6 Steps to reducing inclusion defects.
For today's aluminum foundries, simplicity can be a rule of thumb. Inclusion-related defects often result when a foundry isn't following its rules and procedures, leading not only to reduced productivity and bottom lines, but also to disenchanted customers.
The following article details a comprehensive approach to reducing inclusion-related defects in aluminum castings. In addition to key technical issues, this article focuses on several human resource-related issues for reducing scrap related to inclusions.
The six-step process is:
1 Identify the nature and source of the inclusions as well as the costs involved at the foundry and customer end.
2 Conduct an audit of key incoming raw materials.
3 Perform a process audit.
4 Consider filtration options.
5 Conduct statistically significant trials.
6 Follow-up on savings and continue to monitor for key scrap levels.
At the foundation of these six steps is the importance of developing a multidisciplinary team focused on following this procedure for all casting scrap. Team members should include workers from quality assurance, production, accounting and sales so that the knowledge base understands both the foundry and customer sides of the equation and can prioritize jobs based on their contribution to overall scrap and profitability. This team determines which castings are key problems and then sets measurable time-based goals for scrap reduction.
Throughout this process, 30-minute or less multi-departmental meetings to update the status of various action items are beneficial. These meetings are for communication only and can often be substituted by an email to all team members. Depending on the nature of the scrap, it may be advisable to involve technical support staff from filter or metal treatment equipment suppliers.
Most aluminum foundries have good scrap tracking systems. For maximum effectiveness, identify one or two jobs that directly impact customer quality and in-house profitability and perform detailed analyses of the casting defect, including its location and frequency of occurrence. Scanning electron microscope (SEM) analyses of casting defects can indicate the nature of the defect and possible sources.
In-House Scrap--For aluminum foundries, these defects normally are caused by gross dross inclusions. Some of these may be on the surface but the most problematic are the sub-surface and surface inclusions at critical areas during machining. In some cases, the total scrap rate due to such defects may be as high as 20%, adding significant costs to the process. Some hydrogen related defects are associated with dross inclusions.
Scrap at the Customer--Customers may reject parts due to poor mechanical properties (tensile strength or ductility issues). Cosmetic surface defects also can be problematic. While customer scrap levels are generally significantly lower than in-house scrap, the costs are significantly larger. In some cases, significant valued-added operations (such as machining) may be carried out before the casting is rejected.
Warranty claims based on casting defects also can be passed on to the foundry. These costs can be several times the cost of the original casting.
It is critical to estimate the costs of scrap due to inclusions. Often, cost savings that can be generated because of inclusion scrap reduction will be a powerful motivating force for the team members. Upper management support is often dictated by the potential savings that can be achieved.
Typical Inclusion Sources
The following five types of inclusions often are the cause of scrap in aluminum castings:
* oxide films;
* alumina particles;
* magnesium aluminate (Mg[Al.sub.2][O.sub.4]) spinels;
* grain refiners;
1. Oxide Inclusions
Figure 1 shows a magnified SEM photo of a spent filter cross-section. Fine oxide skins have been retained on the filter (note that this is a 2-D view of the thin and large oxide skins). They are caused by oxidation of aluminum and magnesium.
Impact on Quality--These are often in sheet form and can impact the cosmetic and mechanical properties of the casting.
Possible Sources-Turbulence in the metal transfer systems.
2. Alumina Particles
Figure 2 shows fine alumina particles retained in a spent filter, Either these particles are formed by oxidation or they originate from the electrolytic cells where primary aluminum is produced. Their morphology is fine crystals 10-20 microns in size.
Impact on Quality--Machining problems.
Possible Sources--Poor raw material quality, oxidation during pouring.
3. Mg[Al.sub.2][O.sub.4] Spinels
Spinels (Fig. 3) are large inclusions (sometimes 500 microns in diameter) formed by reactions between MgO and alumina. The reactions between MgO and alumina skins often occur at high temperatures and can be a runaway reaction leading to the breakdown of the stable oxide skin.
Impact on Quality--Hard particles that affect casting machinablity.
Sources--Magnesium addition practice, exposure to high temperatures including flame on the molten aluminum surface.
4. Grain Refiners
Grain refiners (Fig. 4) are added to molten aluminum alloy for superior mechanical properties. Sometimes, they segregate and form large inclusions.
Impact on Quality--Hard dense particles impact machining.
Possible Sources--Improper grain refiner addition properties, incoming material quality.
Salts are added as fluxes to clean the metal. Similar to grain refiners, they can be sources of inclusions.
Impact on Quality--Cosmetic appearance, mechanical properties.
Possible Sources--Poor fluxing process, incoming raw material.
At this stage, it may be useful to involve the customer in the defect identification and corrective action procedures. They often are impressed by your actions (rather than talk) to solve the problem. In addition, customers may have input to solve the problem in a mutually beneficial way.
