Research by the RMIT Centre for Additive Manufacturing, supported by the Defence Materials Technology Centre (DMTC) and Sutton Tools, is breaking new ground in the use of additive manifacturing in the production of custom cutting tools. By Jimmy Toton, Steve Dowey, Songlin Ding and Milan Brandt.
As product design complexity increases and high-performance materials become a necessity, manufacturability generally decreases. These fabrication realities are evident in high-performance components such as turbine discs, blades and vanes made from difficult-to-machine titanium and nickel alloys for the defence and aerospace sectors. Fabrication of these components results in low productivity, and therefore high final component costs.
Custom cutting tools are often required over standard mass-manufactured tooling as the latter may not be capable of performing the task, or the current rate of production is limiting profitability. However, the costs and long lead times associated with custom tooling fabrication often acts as a strain on production and a barrier to their wider adoption.
Additive manufacturing (AM) technologies expand the horizon of manufacturable designs as they have fewer constraints compared with conventional manufacturing methods due to the layer-by-layer manufacturing process. Recently, the knowledge base has matured, to the point where now the capability to manufacture fully functional parts is a reality.
Manufacturers must take full advantage of the opportunities to become or remain competitive, in cases where the current high costs of manufacture are justified. AM has the potential to increase performance, reduce costs and cut lead times for high-value, low-batch, complex-shaped components such as custom cutting tools.
Custom cutting tools, like profile cutters and multi-point tools, increase productivity because they are designed with the inverse geometry feature of the component they are machining, significantly increasing workpiece-tool engagement and hence facilitating higher material removal rates (MRR) compared with standard mass-manufactured cutting tools. They simplify tool paths, potentially eliminating expensive advanced CAM software and saving time in production planning. They also reduce the total number of tools and tool changes needed, minimising production cycles.
Moreover, standard mass-produced tooling may be unable to reach areas in complex components. This is evident in the aerospace and power generation sector, with deep/internal grooving on turbine discs, blisks and engine mounts requiring specialised holders for turning operations.
It is well known that high-performance materials, such as ultra-high-strength steels, titanium and nickel alloys, commonly used in the defence, aerospace and power generation sectors, are classified as difficult-to-machine owing to their low thermal conductivity, and/or excellent retention of mechanical strength at machining temperatures, and/or chemical reactivity with cutting tool materials.
Most issues arise due to the inability of the cutting tools to extract heat from the cutting zone, limiting cutting parameters and hence reducing productivity. However, this problem can be significantly minimised with custom cutting tool geometries that not only increase the MRR but can also, with in-built cooling channels, effectively remove excess heat generated near the cutting zone during machining.
Therefore, the design and fabrication freedom that AM facilitates produces solutions to these problems that are superior to conventional methods such as grinding away large amounts of material from expensive bar stock with limited cooling channel locations, or the initial high expense and long lead times associated with metal injection moulding.
RMIT’s research: Laser metal deposition of cutting tools
There are several approaches to AM custom cutting tools. One involves selective laser melting (SLM) or “powder bed” technology, the second is laser metal deposition (LMD) or “powder fed” technology. SLM builds up finished components layer-by-layer using a laser beam to selectively scan and melt powder coated on a bed according to the sliced CAD data file. LMD is the deposition of a series of overlapping single clads/tracks that make up a single layer. Powder is fed into the melt pool instead of positioned on a bed. The focus at RMIT presently is on LMD.
Properties that influence the performance of cutting tool materials are hot hardness, toughness and wear resistance. Hot hardness is one of the most important properties as it pertains to the ability of the material to retain a high yield strength at elevated temperature, resisting plastic deformation of the cutting edge during loading. Toughness is the ability of a material to absorb energy before fracture – the greater the toughness the better the cutting tool can resist shock loads, chipping and fracturing. The wear resistance of a cutting tool material will vary depending on the underlying wear mechanisms involved, strongly influenced by the workpiece material and the cutting parameters. Notwithstanding, in the context of cutting tools, wear resistance is the material’s ability to resist material loss and therefore changes to the cutting-edge profile over time, effecting the performance of the cutting tool to machine a part to specified tolerances and limiting tool life.
