To cast, or not to cast?

Jon Lawson

Richard Phillips reveals the prime candidates for conversion of fabrications and assemblies to castings

The design and manufacture of a metal component involves a multitude of decision points and will be influenced by considerations such as structural performance, weight, lead time, cost and aesthetics.

Questions relating to how precisely that component is going to be manufactured are often left until the design is at a fairly advanced stage.

This could be the case where there is pressure to get a manufactured product to market quickly or where design engineers have already decided to fabricate an assembly or component due to familiarity with the process.

The reality is that many of these fabrications ¬- particularly components with complex geometries - could be cheaper, lighter and stronger if they were cast in one piece.

A closer examination of the options available for component manufacture can often reveal considerable opportunities for improvements in manufacturing costs, lead times, performance and aesthetics.

Clearly, casting will not always be an option but there are certain component or manufacturing process characteristics that make some components particularly good candidates for conversion from fabrications to castings.

What to look for

A good starting point is to focus on assemblies with complex geometries that are difficult to fabricate.

These could be machined or forged assemblies, particularly where several of these components are welded or bolted together. In addition to being time consuming and expensive, the welding process itself can also create stress and distortion. This is common when wrought sections are welded together.

Casting components in one piece can deliver a homogenous microstructure so performance can be improved as weak points (welds) are eliminated.

Weight reduction without loss of strength is often a consideration for design engineers.

Castings can allow material thickness to transition smoothly throughout a component.

Metal can be placed only where it is needed, which can considerably reduce overall weight without compromising performance. Using just the quantity of metal required to achieve the desired performance also means that casting can use material more efficiently as there is less waste to start with and what there is can be recycled back into the production process.

Casting a complex assembly in one piece can deliver tangible benefits where uniform directional strength is required.

Wrought metal (rolled or forged) exhibits directionality (anisotropy) where a component has strength or ductility in one direction but its transverse properties are lower. Cast products are isotropic, which means that they deliver consistent strength and ductility in any direction.

As castings are typically isotropic, uniformly heat-treated and more stress relieved than a fabrication they tend to have longer fatigue lives and allow more deformation without sudden failure.

Switching from fabrications and assemblies to castings can also deliver a number of manufacturing benefits. For most manufactured components, consistency is vitally important.

Fabricated assemblies are susceptible to human error; many variables can be introduced such as missed welds, inadequate weld joining material, pin holes in weld joints and inconsistency in weld beads or process. Cast components have consistency built in; once the pattern is set, every casting is uniform so human error can be eliminated as a source of inconsistency.

One obvious way to reduce inconsistency is to improve the training and skill levels of production staff involved in fabrication and welding. Clearly, there is a cost to train, maintain and retain a highly skilled workforce. Casting, by contrast, often requires lower-skilled personnel using more automated production processes.

This contrast becomes even more apparent as production volumes increase.

Achieving economies of scale is difficult with fabricated assemblies as the same process needs to be repeated for each assembly using similar materials and labour.

The casting process, however, delivers significant economies of scale once the mould or die has been created. Stripping out many process steps by switching to castings will also speed up the production process.

Finally, design is often an important consideration. More aesthetically pleasing designs are possible using castings as designers can include unique shapes, features and curves.

In the past, the realisation of this potential called for outstanding casting geometry design expertise and experience because shape optimisation calls for complex three-dimensional modelling.

In more recent years, however, powerful 3D modelling programs and casting simulation software have allowed design engineers to quickly produce solutions featuring continuously varying section geometry that fully utilises material strength while also satisfying stress and deflection requirements.

Whether a switch from a fabricated assembly to a single cast component is going to be feasible and whether it will deliver the desired cost, lead time and performance improvements will depend on many considerations, not least of which are the castability of the redesigned component and the production run length.

By working closely with foundry engineers experienced in structural geometry, such as NovaCast, it is possible to fully evaluate and fully cost this kind of project and to realise the considerable benefits conversion can deliver.

Richard Phillips is with NovaCast

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