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Controlling hardness in aluminum castings.

When purchasing aluminum castings, most casting buyers and engineers will specify the quality required based on the final component's end use, typically demanding mechanical properties such as tensile strength, yield and elongation. Occasionally, buyers will ask for a specific Brinell hardness in the casting. A recent discussion with an aluminum metalcasting facility brought this issue to light.

A customer required an 85 minimum Brinell Hardness number (BHN) for an aluminum component sand cast in A356.0 alloy and heat treated to a T6 temper. This standard raises several issues for the metalcasting facility to consider. Where on the casting will the harness reading be taken? Will an aluminum casting have consistent BHN hardness everywhere in the casting or will the hardness vary? What factors would cause variations or lower than necessary hardness values? What process controls can be established to reduce variations and ensure the standard is met?

Hardness is the resistance of a material to deformation, particularly permanent deformation, indentation or scratching. There are several tests that measure hardness, and each type of test has its own defined scale of measure. The Brinell test measures resistance to indentation and is a common technique used to determine the hardness of castings. ASTM E10 describes the Brinell test methods and is the common reference cited for cast metals. The Brinell test is performed by applying a defined load to a small metal ball and measuring the size of the casting surface indentation. The impression diameter is read with a manual scope or optical reader. For most aluminum alloys, a 500 kg test load is applied to a 10 mm tungsten carbide ball for 10 seconds. (Some harder aluminum alloys may require a bigger load and European standards are slightly different so metalcasters should check the casting print for the relevant standard.) The hardness values (listed in E10) are based on the applied load and the diameters of the ball and impression. The test results may be subject to some variation when humans manually read impression diameters or hardness impressions are too close together and affect one another. It is worthwhile to recheck the hardness when a low reading is obtained.

Factors Affecting Hardness

In this case study, the customer request for an 85+ BHN in an A356 T6 aluminum sand casting is not easy to achieve (70-75 is more typical), although this might be more feasible in permanent mold. The metalcaster may want to discuss this requirement with the buyer to find out its basis and how the casting will be used in its final application. Here are a few metallurgical and casting process factors that will contribute to hardness:

Variability: Metal castings are not homogenous, and hardness variations will occur throughout the typical casting, though the variation between locations should not be dramatic. Particularly, the cooling rate of the casting affects dendrite arm size and will have an effect on a hardness test. Casting sections closer to gates and risers may have slower solidification that would lead to larger secondary dendrite arm size and a lower BHN than rapidly cooled areas (with smaller dendrite arm spacing).

Heat Treatment: Poor heat treatment controls can cause softer castings and an increase in variability. The T6 or T61 heat treatment (solution, quench and age) also can be modified to improve the hardness. The primary factor during heat treatment is the magnesium level in the alloy and the precipitating hardening process. The solution heat treat portion of the T6 should redistribute the magnesium and put it back into solution. Factors that could influence the uniformity of this redistribution and the effectiveness of the heat treat process are:

* Solution temperature: The higher the better for A356 (1,000-1,005F). Premium heat treat facilities that can maintain tight temperature controls on their furnaces can go to 1,010F without risk of incipient melting.

* Solution time: The foundry should check part racking (to ensure adequate heat transfer) and test to see if there are cold and hot spots within in the furnace. Confirm all castings are reaching full solution temperature and held long enough.

* Quench rate: This is a critical factor. The delay time from solution bath to quench bath should be less than 20 seconds. Excessive delays can allow the magnesium to come out of solution and start precipitation. It is important that the magnesium has been dissolved into the matrix and finely dispersed.

* Quench temperature: In general, the colder the better, unless the part geometry makes it subject to distortion. Avoid boiling water. How the castings are put onto the rack also can be an issue. Part configuration and improper racking can create air pockets that collect steam, which slows the cooling rate in that area.

* Aging temperature and time: Colder and slower aging provides more uniform results. For example, 315F for eight hours might give the same average hardness as 330F for four hours, but there is more potential for variability at higher temperatures.

Chemistry: To reach the higher Brinell measurement (85+) in sand castings, the metalcasting facility would probably need to increase the magnesium content to the upper limit of the alloy specification (or higher) for a better heat treat response. However, the additional hardness may reduce ductility to below typical A356 levels.

Grain Refinement & Modification: Proper grain refinement (grain size 500-1,000 microns) will help even out magnesium distribution and reduce hardness variability.

Casting Quality: Micro shrinkage porosity can reduce a BHN reading and will increase its variability. If there are significant differences in the BHN near risers or in thicker sections than the balance of the part, micro porosity may be a cause. Foundry personnel should check gating and riser layout to ensure adequate feed metal during solidification.

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Title Annotation:CAST TIP
Author:Robison, Steve
Publication:Modern Casting
Date:Nov 1, 2014
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