Finding new strength in aluminum: fuel efficiency-driven weight reduction issues in heavy-duty transportation have made high-strength aluminum alloys, such as A206, desirable alternatives to A356.

The constant drive to reduce part weight for automotive applications makes aluminum alloys attractive candidates for producing structural components because they are lighter than ferrous metals while featuring the necessary properties to meet the structural requirements. The highly castable A356 alloy has emerged as the most popular aluminum alloy for automotive chassis and suspension components, but its limitations in yield strength, fracture toughness and fatigue strength has kept high-strength- low alloy steel fabrications the norm in heavy-duty transportation applications.

Because the demand for better fuel efficiency in heavy-duty vehicles increasingly calls for lighter parts, the demand for castings made of high-strength aluminum alloys, such as A206, is increasing.

Although 20% more expensive than A356. A206 has superior fracture toughness and fatigue properties. But A206, which typically consists of 4.2-5% copper. is a long feeding range alloy prone to shrinkage defects with a reputation for hot cracking (though a modified version with a lower propensity for hot cracking has been developed). To work through these potential defects, casting optimization technology be comes important.

Casting optimization technology is a powerful tool that can provide the upfront engineering necessary to produce high quality A206 castings.

This article illustrates the efforts expended on the substitution of a large structural steel welded fabrication with an A206 alloy casting for a heavy specialty truck.

Material Selection

Field loading conditions for a large structural casting (LSC) used in the suspension system of a heavy-duty truck warranted a material that could withstand severe loading and fatigue conditions (Fig. 1). Only two materials qualified on a cost basis: austempered ductile iron (ADD and aluminum alloy A206 with T4 and T7 heat treatment.

[FIGURE 1 OMITTED]

Because ADI did not provide any weight reduction over the currently used steel component. A206 was the chosen material. During the material selection process, several important factors affecting fatigue life, such as stress range, residual stress, notches, material properties and corrosive environment, were considered. Table 1 shows a general comparison of the steel fabricated component with the A206 component.

When comparing the mechanical properties of A206 and A356 (Table 2), it can be determined that the yield strength of A206 is twice that of A356, which means A206 can withstand twice the load of A356 without experiencing permanent deformation. In situations where the yielding is exceeded, A206-T6 has the ability to accommodate a much higher stress concentration prior to catastrophic failure when compared with A356-T7.

Steps to a Final Design

The initial prototype design of the A206 aluminum casting closely resembled the geometry of the steel component design that, at the time, performed with a high degree of reliability. The following general methodology was used to arrive at a final optimized design:

1. General wall thickness of the steel fabrication was in the range of 0.25-0.75 in. (0.64-1.9 cm). The sand casting process, with adequate placement of chills, was believed to be capable of producing high-integrity castings.

2. Initial solidification modeling was performed on the casting under the assumption the casting would begin solidification with a constant temperature throughout the casting.

3. The solidification results were analyzed for solidification patterns, hot spots and temperature gradients, and a gating design was developed to cast the LSC. A filling time was determined.

4. Based on solidification modeling results, the rigging system was designed with chills in critical locations. Full-filling, solidification and stress analysis modeling then was carried out, and the results were analyzed and reviewed for subsequent design changes.

5. Prototype castings were produced incorporating chills and risers. Chili and riser placements were optimized using casting process modeling results.

6. Prototype castings were tested to evaluate their fatigue performance using a test rig specifically designed and assembled for the casting.

7. Fatigue test results were reviewed along with stress analysis results. Design changes were made based on modeling results. Design changes also were made to the rigging system, but the gating and filling times were left unchanged. The design was finalized with several chills and risers.

8. Fatigue tests were conducted on several castings. After rigorous testing, the parts were released for installation on vehicles.

Tackling Solidification

Understanding the solidification of A206 before developing a rigging system was essential because the LSC had to conform to the AMS 4236 standard. Typically, thermal gradients in large freezing-range alloys are manipulated to avoid shrinkage porosity. For A206 alloy, a thermal gradient ten times greater than that of steel is required to avoid porosity (Table 3).

More importantly, the thermal gradients to produce high quality A206 castings must be much steeper than those for A356. Therefore, a rigging system that is used for A356 is inadequate to cast A206, and many more chills are required to produce high-quality A206 castings.

Riser design for A206 is extremely important. Because the material is a wide freezing-range alloy, chills and adequate risers must be used for driving solidification and feeding thick sections, and the feeder necks must be wide enough to facilitate adequate feeding.

Additionally, the feeding efficiency of A206 during the critical stages of solidification is much lower compared to that of A356, which must be considered during feeding by the risers as well as when designing the casting (section thickness issues).

Analyzing Modeling Results

A preliminary design of the spring seat of the casting is shown in Fig. 2 next to a solidification result file. Structurally, the spring seat area of the casting bears the brunt of the dynamic load from the main strut and therefore was designated as a design-critical area. The seat region contained several sharp corners, which can be problem areas for hot tears.