In the context of these desired properties and the intended application, the two materials investigated were Fe-C-Cr-Nb-B-Mo (FCCNBM) and Fe-Co-Mo (FCM) based steels, due to their potential to bridge the mechanical properties gap between high speed steels (HSS) and cemented tungsten carbide (WC-Co) grades.
Process parameters and build strategies were developed that facilitate a uniform and repeatable process to fabricate crack- and pore-free cylindrical bars. Geometry complexity was reduced so the focus was on developing a process that yields a material with the desired mechanical properties yet be easily processed via post grinding into cutting tools.
The successful build strategy was designed around a meander nozzle path due to it being more accommodating to implement for cylindrical structures compared to concentric-based methods and faster deposition times compared to raster methods. Using 3D measurements, an optimisation activity was completed which determined suitable track step-over widths and layer heights for uniform layer-by-layer fabrication. This eliminated a common issue of non-uniform layer deposition for LMD-based AM, which causes concave, convex or irregular profiles in the AM component and therefore resulting in a failed build. Lastly, it also produced a build strategy that does not require outer-track/shell deposits, a strategy adopted by many researchers and practitioners for use as a support structure; hence, the developed solution offers reduced nozzle path complexity.
The local and rapid heating/cooling cycles inherent to AM are known to cause residual stress in parts and can lead to crack formation, hence a failed build. The capability to additively manufacture crack-free cylindrical coupons and bars made from FCCNBM- and FCM-based steel powders was achieved by implementing high-temperature substrate heating as a technique to reduce residual stress and supress in-situ age hardening during the LMD manufacturing process, respectively.
The effect of substrate heating on the as-built microstructure and hardness was characterised. The intrinsic heat treatment caused by the cyclic reheating of previously deposited layers, a phenomenon characteristic of the LMD process, was found to enable the precipitation of nanometre-sized particles within grains, thereby increasing the bulk average hardness of as manufactured samples to ~747 HV. Applying high-temperature substrate heating significantly supressed the formation of these strengthening particles and led to no hardness response with an average bulk hardness of ~430 HV.
Post-heat treatment experiments revealed that AM samples respond similarly compared to their commercial powder metallurgy (PM) analogue in terms of hardness. Hardness levels between 900-950 HV were measured depending on the heat treatment cycle applied. Solution treatment was found not to be necessary to attain a high average hardness, due to the in-situ quenching during the LMD process. However, solution treatment was found to significantly reduce the variance in hardness measured, reducing the standard deviation and range by 50%-60%. The AM microstructures were found to be finer than that of the PM.
Following heat treatment, the ground cylinder was machined into a four-flute endmill. Tool life tests consisting of side milling Ti-6Al-4V in the mill annealed state revealed that the FCCNBM-based steel is not a suitable cutting tool material in the as-built state. Endmills made from this material primarily failed via chipping on the outer corner and flaking on the flank faces through a combination of chipping and adhesion wear.
In stark contrast, the milling cutters made from FCM steel performed well achieving a tool life similar to their PM analogue with one AM sample outperforming all conventional tool steels samples. No chipping was observed on AM FCM 3 sample, this being the reason why it had the lowest wear-in stage compared against all cutting tools. Further work is needed to quantify the fracture toughness of these materials as the cutting tool tests show qualitatively that the AM material has a high fracture toughness, desirable in-milling and broaching operations.
The research completed at the RMIT Centre for Additive Manufacturing has demonstrated that AM FCM steel can be used as a cutting tool material when applied to difficult-to-machine Ti6Al4V. Work is underway to develop the capability to use AM to create near-net-shape complex custom cutting tools that maintain the desired microstructure and consequently mechanical properties. This may lead to a print-to-order capability, which can increase performance, reduce costs and cut lead times for high-value, low-batch, complex-shaped custom cutting tools.
The authors would like to acknowledge the support of the DMTC. The DMTC was established and is supported under the Australian Government’s Defence Future Capability Technology Centres Program. They also thank Alan Jones (from RMIT University) and Sutton Tools.