Marrying Almag 535 to the permanent mold process.

Inside This Story

* The impressive characteristics Almag 535 can achieve in the sand casting process are all but lost in other processes, such as permanent mold. By combining the characteristics of the permanent mold process with the alloy, metalcasters can achieve the best of both worlds.

* Detailed within are the results of a recent trial to determine the feasibility of using Almag 535 in the permanent mold casting process.

Components cast for use in automotive, marine or military applications often require special attention. Because of the unique nature of their end-use, better corrosion resistance against mild alkaline and salt spray exposure, dimensional stability, and a good combination of strength, ductility and shock resistance is often demanded of these types of castings.

Those qualities can be found in aluminum-magnesium casting alloys, the most common being aluminum-magnesium alloy 535.0 (Almag 535). However, the alloy is used almost exclusively in the sand casting process because it exhibits poor hot tearing resistance when poured into metal molds.

Energy savings as a result of good mechanical properties and dimensional stability in the as-cast condition along with more cost-effective heat treatment options make Almag 535 an attractive choice--for sand casters. But castings for the automotive, marine or military industries often require characteristics beyond the scope of sand casting.

Because of the inherent benefits of using metal dies, the permanent mold process could help unlock new opportunities for the use of Almag 535. The alloy is not used in permanent mold applications because it is prone to hot shortness and poor fluidity due to its long freezing range. Further, when compared to the aluminum-silicon alloys, aluminum-magnesium alloys are dross forming, require more attention in gating design, and greater temperature gradients are required to produce sound castings.

But by developing appropriate permanent mold casting technology for alloy 535.0, metalcasters can harness the benefits of better surface finish, precise and consistent dimensional tolerances and improved mechanical properties that are inherent to permanent mold casting.

This article examines recent trials performed to evaluate the castability of selected prototype castings of aluminum alloy 535.0 in permanent molds.

The Trials

A series of test castings was produced using Almag 535 in three different prototype molds. The alloy was melted under controlled conditions and degassed as needed. The pouring temperature was kept at 730C (1,346F), which is a 100C (212F) superheat. Some of the test castings were grain refined while others were not. The prototypes then were cast in one of three molds--a plate, a bushing or a rocker arm (Figs. 1-3).

[FIGURES 1-3 OMITTED]

The plate and bushing were geometrically simple while the rocker arm was more complex. The plate and bushing molds, as well as a solid cylinder core, were fabricated from a copper-beryllium mold material. The rocker arm mold was machined from gray cast iron plate castings.

The rocker arm and bushing castings both required the use of metal cores that were mounted and/or inserted into the mold cavity. A solid rectangular core with a small taper was used for the rocker arm casting in the preliminary casting trials. The core was later hollowed out to reduce the heat sink during pouring and used in subsequent casting trials.

Plate and Bushing Castings

All of the castings poured from the plate and bushing tooling showed good surface finish results. The 6.4-mm thick plate castings with the gating system are shown before and after the addition of a grain refiner (Fig. 4). Hot tear cracks were observed at the top edges of the plate poured from unrefined metal but were not present after grain refinement. The five plate castings from the unrefined metal all showed hot tear cracks.

[FIGURE 4 OMITTED]

The 13-mm thick bushing casting showed mixed results. Some components were cast defect-free while a hot tear crack was observed under the ingate section opposite the casting ingate (Fig. 5) in others.

[FIGURE 5 OMITTED]

In addition to the hot tear cracks, the bushing castings were difficult to eject from the mold cavity with the 1.5-degree core taper at the maximum operating ejection force possible in the casting machine. A more generous core taper would be required for the casting of this type of component in alloy 535.0 for this particular mold design.

In addition to the lack of an adequate core taper, thermal expansion of the core can possibly force the casting toward the mold wall and reduce the effectiveness of the core taper. The use of metal core material with a lower coefficient of thermal expansion than the mold material could alleviate this problem. It is noted that copper-base alloys are not usually used as mold materials for the casting of aluminum-base alloys. This suggests that copper alloys, such as beryllium-copper, could be potential mold materials because their high thermal conductivity could lead to a higher solidification rate and improved casting soundness.

Rocker Arm Casting

Twenty-eight rocker arm castings were produced from alloy 535.0 and 13 more were cast in aluminum alloy 356.0. The casting with the gating system is shown in Fig. 6. In the four castings poured using a solid metal core, horizontal and vertical hot tear cracks were observed on the surface that solidified against the metal core (Fig. 7). In some cases, a surface shrinkage could be observed with the hot tear crack, suggesting a lack of feeding in that region. In the castings produced with a vertical hot tear, the hot tear regions were close to the area where the sprue made contact with the casting and also areas where there is a change in section thickness.

[FIGURES 6-7 OMITTED]

It also was observed that pouring at a lower superheat reduced the hot tearing tendency. Three of the seven castings poured at a 70C (158F) superheat showed fewer hot tear cracks.

The seven castings poured from alloy 356.0 using the same solid metal core also showed hot tear cracks.

In addition to hot tears, extraction of the castings from the mold cavity presented problems as the castings tended to stick with one side of the die. The graphite wash on top of the underlying insulating coating was not an effective mold release agent. Hence, the selection of appropriate mold coating and its application are important to the successful permanent mold casting of this alloy.

After the initial casting trials, a hollow metal core was used to reduce the rapid heat loss to the core to see if this could lead to improved feeding at the ingate and reduce hot tear cracks. Seventeen castings were poured from alloy 535.0 and six from alloy 356.0 to test the effect of enlarging the pouring sprue and the use of the hollow metal core. The incidence of hot tear cracks and surface shrinkage were reduced but minor hot tear cracks were still observed. Five of the six castings poured from a melt prepared from 6% magnesium show reduced hot tear cracks, and one casting was free of hot tear cracks.

The majority of defects in the rocker arm castings were below the sprue, which was gated into the main without body of the casting an ingate into the casting cavity. The shrinkage and hot tear cracks were associated with the lack of a generous ingate into the casting cavity and the lack of adequate metal to feed the shrinkage.

The shrinkage and hot tear defects could be eliminated by introducing a basin below the end of the pouring sprue that serves as a riser. Also, by using a hollow metal core, grain refinement and a low magnesium level, hot tear cracks were reduced.

Encouraging Results

The results of the trials show the potential for the permanent mold casting of Almag 535. A few modifications will have to be made, however. Hot tear cracks were prevalent but were easily eliminated through grain refining, casting design and the use of a metal core. Melts of alloys 535.0 and 356.0 were generally degassed to 0 or 1 standard quality, showing that the same degassing process could be used.

Casting soundness and productivity were increased by using copper-base of using Almag 535 in permanent mold casting. In doing so, metalcasters can combine the surface finish characteristics, consistent dimensional control and improved mechanical characteristics inherent in the process with the good mechanical properties, dimensional stability and excellent corrosion-resistant properties found in Almag 535.

Mahi Sahoo is the manager of casting technology, Yemi Fasoyinu works as a research scientist, and Denis Cousineau serves as a technician at CANMET/MTL, Ottawa, Ontario, Canada.

For More Information

"Mechanical Properties and Metallography of Al-Mg Alloy 535.0," F.A. Fasoyinu, J.P. Thompson, D. Cousineau, J. Barry and M. Sahoo, AFS Transactions, 2003, paper 03-115.

"Gravity Permanent Mold Casting of Al-Mg Alloy 535," F.A. Fasoyinu, J.P. Thompson, D. Cousineau, T. Castles and M. Sahoo, AFS Transactions 2002, paper 02-145.