Table 1. Correction Factors for Common Alloys Alloy
metalcasting facility: 1. In the transfer ladle, used to
convey metal between melting and holding furnaces. 2. In crucible furnaces, usually just
before the molten aluminum is cast. 3. In an in-line system, when the metal is conveyed to holding furnaces through a launder. Te first two options are most
common and the degassing operation for both is typically accomplished using a rotary impeller degasser (RID). In practical terms, all rotary degassers are not created equal. It is important to have an optimum head design to produce highly efficient, small bubbles. Significant cost savings may be realized from shorter treat- ment times and reduced gas usage. In the past, the metalcasting industry has gravitated toward simple head designs, which are less costly to machine but produce larger bubbles. Tis path presents a false economy due to reduced efficiency.
Adjusting Process Parameters Once the RID unit is fully
lowered into the liquid metal with the shaft in place, the degassing operation can begin. The best shaft location is slightly off the centerline of the crucible or ladle to help avoid
206 308 319 355 356 357
380 390 413 512 535 713
Alloy Composition Al-4.5Cu
Al-5.5Si-3.5Cu Al-6Si-3.5Cu Al-5Si-1.3Cu Al-7Si-0.3Mg Al-7Si-0.5Mg
Al-8.5Si-3.5Cu-2Zn Al-17Si-0.8Cu Al-12Si-0.8Cu Al-4Mg-2Si Al-7Mg Al-7.5Zn
C
0.72 0.57 0.56 0.68 0.67 0.68
0.51 0.39 0.54 0.95 1.18 1.00
flow increases, the size of the bubbles should increase. Te desired result is a good disper-
sion of small bubbles while maintain- ing a relatively quiet surface. When an optimal combination of flow rate and shaft rotation speed is found, note the parameters for future use. Also note the total degassing time, which nor- mally falls somewhere between four and eight minutes, unless the tempera- tures are very high or the amount of gas needed is low.
Gas Measurement Tere are two primary methods of
vortex formation with its circular rotation in the liquid metal. An offset distance of 2-4 in. from the centerline is usually sufficient. The use of a baffle plate also is a good idea, because the baffle opposes the circulation movement of metal and reduces vortex formation. With the RID in the proper loca-
tion, the unit should be turned on and the shaft speed should be set to 300 RPM. Te inert gas flow also should be on and operators should then adjust the gas flow rate and shaft speed. Gas flow should be increased until see gas bubbles are visible as they float to the surface of the liquid metal. As the gas
gas analysis, sampling techniques and in situ methods. Sampling techniques may be further divided into two classes. In the first, a liquid sample is withdrawn and introduced directly into the measuring instrument before solidification takes place. In the sec- ond, a sample of liquid is poured into a specially designed mold and the solid sample is analyzed. Te methods of analysis are shown in Table 2. All three methods of analyzing a liquid sample somewhat depend on the inclusion content of the melt, because these inclusions nucleate gas bubbles. Fig. 2 shows a pore in an A356 casting that has nucle- ated on oxide films. In spite of this
Table 2. Gas Analysis Methods Sampling Techniques
Analysis of Liquid Samples • Straube-Pfeiffer (reduced pressure) Test • First Bubble Technique • Vacuum Extraction during Solidification
Analysis of Solid Samples • Vacuum Subfusion Extraction • Vacuum Extraction from Remelted Sample
• Carrier Gas Extraction from Remelted Sample
In Situ Techniques • Immersible Probe Technique • Electrochemical Determination • Recirculating Gas Method
• Direct Pressure Measurement (DPM) Fig. 2. The pore in this A356 alloy casting is shown in 200x magnification. August 2015 MODERN CASTING | 31
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