Additively manufactured Fe-25Co-15Mo maraging steel cutting tools for aerospace machining via laser metal deposition

Processing difficult-to-machine materials such as titanium alloys is a significant problem to industry.  Their desired application properties are directly the reason why they are difficult to machine, resulting in low productivity and high costs.  Custom cutting tools have been one solution to this problem, as they increase productivity via improved material removal rates (MRR), eliminating the number of machining operations and lowering the number of cutting tools required and tooling setup time.  However, the increased costs and long lead time associated with the design and manufacture of custom cutting tools are still an issue for industry and preventing their wider adoption.  Additive manufacturing (AM) technologies have demonstrated capability to manufacture fully functional parts in a variety of applications.  One method of metal AM is laser metal deposition (LMD) which has the potential to reduce production time and cost for low volume and high value components.  The expansion of metal powder providers has also enabled powder compositions not currently available to cutting tool manufactures in bar form.  Therefore, an opportunity exists for the cutting tool industry to apply metal AM to complex/custom cutting tools, such as profile cutters, using powders which may potentially offer increased thermo-mechanical properties, further increasing their MRR i.e. productivity.  Carbon free Fe-25Co-15Mo maraging steel has been shown to have mechanical and thermal properties advantageous as a cutting tool material for difficult-to-machine workpieces such as titanium and nickel alloys and stainless steels.  This maraging steel alloy has been shown to have a higher hot hardness, temper resistance and thermal conductivity compared to high performance powder metallurgy (PM) high speed steels. Within this context, this research project investigated and developed the capability to additively manufacture crack and pore free bar stock from Fe-25Co-15Mo maraging steel via LMD using build plate heating.  Post AM vacuum heat treatment conditions which produced appropriate hardness and microstructures suitable for metal cutting applications were then determined.  Metal cutting tests were performed to characterise the tool life and wear performance machining titanium alloy Ti-6Al-4V.  Hardness, microstructure, and wear performance metrics were measured and compared against a conventionally manufactured PM Fe-25Co-15Mo maraging steel analogue. This work used pre alloyed Fe-25Co-15Mo powder produced via gas atomization to develop process parameters and build strategies for the AM of crack and pore free cylindrical coupons followed by cylindrical bar stock with and without build plate heating.  Focus variation-based microscopy was used to characterise the interaction of multi-track and multi-layer LMD deposits utilizing multiple tool path strategies.  Their porosity was studied using 3D computational tomography.  The hardness and microstructural response were characterised using Vickers hardness indention, scanning electron microscopy (SEM) in back-scatter mode, transmission electron microscopy (TEM) in bright field, dark field, and selected area diffraction (SAED) modes and energy dispersive X-ray spectroscopy (EDS) analysis.  The effect of vacuum heat treatment on the hardness and microstructural response of additively manufactured and PM produced bar stock was characterised using Vickers hardness indention, SEM in back-scatter mode and X-ray diffraction.  Lastly, the effect of AM and PM manufacturing methods and various heat treatment conditions on cutting tool wear performance were characterised via side milling titanium alloy Ti-6Al-4V in a down milling configuration under emulsion coolant conditions using focus variation-based microscopy. Characterising the effects of LMD process parameters and build plate heating in terms of the flow rate and traverse speed ratio F/S, laser power and the square root of traverse speed ratio P/√S, and single-track aspect ratio parameters facilitated the development of a build strategy for pore free cylindrical coupons and bar stock.  A hybrid tool path strategy consisting of a zigzag infill with a single outer contour was shown to produce uniform single layers when applied with a 30% overlap factor for the infill and between the infill and outer contour in terms of layer coverage and layer height uniformity.  Applying a layer rotation of 71 ° has been shown to distribute minor non-uniformities evenly as components were built.  Age hardening was observed throughout all samples produced with no build plate heating except the topmost layers.  This process occurred via the intrinsic heat treatment (IHT) effect found during the LMD of maraging steels.  Sever cracking was observed throughout all age hardened regions, with the severity in cracking observed to increase as the number of heating cycles increased.  Maintaining the build plate temperature at 500 °C during a build was observed to suppress in situ age hardening and crack formation.  It is likely that cylindrical coupons and bars built in this work were maintained above the Ms and Mf temperature during the build process which suppressed the IHT effect.  Incorporating a build plate temperature of 500 °C produced an as built microstructure consisting of irregular lath martensitic sub grains within large irregular prior austenite columnar grains growing in the direction of heat flow incorporating epitaxial growth.  The remelting and repeated reheating of prior layers produced a hypereutectoid microstructure along the prior austenite grain boundaries consisting of µ-phase particles when using a build plate temperature of 500 °C with no interlayer dwell time.  Notwithstanding, the martensite was found to be super saturated with molybdenum. Solution treatment of AM Fe-25Co-15Mo cylindrical bars at 1150 °C for 15 minutes facilitated recrystallization which formed an equiaxed microstructure with globular primary µ-phase particles, an effective microstructure for cutting tools.  As built and solution treated hardness measurements were identical to the PM analogue in the as received and solution treated condition.  Aging at 550 and 600 °C for 60 minutes produced a similar hardness response for both manufacturing methods with mean hardness between 888 – 937 HV0.3.  AM Fe-25Co-15Mo maraging steel bars via LMD, post heat treated and ground into square endmills were observed to side mill titanium alloy Ti-6Al-4V as well and better than their PM analogue, with post heat treatment conditions shown to be more significant than the consolidation method.  The non-solution treated AM sample, i.e. as built microstructure, was qualitatively shown to have the highest fracture toughness with no micro chipping of the outer corner observed.