Reducing Lead Time and Increasing Flexibility in High Precision Manufacturing – Integrated Laser Hardening and High Accuracy Near Net Shape Finishing

The current ongoing evolution towards mass customization, resulting in the production of small lots or prototypes and single unique items, requires a different strategy compared to classic high volume manufacturing. In order to deal with the increasing product range and decreasing time to market, the flexibility of current manufacturing process chain needs to increase.

A conventional manufacturing process of a high quality component consists of an initial machining step, for example a milling and/or turning operation, starting from bulk material (block, cylinder). In a next step, the part is unclamped, transported to a heat treatment facility in order to attain the required material properties (hardness, residual surface stresses, microstructure,…). This step introduces inevitably deformations to the component, which is why a finishing operation is performed after the component is transported, re-clamped and re-aligned. In this PhD thesis 2 alterations to this conventional process flow are investigated.

A first innovation investigated in this thesis, is combining the machining, heat treatment and if required a hard finishing operation in 1 single machine and setup. This omits the need for logistics back and forth to the hardening facility. Re-clamping and re-aligning of the component is no longer needed for the hard finishing operation, which reduces the total lead time and errors introduced during these labor intensive operations. This is realized by integrating a laser into a machining center, allowing to perform both the machining operations as the heat treatment in one setup. In this thesis, 2 different setups are developed and described.

The first setup, uses a 500 W Nd:Yag laser as a heat source. A fixed spot optical design is made using basic optics calculus and is updated using ray tracing simulation. A mechanical design is made and realized. The integration of the laser as a tool is done into a 5-axis milling machine. A spotsize of 1.2 mm diameter or more (if out of focus) is moved across the surface of the component by moving the machine’s axis. The process of integrated laser hardening is investigated on C45 steel by performing a Design of Experiment, using 3 different feed rates and 4 different spotsizes. High hardness values, above 750 HV and a hardening depth of about 0.2 mm are achieved. Thanks to the high cooling rate during the heat cycle in laser hardening, the hardness exceeds hardness values achieved by conventional hardening techniques. A second design of experiment, investigating multiple laser tracks next to each other, in order to cover a larger area, is performed. From these experiments, it is concluded it is impossible, no matter what overlap/inter-track distance is used, to achieve a uniform hardness along the cross section of multiple scan tracks. This effect is identified as a tempering/softening effect, in which the second track tempers a part of the first track. Finally, a die and mold component, prone to rapid wear, made of a high alloyed steel, is laser hardened using the developed setup, proving the industrial relevance of this work.

In order to achieve a uniform hardness across a larger area, a new setup is developed on a 11-axis machining center. Instead of using a fixed spot hardening strategy as in the previous setup, a very small spot (0.8 mm in diameter) is scanned across the surface at high speed using a rotating mirror, while moving slowly in the feed direction by moving the machine’s axis. The optical and mechanical design and realization of this setup are described. A control system, controlling laser source (500 W diode laser), mirror actuator and auxiliary equipment, while monitoring the actual temperature and machine position, is developed. A software tool which allows to program the hardening operation of complex shaped components, is developed and the strategy is illustrated on an example. The process of integrated scanning laser hardening is investigated on a low alloyed steel (C45) and a highly alloyed steel (40CrMnMo7). The samples showed no tempering effects, resulting in a uniform hardness across the scan width. The robustness of the process and control is verified by testing contaminated samples (oil, steel/aluminum chips, emulsion) and samples with varying scanning width. Two cases are described as an example of industrial applications for this technology.

The second innovation tackles the machining of near net shapes, which is significantly different to machining from bulk material. In this thesis, 2 types of near net shape machining are investigated: the machining of gears from forgings and the finishing of large assemblies built by additive manufacturing.

The machining of gears using the conventional processes requires dedicated, design specific tools. The lead time and cost of these tools are high, which is an issue for prototypes and small batch production. Three different alternative production technologies are investigated. A multi-axis milling strategy using standard end mills is developed and an automotive gearbox gear is made to high quality grinding specifications in less than 24 hours, while the lead time for conventional production is in the order of 13 weeks. This machining process is ideal to combine with the integrated laser hardening. Since the distortions introduced by laser hardening are limited, a hard finishing operation might be omitted, resulting in a hardened high accuracy gear manufactured in hours/days instead of weeks.

The finishing of large assemblies, built using additive manufactured components, to a very high accuracy is quite different to machining a small component from bulk to the same accuracy. By investigating and reducing errors introduced by CAD/CAM, machining, in-machine mounting (clamping and aligning) and machine condition (position accuracy, flatness table and thermal stability), the achieved tolerance is reduced significantly. This is verified on a demonstrator part.