Semi-solid casting process shows muscle for Mercury Marine.

Inside This Story:

* Diecasting is a popular process for casting aluminum components due to its high-volume production capabilities at a low cost, but that does not mean the process is perfect.

* Semi-solid metalcasting (SSM) is an alternative to diecasting that produces castings with improved material properties, but at higher costs.

* Slurry-on-demand, a new SSM process, is capable of producing parts with the material properties associated with SSM at costs similar to diecasting.

When a company wants aluminum cast components fast and at low cost, high-pressure diecasting often is the chosen process. But, one of the main drawbacks to diecasting is the amount of volumetric shrinkage that can occur, which causes internal porosity and, subsequently, weaker castings. During the solidification process of die castings, 6% volumetric shrinkage occurs. In heavy cross-sections or isolated areas, the amount of porosity or shrinkage that results can be several times more severe.

A new process has emerged as an enhancement to diecasting and permanent mold casting for applications requiring higher mechanical properties. It promises to decrease shrinkage and cycle times, save money and improve the mechanical properties of the aluminum castings. This process, termed slurry-on-demand, is a derivation of the semi-solid metalcasting (SSM) process, which was introduced to the industry 20 years ago as a way to utilize the high-production speeds of diecasting while maintaining the mechanical properties of permanent mold.

Often referred to as thixocasting, semi-solid metalcasting traditionally utilizes a metal ingot that is heated through several temperature stages in induction coils until it reaches a 50-50 liquid-to-solid ratio. With a 50% solid base, the SSM charge, once plunged into the die to form the casting, requires the removal of less heat to solidify, and thus, less time to cool, resulting in less shrinkage after casting. But, similar to high-pressure diecasting, thixocasting has its limits.

One major disadvantage of thixocasting is the size limits for casting due to the small nature of the billets. Billets (in the semi-solid state) can be made to dimensions no more than 3 x 4 x 4 in. (7.62 x 10.16 x 10.16 cm)--any bigger and they collapse under their own weight. This allows the thixocasting process to work well for parts less than 15 lbs. (6.8 kg), but diecasting is required for anything larger.

It also was found that the induction heating process of the billet sometimes could cause problems. For example, Mercury Marine, Fond du Lac, Wis., was developing a swivel arm bracket (Fig. 1) using the thixocasting process when it discovered that castings had a low impact strength.

[FIGURE 1 OMITTED]

It was found that when the casting temperature was lower than required in one of the billet induction coils, "knit" lines subsequently occurred in the cast component, resulting in low impact strength. Knit lines are formed when two divergent semi-solid metal flow fronts do not recombine completely. But if the casting temperature was too high, about 15% of the billets "slumped" in their induction coils because they were not solid enough to maintain their form, and therefore, had to be scrapped. The unreliability of the billet heating took away from the time saved due to a quicker cooling process.

Building Strength

Slurry-on-demand eliminates the need for induction heating of billets. Instead, molten metal is ladled out of the same type of furnace used in diecasting and poured into a crucible. As the metal cools in the crucible, a magnetic stator stirs the melt, preventing dendritic growth in the slurry. Rather, the nuclei formed in the melt have a round structure and make up the 50% that is solid. The liquid eutectic phase comprises the other 50% of the semi-solid slurry. A robot then picks up the gelatinous metal and takes it to the shot chamber, where it is plunged (at a slower speed than with diecasting) into the cold die.

The slurry-on-demand process supplies four main benefits for aluminum castings:

Quicker Cycle Times--Cycle times hinge on waiting for the 100% liquid metal to cool. This can be a prolonged period of time considering the amount of superheat and heat of solidification that must be extracted during the shot cycle. With slurry-on-demand, rather than wait for 100% of the metal to cool to a solid, the metal solution is already half-way to its solid form. Less heat input means cycle times can be significantly reduced.

Reduced Shrinkage--Molten aluminum shrinks 6% as it solidifies through the liquidus and solidus phases and down to room temperature. In slurry-on-demand, less molten aluminum needs to solidify, reducing shrink to 2%, so castings have less porosity, greater strength and greater ductility with slurry-on-demand.

Longer Die Life--Because molten metal must cool in order to achieve solid state, a 50-50 slurry solution is cooler than completely molten metal. Because of the lower temperature, the metal in the slurry does not solder, or alloy, with the metal of the die, which can happen in diecasting. The lower temperature also reduces the amount of thermal fatigue seen by the die. Both of these benefits significantly increase die life over diecasting.

Improved Mechanical Properties--The aluminum alloy used in diecasting typically contains iron in order to prevent soldering with the steel in the die tooling. However, adding iron significantly reduces the mechanical properties of the aluminum. Slurry-on-demand, due to its lower temperature, does not require the added iron to prevent die soldering, so the aluminum maintains higher mechanical properties compared to alloys in the permanent mold casting process.

Mercury Marine has used the process to cast a wide range of parts from thin-walled propellers to a 27-lb. (12.25-kg) four-cylinder engine block. In addition, Mercury currently is using slurry-on-demand to cast a 7-lb. (3.18-kg) swivel bracket. In all of these components, Mercury Marine has seen significant reductions in cycle times and improvements in material properties.

Slight modifications are required to existing die casting equipment to incorporate slurry-on-demand. The process requires the same type of furnace and die tooling equipment already present for diecasting, as well as the slurry maker and a robot to bring the slurry to the shot chamber (requiring a minimal footprint).

Table 1. Mechanical Property Data of Molding Methods and Alloys

                            Ultimate
                            Tensile      Yield      Elongation
                            Strength    Strength       (%)

Slurry-on-demand            314 Mpa     223 Mpa        9.0
356-T6 alloy                 45 ksi      32 ksi

Slurry-on-demand            255 Mpa     186 Mpa        8.0
356-T5 alloy                 37 ksi      27 ksi

Pressure lost foam          258 Mpa     181 Mpa        6.0
356-T6 alloy                 37 ksi      26 ksi

Squeeze casting             300 Mpa     223 Mpa        9.0
356-T6 alloy                 43 ksi      32 ksi

Permanent mold              255 Mpa     150 Mpa        5.0
356-T6 alloy                 37 ksi      22 ksi

Traditional lost foam       227 Mpa     165 Mpa        3.5
356-T6 alloy                 33 ksi      24 ksi

High pressure diecasting    331 Mpa     166 Mpa        1.5
380 alloy                    48 ksi      24 ksi

Adam Kopper is a development engineer for Mercury Castings, Fond du Lac, Wis. Raymond Donhue is senior director of metals and foundry technology at Mercury, and David Olson is genera/manager of Mercury Castings. Stephen Midson is president of The Midson Group, Denver.

For More Information

"Continuous Rheoconversion Process for Semi-Solid Slurry Production, " M. Findon and D. Apelian, 2004 AFS Transactions, Paper No. 04-055.

"Semi-Solid Processing of Hypereutectic Alloys," D. Saha, D. Apelian, and R. DasGupta, 2004 AFS Transactions, Paper No. 04-057