechanical properties. Solidification rate also influences the size of intermetallic phases – very fast cooling rate minimizes negative effect, for example, of phases Al5FeSi/Al3Fe.
 Aluminium Casting Technology, American Foundry Society, Illinois, 2001, 356 p.