[FIGURE 2 OMITTED]

Hot tearing is a phenomenon in which an area of the microstructure may become devoid of liquid metal during the final stages of the solidification due to local straining of the material. This can lead to the deformation of local microstructure. If this area is not compensated and is deficient in liquid metal, it will result in a hot tear. Hot tears can manifest internally and cause structural weakness within a component.

As the design was being reviewed, it was decided to move forward with the production of a few cast prototypes by placing large chills in the thick sections that showed local hot spots. A gating system and riser placement also were finalized for the production of prototype castings.

The maximum principal strain rate results showed the corner areas of the spring seat experienced higher strain rates than the rest of the casting during solidification (Fig. 3). Fatigue tests on prototype castings showed excellent correlation with the simulation results. External inspection of the casting showed cracks initiating in areas of high principal strain rates. The crack propagation path during fatigue testing and final failure of the castings showed that failure occurred in high hot tear areas predicted by the simulation (Fig. 4).

[FIGURES 3-4 OMITTED]

Microscopic characterization of the fracture surfaces using scanning electron microscopy (SEM) revealed strong evidence of hot cracking in the microstructure adjacent to the corners in the spring seat region of the casting.

Reaching a Successful Design

Final design modification to the spring seat (Fig. 5) showed several key advantages:

* casting and solidification characteristics of the component were improved;

* the design incorporated large well-rounded corners to reduce/eliminate hot tearing;

* the addition of flanges allowed for efficient placing of ingates and risers;

* modified geometry allowed for more efficient placement of risers;

* casting integrity was superior to the original design.

[FIGURE 5 OMITTED]

The general rigging system (without the gating system) is shown in Fig. 5. Placement of chills in the vicinity of the spring seat and the use of large risers to feed the spring seat area resulted in the casting integrity conforming to AMS 4236 standards. Minor changes were necessary to the chills and feeder neck, and the riser diameter before the rigging system was approved for the manufacture of parts used for fatigue testing.

The casting optimization process also involved performing a residual stress analysis on the final design with the full rigging system. The design changes to the spring seat with well-contoured smooth corners resulted in a decrease in maximum principal strain rate values and hot tear criteria. The inner corners of the spring seat did not show any areas of maximum principal strain rate that were considered to be of concern. The hot tear criteria values showed a dramatic drop for the new design.

These results demonstrated the capability of casting process modeling tools for the prediction of flaws in the design. Improvements in the design were tracked easily with respect to the original design as iterative design changes were made. It was this design that eventually passed the fatigue test and was released for production.

For More Information

"Recent Developments in the High Strength Aluminum-Copper Casting," G. K. Sigworth and F. DeHart, 2003 AFS Transactions (03-135), p. 341-354.

Winston Sequeira is an applications manager at Magma Foundry Technologies Inc., Schaumburg, III. Vadim Pikhovich is a senior metals/process engineer for Oshkosh Truck Corp., Oshkosh, Wis. David Weiss is the vice president of engineering for Eck Industries Inc., Manitowoc, Wis.

Table 1. Comparison of Steel Fabrication with Cast Aluminum Alloy A206

                                   Fabrication        Comment on
 Material    Weight comparison       issues          fatigue life

HSLA steel         1.00          Assembled from       Function of
                                 wrought shapes     weld integrity

   A206          0.45(55%           Near net          Function of
              weight savings)    shape component   casting integrity

Table 2. Comparison of Typical Mechanical Property Data for A356 and
A206 Alloys (unless otherwise specified in the table, all values for
A356 and A206 are for T6 and T7 conditions)

                                           Ultimate Tensile
Material    Yield Stregth     Elongation       Strength

  A356     30 ksi (207 MPa)      10%       41 ksi (283 MPa)
  A206     59 ksi (405 MPa)       6%       65 ksi (455 MPa)

           Fracture Toughness         Fatigue Endurance
            ([K.sub.Q]) MPA     Limit at 5 x [10.sup.8] Cycles
Material   [square root of m]       (T71 Temper Condition

  A356             17                  8.5 ksi (59 MPa)
  A206             28                  23 ksi (159 MPa)

Table 3. Comparison of Measured Thermal Gradients Needed to
Avoid Shrinkage Porosity

       Alloy/Castings           Measured Gradient Required
                                    to Avoid Shrinkage
                                 Porosity ([degrees]C/m)

     Steel: 5-cm plates                     40
      Steel: 10-cm bars                     40
 Al-7Mg: 5-cm plates (A356)              110-375
  Al-7Mg: 10-cm bars (A356)              110-490
Al-4.5%Cu: 5-cm plates (A206)              400
Al-4.5%Cu: 10-cm bars (A206)               650