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AFS Technical Department, Schaumburg, Illinois

CASTING ANSWERS & ADVICE Achieving Consistent Hardness Readings


Our customer requires a Brinell hardness number (BHN) of 85 on an A356-T6 sand cast-

ing, and our tests show mea- surements as low as 57 under the riser. Should we expect to achieve minimum BHN every- where in the casting? Can you suggest potential root causes for the low hardness values under the risers?

According to the metal- caster, this aluminum part typically achieves a BHN of 80 or better throughout the part. A consistent 85+

BHN, even in T6 or T61, is not easy to achieve in sand cast A356. A more common hardness would be 70-75 in sand casting and 85+ in permanent mold casting. While hardness variations can occur

in aluminum castings, it is unusual to see such a dramatic variation within the same part. The hardness obtained in T6 or T61 (solution, quench and age) comes primarily from the magnesium level in the alloy chemistry and the precipitating hardening process. In the as-cast condition, a casting would be expected to show reduced hardness in slower cooling rate areas (like in sections attached to a riser), since the magnesium can be segregated and not as uniformly dispersed. The solution heat treat portion of the T6 would typi- cally redistribute the magnesium and put it back into solution. The slower solidifi cation rates under the riser would somewhat lower the BHN. However, it should not yield the dramatically lower hardness this metalcaster reports.

Recommendations Several issues could infl uence such

a hardness differential. It is likely some MODERN CASTING / June 2010

If the riser does not adequately feed the solidifying casting, it might leave micro-shrinkage under it.

level of porosity exists under the riser. If the riser does not adequately feed the solidifying casting, it might leave micro-shrinkage under the riser or in the riser pad and extending into the casting surface, which would create a low BHN reading at that location. After checking for porosity, consider the fol- lowing casting issues to help tighten the disparity in hardness in the casting: • Grain Refining: The slower so- lidifi cation rate under the riser can create a coarser grain structure. The metalcaster should consider grain refi nement and silicon modifi cation. Proper grain refi nement will help even out the magnesium distribution and produce more consistent BHN levels, and fi ner grains eliminate BHN error readings due to element distribution and grain boundaries.

• Gating and Risering: On a part with mechanical property requirements, side risers or chilling might be better options than a riser on top of the casting. Although heat treatment alone might

not be solely responsible for the dramat- ic difference between the BHN under the riser and elsewhere in the casting, the T6 process can be a contributing factor in hardness issues. Of particular concern are the time and temperature parameters of the heat treating process and magnesium chemistry: • Solution Temperature and Time: Use higher solution temperatures for A356 (1,000-1,005F). Premium casting producers with tight tem- perature control in their heat treating furnaces can go as high as 1,010F. Higher temperatures will risk in- cipient melting. Roughly one to two hours at temperature are required for magnesium to go into solution. The remainder of the solution time is required to round the eutectic silicon particles.

• Quench Rate and Temperature: The delay time from solution bath to quench bath should be less than 20 seconds. If delays occur in the quench time, the magnesium will not be locked into solution and precipitation may begin. The colder the temperature of the quenchant, the better. However, exces- sive casting distortion can occur if the quench media is too cold. If distortion is not a concern for a particular casting, it is best to avoid boiling water and use a colder temperature. Part confi guration also can cause problems, creating air and steam pockets that slow the cool- ing rate in a local area. Glycol (10%) can help avoid this issue. How a part is put onto the furnace rack also can cause inconsistencies.

• Aging Temperature and Time: More uniform results can be attained us- ing colder temperatures and slower times. For example, 315F for eight hours deliver the same average hard- ness as 330F for four hours, but the potential for more variability arises at higher temperatures.

• Magnesium Chemistry: It is not un- usual to achieve 93-100 minimum BHN in a section thickness of 1.5 inches with a 0.36-0.4 magnesium level. A level below 0.3% might not be suffi cient to produce an 85+ BHN. If a sand casting requires 85+ BHN, push the magnesium level to the upper limit (although it may reduce ductility to below typical A356 levels).

• Heat Treat Furnace Controls: Poor heat treatment furnace controls, such as inconsistent racking, hot or cold spots in the furnace, etc., also can cause hardness problems. All cast- ings should be held at the solution temperature for the designated time. Thermocouples on the casting will determine when the casting reaches solution temperature.

MC 41

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