MARKETING MATTERS CAST TIP
Temperature Control in Melting Aluminum G
STEVE ROBISON, AFS SENIOR TECHNICAL DIRECTOR
ood foundry practice involves controlling all casting parameters and
methods to minimize variations and produce high quality compo- nents that meet customers’ needs. When it comes to aluminum, pour temperature is critical in achieving quality castings. Relatively simple to manage, melt temperature also is easy to neglect.
Molten aluminum is temperature
sensitive. Higher temperatures exac- erbate 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. (Significant increases begin at about 1,250F, or 677C, for hydrogen solubility and about 1,450F, or 788C, for oxide formation.) Temperature also affects fluidity and filling, as well as shrink- age during solidification. Some cast- ings are tolerant of tempera- ture 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. Te 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, must 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 ne- glect, the thermocouple can attach to the crucible wall via an oxide bridge (Fig. 1). In such a case, the measurement would be distorted by the oxides, and the metalcasting facility would lack accuracy in de- termining the molten aluminum’s temperature. Consistency: Te thermocouple
measurement should be taken in a consistent manner. Tis is espe- cially true for handheld pyrometers because the thermocouple tip can be inserted at different depths and/or locations in the crucible. Te temper- ature may vary somewhat throughout the molten aluminum bath, so be as consistent as possible. Always insert
the thermocouple tip 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 per- sonnel can pour the casting anywhere between 1,240–1,260F (671–682C). Te targeted pour temperature and minimum/maximum range should be clear to melting and pouring personnel, and controls should be in place to assure compliance. Many metalcasting 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 acceptable to pour. For metalcasting facilities using handheld pyrometers, it is critical to understand the furnace temperature profile, because some fur- naces will continue to climb in temperature even after the combustion and blowers have been turned off. In this case, if a delay occurs be- tween the last measurement and the pour, the tempera- ture may differ significantly. Take the temperature imme- diately before pouring. As with all process con-
Fig. 1. Insufficient furnace cleaning can lead to the formation of an oxide bridge between the crucible wall and thermocouple, result- ing in incorrect melt temperature measurements.
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 defects—and maximize ideal casting properties.
August 2013 MODERN CASTING | 49
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