Casting questions & answers.

Q The main electrical fuse panel in our aluminum foundry is near the cleaning and finishing room. We have heard that aluminum dust can be hazardous and flammable. Should we be concerned about the potential to accumulate aluminum grinding dust in and around the panel?

Background: Metals such as aluminum, titanium, magnesium, lithium and zirconium can be flammable in certain forms and under particular circumstances. For foundries, aluminum and magnesium should cause the most concern. For the most part, magnesium foundries are aware of the dangers their metal can cause, but aluminum foundries may not be aware of how dangerous aluminum can be.

Aluminum poses no flammability risk in its solid form and is not highly reactive in its liquid form, but under certain circumstances it can be reactive in the powder form. For example, an accumulation of aluminum grinding dust could present a fire hazard in dust collector systems, hoppers and duct systems that do not completely empty. In addition, accumulations of finely divided aluminum alloys and metallic contaminants can ignite if they come in contact with electrical panels.

Evaluating the risk of combustible metals requires specific knowledge of the process and the facility in which the metals are used and stored. Metal dust can cause a particular danger because metal fires burn rapidly and at high temperatures, destroying structural materials faster than lower temperature fires.

Recommendations: Accumulation of aluminum dust or fines should be avoided. All electrical controls and electrical boxes should meet National Fire Protection Assn. (NFPA) and National Electrical Code requirements. (For further information, see NFPA 65--Standard for the Processing and Finishing of Aluminum.) Foundry personnel should be aware of potential dangers and should know where to locate extinguishing materials in case of fire. The local fire chief should be advised of the potentially combustible metals.

Q We are experiencing cracking during load testing on a 296 aluminum alloy casting. The component had been cast in A356 aluminum alloy without substantial problems. How do the two alloys compare and can the alloy change be a part of the problem?

Background: The choice of casting alloys is an important step in the casting engineering and design process, Care must be taken to match the critical property requirements of the final component with the mechanical properties and heat treatment process of the casting alloy to ensure compatibility. Changing or modifying alloys of components in production can result in either enhanced or deteriorated casting properties and component performance.

The castings in question are made via permanent mold tilt pour casting and undergo a T-6 heat treatment. Because the foundry pours a large amount of 296 alloy and has a long history of success with the alloy, it sought and received permission from the customer to convert the casting to this alloy.

The parts are drilled, tapped and then assembled in the field using a torque up to 45-foot lb. During torque testing, however, the 296 aluminum components cracked. The foundry tested different heat treatment options for 296 and has done stress relieving--all without significantly reducing the crack-failure rates. The tapping operation also has been modified to eliminate stresses.

Recommendations: Several aluminum alloys have been developed that provide high strength and ductility combinations. A look at the alloys (Table 1) shows a considerable difference in typical mechanical properties--especially elongation. Elongation measures the distance between two gauge points before fracture during tension. A casting with lower elongation is less resistant to internal stresses.

Although the strengths between A356 and 296 alloys are comparable, the difference in elongation is considerable and may be responsible for the cracking during high torque. Some improvement in elongation can be obtained in 296 by changing to a T4 temper (causing some strength loss), but this will not raise the elongation to the A356 level.

Editor's Note: The castings are now being produced in A356 T6. The cracking problem has been eliminated and the castings are now successfully meeting all component requirements.

Table 1

Mechanical Properties of 296, C355 and A356 Aluminum Alloys Cast via
Permanent Mold

Alloy/      Ultimate         Yield       Elongation
Temper   Strength (ksi)  strength (ksi)     (%)

296-T6         40              26            5
296-T4         37              19            9
C355-T6        48              28            8
A356-T6        41              30           12

Source: Aluminum Casting Technology, AFS.