The demand for weight and material reduction in industries like transportation continues to drive innovations in engineering and advancements in material development. Researchers are evaluating new aluminum alloys for their performance in high temperature applications, while on the iron side, the industry is developing ways to cast thinner walls to achieve weight reductions. 

3 mm Walls in Sand Casting

In a 2022 AFS Transactions paper, “Opportunities to Fully Exploit Ductile Iron Castings in Lightweighting of Vehicles,” the authors explored ways to achieve carbide-free 3-mm walled ductile iron castings and organic designs. Although ductile iron is relatively low cost and exhibits a high strength-to-weight ratio, barriers exist to its wider use for lightweighting—namely the ability to produce very thin sections.

High volume green sand foundries in general will not quote as-cast ductile iron castings with sections under 5-mm (although this is changing) because of the risk of brittle carbides forming on the edges. In the 2022 study, various techniques were explored to make thinner walls without carbides. The researchers tested various types of molding sand and alloy chemical compositions on 3 x 28 x 100 mm “finger” castings (see Figure 1).  Results were studied for the chemistry effect on carbides and the impact on mechanical properties.

Within the limits of the investigation, the following conclusions were made:

• Carbide-free sections with good tensile properties can be achieved using high-silicon content alloys.
• The use of non-crystalline fused silica sand molding material was found to be necessary to produce 3mm-thick carbide free ductile iron.
• These alloys have excellent austempered mechanical properties even with a high Si content.
• These alloys also have modest low temperature impact properties even with the addition of up to 4% NI, but they are not high enough for some industrial applications.
• Thin section, carbide-free ductile iron castings were produced having a range of nodule counts. Also, the austempered mechanical properties were standard or above average for grade 2 ADI.

It does not seem that previous findings regarding the importance of nodule count on carbide depth hold true with these alloy compositions, but it is possible that a new frame of reference for what constitutes a proper nodule needs to be produced for these thin sections. Traditional nodule measurements may be missing significant numbers of nodules that should be measured.

There could be more avenues for the advancement of ductile iron to a further range of applications that include taking advantage of its high strength-to-weight ratio using thin sections.

Lost Foam Possibilities

Walls thinner even than 3 mm may be possible in other casting processes, like lost foam and nobake sand. The U.S. Department of Energy has stated a goal to achieve castability of iron down to 2.5 mm by 2050 in its “Light-Duty Vehicles Technical Requirements and Gaps for Lightweight Propulsion Materials Report.” The DOE has also specified goals to reduce the weight of iron steering knuckles by 35% and iron castings for brakes by 50% by 2025.

Skuld LLC in partnership with The Ohio State University completed a series of experiments as part of the DOE’s Small Business Innovation Research program that investigated the use of lost foam to obtain carbide-free microstructure in sections as thin as 1 mm and the ability to maintain section thickness tolerance. Lost foam and nobake sand casting samples were analyzed in the experiment for comparison. A summary of the massive carbide microstructure measurements is shown in Table 1.

Results from the study led the researchers to conclude the old guidelines to avoid using ductile iron below 6 mm no longer apply and multiple process options are available. Both nobake and lost foam casting proved to achieve carbide-free thin sections. Lost foam also provided better tolerance control. Additional research to achieve 1-mm thin-walled castings is ongoing. 

Hollow Designs

A few years ago, Ford Motor Co. challenged its design team to reduce weight in its CD4 platform; one of the components it redesigned was a lower control arm. Often, lightweighting though a change in material is achieved by moving from steel or iron to aluminum. But in this case, the original aluminum cast part was redesigned as a ductile iron component.

Waupaca Foundry collaborated with Ford to design engineer a high strength, light weight iron casting that reduced cost, achieved a 25% weight reduction, and alleviated supply chain concerns.

The first-draft concept showed a considerable gap to achieve weight reduction targets. The team used design optimization tools on various configurations to evaluate performance. Once a new , hollow-design concept emerged that was more effective at reducing weight, the team further refined the design for production feasibility. 
Mating part interfaces for the control arm were carefully detailed to enhance durability and ensure proper assembly function. Surface profiles were shaped to fit into the allowable packaging space. Once the program weight and cost reduction targets were achieved, the project was approved to continue with tooling construction and production launch.

CGI Considerations

Compacted graphite iron’s microstructure features graphite particles that cluster into the iron matrix and provide increased mechanical properties compared to gray iron and improved thermal conductivity compared to ductile iron. It has become a popular material choice for V-diesel passenger cylinder blocks and heavy-duty cylinder blocks and heads but not yet small in-line engines. Sintercast and Tupy initiated a study that converted an aluminum 1.2 liter three-cylinder engine to CGI and shared the results in the 2022 paper, “The Petrol Engine Cylinder Block Reinvented: Cast Iron With the Same Weight as Aluminium.” The new engine design was also upgraded to a 48-volt hybrid configuration to demonstrate the potential of CGI in those applications.

The Tupy foundry group has more than 20 years of experience in CGI product development, foundry production, and manufacturing experience and applied this knowledge to retrofit the 1.2 liter, three-cylinder engine. The successful CGI engine featured 2.7 mm wall thickness and achieved tensile strengths between 550 and 600 MPa.
In addition, high strength iron can result in smaller parts, in terms of space. Although the more compact iron component might not necessarily be lighter than its aluminum counterpart, it allows the ancillary components around it to also be smaller, which could lead to offsetting weight reductions.  CS

Click here to view the article in the digital edition of September/October 2023 Casting Source.