Auditing Incoming Raw Materials
Casting defect sources can be related to the incoming raw materials, While pre-alloyed ingot (certified by the supplier) is the best source for clean metal foundries often must use internal returns, external scrap and unalloyed ingot for molten metal due to cost considerations. When this is the case, alloy additions, fluxes and grain refiners are critical components to quality metal.
Foundries should contact their raw material suppliers to perform an audit of incoming raw materials as well as melt deck procedures to ensure adherence to established quality standards.
A detailed review of inclusion measurement methods such as LAIS and PODFA are available in published literature, It is recommended that a baseline measurement of inclusion levels be carried out by every aluminum foundry, and that these inclusion levels are monitored at frequent intervals.
Melting practices and furnace cleaning have a considerable impact on metal quality. It is essential to have a regular melt skimming procedure. Small amounts of halogen-based fluxes should be mixed with the molten aluminum and resultant dross that forms should be skimmed after 10-15 mm, Hot cleaning of the melting and/or holding furnaces should be performed at least once a week.
Metal transfer often is overlooked as a possible cause for metal oxides. Review the equipment design and practice to determine possible causes of oxidation.
Degassing also is a critical step. Auditing this process involves checking the level of surface turbulence and the bubble type emerging from the melt. Optimal speeds and flow rates are essential for effective operation of the degasser, In addition, when nitrogen is used as the degasser, it produces wet dross while argon produces dry dross. The equipment supplier should be invited to periodically inspect the system and make recommendations.
While audits often conjure up images of special teams descending upon unwary personnel, the opposite approach is essential. Production and quality team members should be encouraged to openly discuss possible causes for the inclusions with auditors.
The tough metal quality demands on today's foundries often demand metal filtration during casting. In aluminum foundries, the most common filtration methods are:
* large filters in the holding furnaces;
* filters in riser tubes;
* filters in gating system.
Filters in furnaces--This method is good for retaining spinels and other large inclusions found in the furnace (Fig. 5). Due to the dynamic nature of inclusion retention, it is possible that inclusions can be released, It is recommended that these filters be changed at least once every week and spent filter analyses be performed to determine the nature of inclusion loading and the types and sizes of inclusion material retained in the filter.
Riser tube filters--Most automotive foundries have six to eight riser tubes to fill each casting in a typical low-pressure system. The use of filters can be effective in removing large inclusions (200 microns or larger). Typically, these filters are used in riser tubes for 20-24 hrs. The roundness of the filter and dimensional control are important. Easy placement and removal of filters without damage to the riser tube is essential as special gasket materials are available for such applications (Fig. 6).
The method of inclusion retention is dynamic since the molten metal stream is pumped periodically through the filter into the casting and permitted to drain back into the holding furnace. Depending on the cleanliness of the metal, it is possible that large inclusions are broken into fine inclusions and introduced into the casting cavity. As a result, spent filters must be examined for inclusion retention.
In-mold filters--This method is popular in gravity permanent mold casting. A filter print is placed in the gating system. The filter is sized large enough so as not to act as a constraint to the metal flow (the filter area is 5-6 times the choke or the minimum gating cross-section area). Very fine pore filters are used (#15 or #25) since the inclusions that need to be retained are fine.
The filtration is effective since the filter is close to the final casting. A key factor in reducing inclusions is related to the reduction of turbulence in the molten aluminum stream. This reduction in turbulence can reduce the amount of dross formed. See the "For More Information" section at the end of this article for more filtration information.
In many cases, solutions for reducing inclusions are related to two or three factors (interaction effects). In pursuit of these factors, controlled experiments and trials must be run to isolate those factors and develop solutions. Quality assurance personnel can be key because they have taken experimental design courses. In addition, fractional factorial/screening experiments also are valuable for pilot tests.
During these trials, care must be taken to understand the interactions between various key variables. Large-scale production runs (1-2 weeks) must be performed to verify the findings of the pilot study. Then, and only then, can the corrective actions be made in wide scale production.
The key to any process changes is to document the cost savings they achieve, Therefore, when future need arises for another investigation, management buy-in will be easier to receive. In addition, these cost-savings can be documented for your customer to show the added benefits you are supplying.
Reduction of foundry scrap is a challenge. While it may be viewed as a low priority, work in this area will contribute to lower costs, improved productivity, increased profitability and superior customer satisfaction in the long run.
For More Information
Visit www.moderncasting.com to view "Ceramic Foam Filter Technology for Aluminum Foundries," J.R. Schmahl and Nathan J. Davidson, MODERN CASTING, July 1993, and "The Dilemma of Assessing Your Aluminum Melt's Cleanliness," M. Lessiter and W.M. Rasmussen, MODERN CASTING, February 1996.
"Improving molten metal quality for die casting--Part 1 & 2," D.V. Neff, Die Casting Engineer, January/February 1991 and March/April 1991.
About the Authors
Luiz Martins is manager--aluminum melt treatment sand and Sudesh Kannan is technical service manager for Selee Corp. Both provide technical support for inclusion reduction to a variety of foundries across the globe.
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|Title Annotation:||aluminum castings|
|Date:||Mar 1, 2003|
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