casting 101 C
Temperature Control in Melting Aluminum
Steve Robison, AFS Senior Technical Director W
hen it comes to aluminum, pour tempera- ture is critical in achieving
quality castings. Melt temperature is relatively simple to manage, and your suppliers should have appropriate controls in place.
Molten aluminum is tempera-
ture sensitive. Higher temperatures exacerbate the formation of oxides and hydrogen gas. Both the solubility of hydrogen in molten aluminum and the thickness of the melt’s aluminum oxide layer increase exponentially as temperatures rise. (Signifi cant increases begin at about 1,250F, or 677C, for hydrogen solubility and about 1,450F, or 788C, for oxide for- mation.) Temperature also aff ects fl u- idity and fi lling, as well as shrinkage during solidifi cation. Some castings are tolerant of temperature variations, while others require strict controls.
Measuring for Accuracy Temperature generally
is measured by a pyrometer with a thermocouple that typically sits in the molten metal. T e keys to precise measurements are the three “Cs”—calibration, cleanliness and consistency. Calibration: Because a
metalcaster’s only indica- tion of the melt temperature depends entirely on the measuring device’s accuracy, pyrometers and thermo- couples, whether in-pot or handheld, should be checked and calibrated regularly. Ad- ditionally, many facilities in- corporate an in-house check, using a handheld pyrometer to verify the in-pot thermo- couple reading. Cleanliness: The ther-
mocouple cover should be free of oxide buildup, which can distort the temperature reading. When the furnace or crucible is cleaned, the in-pot thermocouple also must be removed and cleaned to prevent buildup. In severe cases of neglect, the thermocouple can attach to the crucible wall via an oxide bridge (Fig. 1). In such a case, the mea- surement would be distorted by the oxides, and the metalcasting facility would lack accuracy in determining the molten aluminum’s temperature. Consistency: T e thermocouple
measurement should be taken in a consistent manner. T is is espe- cially true for handheld pyrometers because the thermocouple tip can be inserted at diff erent depths and/ or locations in the crucible. T e temperature may vary somewhat throughout the molten aluminum bath, so it is best when the thermo- couple tip is inserted to the same
depth and in the same location in the bath (say, in the center rather than near the crucible wall).
Staying in Range Most metalcasting facilities will
set a target pour temperature for each casting and establish a correlating temperature range, such as ±5–10F. If an individual casting calls for a pour temperature of 1,250F (677C), a range of ±10F means melt person- nel can pour the casting anywhere between 1,240–1,260F (671–682C). T e targeted pour temperature and minimum/maximum range should be clear to melting and pouring person- nel, and controls should be in place to assure compliance. Many met- alcasting facilities incorporate sensors and visual aids, such as a red light when the temperature is out of range and a green light when it is accept- able to pour. Metalcasting facilities using handheld pyrometers should pay attention to the furnace temperature profi le, because some furnaces will continue to climb in temperature even after the combus- tion and blowers have been turned off . In this case, if a delay occurs between the last measurement and the pour, the temperature can diff er signifi cantly. As with all process con-
Fig. 1. Insuffi cient furnace cleaning can lead to the formation of an oxide bridge between the crucible wall and the rmocouple, resulting in incorrect melt temperature measurements.
46 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2013
trol factors, proper training and clear, written procedures are crucial to consistency and compliance. Pouring temperature is within the control of melt personnel. Diligence and a few simple engineering controls can minimize the potential for temperature-related de- fects—and maximize ideal casting properties.
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