Simple Solutions That Work! Issue 12

In all cases, the scrap must be cleaned. Cleaning is typically performed by sand blasting followed by air blow-off. Many times dross must be removed by sawing or grinding. Adding scrap castings to the melt that have sand and dross on the surface will result in oxides and inclusions in the casting increasing the foundry scrap rate. STEP 2 – CRUCIBLE AND LADLE CLEANLINESS Crucible Cleanliness - Allowing aluminum and dross to build up on the crucible walls should not be acceptable in any aluminum foundry. This build up of oxide saturated material will frequently break loose releasing oxide inclusions into the melt which ultimately end up in the casting. Every aluminum foundry should implement cleaning procedures to ensure the crucible and melt furnace is free of dross build-up. Ladle Cleanliness - The aluminum skin which forms on the walls of the pouring ladle should be removed prior to each pour. The surface of the this skin contains an oxide layer. The skin will remelt when combined with the new metal for the pour, releasing oxides into the casting. STEP 3 – DEGASSING To control hydrogen porosity in the casting, foundries must control their degassing process. Often aluminum foundries will degass for set period of time, for example 15 minutes. However, using a set degassing time as a foundry control is not always effective. The hydrogen content of the melt is effected by many external factors such as melt temperature, melt quantity, daily humidity, and raw material. Each of these factors will contribute to the amount of hydrogen in the molten aluminum creating a moving target for the degassing process. Fortuntely, there is a quick and easy test to determine the gas level in the melt, Reduced Pressure Testing. Reduced pressure testing is performed by solidifying a sample of the melt under a vacuum and then analyzing the solidified sample to a target density. In the past, the sample was analyzed by manually sawing, polishing, and visually comparing to a chart. Now days, equipment like the Palmer PAS3000 and PAS5000 have vastly improved the accuracy and speed of the RPT test. Using this type of equipment reduces the time to analyze the sample to about 30 seconds with no sawing, polishing, or judgement calls required. Just place the sample in the machine and the density result is displayed and the data stored. STEP 4 – TEMPERATURE CONTROL Controlling your pour temperature is critical to reducing variation in casting scrap. Foundry procedures should specify a pour temperature for each casting produced. Residual sand and dross on scrap castings should be removed prior to placing in the furnace. The dross build-up on this melt furnace will certainly add to the foundry scrap rate. The same melt furnace after cleaning will result in less casting scrap. This skin contains oxides and should be removed prior to filling the ladle. The viscosity of the aluminum melt changes with temperature. As the viscosity of the aluminum changes, the flow of the metal within the mold changes along with the rate at which solidification occurs. Since many casting defects are influenced by metal flow and solidification rate, it is impossible to consistently control scrap without having control of the pouring temperature. Tips for Controling Pour Temperature • Implement process procedures for controling pour temperature • Include part specific pour temperature in procedures. Parts with thin walls may require a higher temperature than thick wall parts. • When using a furnace with integral melt temperature control, ensure that the thermocouple and controller is on a calibration schedule. • When using a furnace without a integral temperature control, such as an on/off gas melter, ensure the operator has a calibrated handheld thermometer with an immersion thermocouple. • In the case of an on/off gas fired melter, it is more difficult to control the pour temp, however it can and should be controlled with detailed operating procedures. Contact: BRAD HOHENSTEIN blh@porositysolutions.com 35 BACK TO BASICS