Austempered Ductile Iron and Aluminium Alloys Do Not Always Cross Paths in Lightweighting

Contributed by: Dipanwita Gupta

Lightweighting is the order of the day. Globally, transportation industry players are spending dollars to hit upon that one perfect technology that enables them to reduce weight of their next outgoing vehicle model so that it meets the now stricter sustainability rules. In packaging too, brands are racing against time to attain that perfect thickness of their products through down-gauging only to meet demands of the discerning modern-day consumers. In the process of doing so, they are laying their hands on a variety of materials all of which are results of very high-end research and characterised by a wide range of distinctive physical and chemical properties. Austempered Ductile Iron and Aluminium Alloys are the two most popular materials that are being used today for this purpose.

There is a general notion that these two materials compete with each other in the lightweighting space, but that is only partially correct. Barring a few common areas of application, ADI and aluminium alloys do not always cross their paths when it comes to lightweighting. They have their own defined areas of usage and based on the design considerations and technology used they are chosen by the mechanical designers quite early in their design process.

In the paper titled ‘Designing with Austempered Ductile Iron (ADI)’ authors J.R. Keough and K.L. Hayrynen write, “The mechanical designer has a tough job. He/she must be able to satisfy the physical performance, aesthetics, and the cost of the component or system. The range of material/process choices has broadened dramatically in the past several decades. While steel properties have been rather well defined for over 50 years, the properties of materials like the various aluminium alloys, composite materials, ceramic materials and polymers has been evolving as the information “blanks” are being filled in with experimental and experiential investigations.”

Austempered Ductile Iron and its applications

ADI components are usually made by welding several steel elements together into a single, durable and sustainable piece. This allows the customer to save on production costs, increase quality and solve problems related to the limitations of steel. Thanks to cast iron’s characteristic ease of casting, ADI expresses its maximum potential in creating complex shapes that allow the material to be applied only where needed, therefore reducing waste.

Thanks to the use of ADI components vehicles are lighter, because the specific gravity of Austempered Ductile Iron is 10% less than that of steel. In addition, ADI is also a wear-resistant material that doesn’t require surface treatments and promotes noise reduction thanks to the “damping” effect of the graphite and its “ausferritic” structure.

Zanardi Fonderie, a world leader in production and research related to austempering of ductile iron, recently organized “ADI Days” in Italy, in collaboration with EIT Raw Materials. The two-day event was essentially dedicated to austempering as a substitution technology. Industry experts from renowned companies and organizations such as CNH Industrial, Magneti Marelli suspension systems, Bonfiglioli Mechatronics, Industrie Cometto, and researchers at the University of Padua, the University of Cassino and RWTH Aachen participated in the ‘ADI Days’ and shared knowledge on this technology among industry peers.

Zanardi Fonderie offers ADI as an alternative to cast and forged steel components used in the suspension mechanisms for commercial vehicles, agricultural machinery, railway cars and for mechanical transmission parts. One key characteristic of Zanardi ADI is like all cast iron, it is completely reusable when a component reaches the end of its lifecycle.

Major industrial players such as Fiat, Iveco, Berco, Caterpillar and Bonfiglioli have been using ADI castings from Zanardi Fonderie for years to enhance their competitive performance.

Fabio Zanardi, President of Zanardi Fonderie says, “All phases of ADI production are integrated in our factory at Minerbe. Daily production and controls regarding the related processes are ‘founded’ on study, research, engineering and innovative development: this is the way we ensure quality, competitiveness and innovation.”

Relative advantages of aluminium (alloy):

Aluminium scores high essentially due to its low density and superior manufacturability. ADI’s density is 2.4 times that of aluminium.
Aluminium has no ductile-to-brittle transition temperature; so, performs better in low temperature conditions.
The limiting factor with ADI in designing for stiffness is the minimum section size achievable. In conventional sand moulding, the minimum ductile iron/ADI design thickness would be about 5mm generally and 3mm in specific area. So, it cannot replace an aluminium die casting where it is used with a 2.5mm wall thickness.
Aluminium is 100 per cent FCC (face-centred cubic structure) and that is why the properties of an airplane’s skin and wings do not deteriorate at ‑60degree Celsius during high altitude flight. ADI has FCC austentite as one of its principal constituents and the lower strength grades of ADI have the most ductile to brittle profile. In fact, ADI maintains nearly 70 per cent of its room temperature fracture toughness at ‑40degree Celsius.

Cost-benefit trade-off

The desirability of a product is judged on the basis of the expected benefit relative to the cost. There’s no denying the fact that ADI, which has much lower cost per unit mass than steel or aluminium, offers mechanical designers an economical alternative to aluminium castings, forgings, and weldments, but when actual lightweighting ability of aluminium is taken into consideration it far outdoes ADI’s cost advantage.

Nevertheless, competition is not always between the materials but between ‘how’ and ‘in which circumstances’ they perform. As Francesca Bruni, President of ArtValley Association, rightly puts it, “In dealing with a complex project, there’s only one technology to use: The right one! If I need, for example, high fatigue resistance, I can’t discount ductile iron, but it would never be used, if possible, in an airplane where aluminium is the right choice!”