Adaptive Repair For Turbine Blades

BY ROBERTA PRANDI

Hamuel Maschinenbau GmbH & Co. KG, which offers machine tools for turbine blade and blisk manufacturing, recently added the HSTM Hybrid machine designed to offer adaptive and additive manufacturing in one machine.

The company, based in Meeder, Germany, said that in this case, hybrid means a machine that combines high-precision and dynamic five-axes simultaneous machining, laser metal deposition, in-process measuring, in-process control devices and adaptive milling. The HSTM Hybrid system can be expanded by including a 3D scanning system, buffer storage, a coordinate measuring machine (CMM) system and a fully automated part handling system, the company said..

Hamuel said growth in the energy sector and in the maintenance, repair and overhaul (MRO) markets means more nickel-based alloys, super alloys, titanium aluminide (TiAl), and stainless steel are being used in turbine components. The complexity of part geometry and the demand for higher product quality, shorter production time and lower reject rates will lead to a new production and process strategies, the company said. New technologies like laser powder cladding increasingly will be used to repair damaged parts as well as to produce complex parts and geometries not possible by conventional manufacturing processes.

In component repair operations, the HSTM Hybrid machine can remove the damaged area by milling off the worn portion and then rebuild that portion by adding material without moving the component to other machines. This helps ensure accuracy, repeatability and reproducibility, Hamuel said.

The HSTM Hybrid machine incorporates a laser cladding head that is stored in the tool magazine like all the other tools and can be exchanged like any other tool. The compact design of the laser cladding head permits its very simple use in the usual area of a standard milling machine, the company said.

Hamuel said all tool units can be clamped in the milling spindle of the HSTM machine with the same standard HSK63A interface. The laser cladding head guides the laser beam, the powder and the protective gas. In general, any kind of laser welding material can be used. The positioning of material and laser focus are designed to minimize the heated area and thermal deformation a minor factor, the company said.

The laser cladding head in operation on a turbine blade in the HSTM Hybrid machining center: the head guides the laser beam, the powder and the protective gas in a fully automated process.

This kind of manufacturing insures the accuracy of the five-axes machine for the positioning and movement of each individual axis, as well for five-axes simultaneous machining. In terms of welding accuracy, Hamuel said that the work piece itself never changes its position. The laser cladding head is positioned through the HSK63A tool interface, and this configuration allows for highest repeatability.

Hamuel calculated that a conventional repair process of a turbine blade has about 13 steps, including the removal of the damaged areas, the welding of new material by laser cladding and the rework of that area by milling and polishing. These steps are mostly done manually with some robotics for certain steps. In the traditional process, there are several steps where a deviation can occur, the company said. For example, the clamping of a work piece at another work place, imperfections at the datum faces, and the machining tolerances. With Hybrid manufacturing, the part is fixed in the same position at the beginning of the process and remains there until the last operation is completed—which also reduces waiting time for the next process step, the company said.

An automated process requires an adaptive process chain, because each blade is a specific one. This is why the Hybrid machine by Hamuel is equipped with a program that the operator selects for the type of blade to be repaired and the entire repair sequence is controlled by that program.

After the damaged turbine blade has been clamped in the HSTM, the component is measured. This can be done with either a tactile or optical measuring sensor and can capture the complete component or individual segments. These measurements are passed automatically from the machine to a connected adaptive computer-aided machining (CAM) system. The original component geometry and the measured actual work piece geometry in the machine are compared. The adaptive software uses this information to create a modified numerical control (NC) program for the next processing steps.

First, defective portions are removed by milling in order to generate a good surface for the laser deposition welding. Using the laser unit, the missing material is applied on these surfaces. This operation, too, is controlled by the standard CNC of the machine and a standard NC program. The resulting post-weld geometry is measured again. Then another measuring cycle is started, the measured results are sent to the connected adaptive CAM system, and the milling program for finishing is created from these data to achieve a smooth and stepless transition between the already existing part surface and the material added in the damaged areas. The repaired area can be polished to make it even smoother. For this purpose a polishing tool can be inserted into the standard interface at the milling spindle.

The elimination of idle time between the individual machining operations can reduce overall lead time dramatically, the company said. The creation of a fully automated process also permits comprehensive documentation and traceability. With the HSTM Hybrid machine and its automated process components can be reinforced or enhanced selectively—in areas subject to higher wear additional or more resistant materials can be applied in very thin layers.

Hamuel said the new machining system can apply a different material in specific areas under very high stress, a process that makes the component stronger and more reliable. These wear-protection and enhanced materials can be applied very specifically and in very thin layers, so that the work piece geometry does not need to be changed to obtain the required strength.

The relationship between the base substrate and the added material plays of course an important part. One of the core points of the process is the compatibility of the various material combinations. Numerous studies of different variations and combinations have been analyzed by Hamuel, so that the process meets even the most stringent quality criteria.

Hamuel said that the optimum integration of each step in the entire process chain was crucial for the success of such a system. The know-how from different partners was brought together for each area of the process in the HSTM Hybrid machine; for example the adaptive milling software has been implemented together with DELCAM, and any other software can also be integrated, such as BCT Online. Hybrid Manufacturing Technologies have contributed to the development of the laser unit.