Simple Solutions That Work! Issue 15

C ontrolling the levels of hydrogen porosity in aluminum castings is critical to nearly all aluminum foundries. BRAD HOHENSTEIN President Porosity Solutions ARTICLE TAKEAWAYS: • Eliminate subjective visual methods to evaluate hydrogen gas porosity • Use specific gravity measurements as GO/NO GO targets for operators • Increase productivity and reduce costs by optimizing degassing time CONTROL OF HYDROGEN POROSITY BY OPTIMIZING ANALYSIS OF RPT SAMPLES Not only is porosity a major contributor to internal scrap rates, in many cases it is the number one reason for customer returns. Typically, when a part is rejected by the customer it is after machining. This means in addition to getting charged back for the casting, the foundry is also charged for machining and processing costs. The cost of improper control of hydrogen porosity is high, however developing a process to control hydrogen porosity need not be a costly venture. MEASURE AND CONTROL Measuring hydrogen content in the aluminum melt basically falls into two categories: Direct hydrogen measurement of a liquid sample or analysis of a RPT sample solidified under vacuum (Reduced Pressure Test). Of the two methods to analyze the hydrogen content in the melt, the 38 reduced pressure test (RPT) is the least expensive, most robust, and by far the most widely method used. When used as part of a structured process to control porosity, the RPT is an extremely effective tool. Unfortunately, many foundries do not use the reduced pressure test to its full capability. When evaluating RPT samples, most foundries cut the part on a band saw, belt sand the cut surface, then visually compare the prepared surface to a chart. Replacing this visual method with a system to measure sample density (specific gravity) is a safer, quicker, and more accurate method to analyze the RPT sample. Using a specific gravity measurement to analyze the RPT sample allows the foundry to have a target number for their alloy and casting process thereby eliminating subjective judgement calls on whether the melt is ready to cast. Within 30 seconds the operator will have a specific gravity measurement which is their Go/No-Go number. No more subjective decisions based on a visual comparison to a chart. CASE STUDY 1 A major manufacturer of precision cast parts for the automotive industry was experiencing a high level of customer returns for porosity detected by the customer on the machined surface of the part. In some cases, the return rates would be over 20% but as typical with hydrogen porosity, this would vary from lot to lot. A porosity analysis system was purchased by the company to measure specific gravity of the RPT samples. The first step in developing foundry process control procedures for hydrogen porosity is to correlate the specific gravity of RPT samples to good and bad castings. Using this method, it was determined that a specific gravity of 2.58 or higher yielded an acceptable casting while RPT samples with specific gravity readings under 2.58 meant the melt may not yield a good part. Knowing this information, the target for a newly degassed melt was set at 2.62 with a minimum of 2.58 required to cast. With the specific gravity target defined, a standard degassing time could be determined. In this case it was 20 minutes of degassing for each holding furnace (16 furnaces in total). Metal from the degassed furnaces would be used and replenished throughout the shift and then degassed again to start the next shift.