Laser Cladding of Worn Cylinder Holes

Laser cladding is a proven technology for servicing or optimizing engine components, and it is an indispensable technology due to its reliable repeatability, low thermal element stress and low cost mechanical modification. There is now a new device on the market, iClad®, that can be used to cladding internal contours or blind holes to a level that is not possible with conventional machining heads. This micro-machining is suitable for cladding holes as small as 26mm in diameter and machining depths of up to 500mm. Now, for the first time, its manufacturer Pallas (Wrselen, Germany) has equipped the iClad with a rotary machining head that can be used to repair worn cylinder holes in diesel train engines. Machines equipped with rotary functions have unrivalled potential for a wide range of applications, repairing the internal contours of non-rotationally symmetric open components with eccentric holes or 50 mm or more in diameter, with machining depths of up to 500 mm.

Belgian Laser specialist Laser Coating Diamond Technology (LaserCo DT), based in Strpy-Bracquegnies, Belgium, protects metals from wear through laser cladding and diamond coatings. A Belgian railway operator commissioned the company to repair worn cylinder holes using laser cladding technology on the reverse points of the upper and lower pistons. Hugues Desmecht, the company's managing director, asked the Fraunhofer Institute for Laser Technology (ILT) in Aachen, Germany, to carry out a feasibility study. In connection with this research, ILT worked closely with Pallas (www.pallaskg.de), which earlier developed iClad and is now marketing the system. In this study, ICLAD-based laser melting was used to repair two of the six damaged cylinder holes.

The cost of replacing the engine is about $20,000. Considering the large amount of money and time consumed during transportation, laser cladding is recognized as a more time - and cost-efficient alternative. For sufficiently large apertures, such as 100 mm in diameter or larger, a variety of conventional machining heads are available on the market to provide machining of external or internal profiles. Now, with the advent of the small rotating iClad head, it has become a reliable new technology for repairing damaged cylinder holes, which was not possible with previous standard machining heads. LaserCo DT and the Belgian rail operator therefore have high expectations for the feasibility study carried out by ILT.

Each iClad head consists of three modules: powder feed port, body and fiber plug. In contrast to conventional machining heads, iClad machining heads and components operate within a few millimeters of each other. In standard machining heads, this distance reaches 20 mm. Depending on the processing point, the laser beam can exit at an Angle of 30 or 90. Blind holes can be easily coated with a 30-angle machining head, thanks to the laser's operating point in front of the machining head. When it is necessary to coat through holes, a 90 Angle machining head is used. The narrow internal rotary machining head has additional functionality as it can be inserted into the hole for cladding from the side or top - down to a depth of 500 mm. The main criterion for industrial applications is the continuous heat load capacity of the processing head. The efficient water cooling method used prevents overheating of precision components. The internal protection system of the machining head prevents it from being contaminated or damaged by the deposition of powder particles. In addition, the optical path the laser beam travels through is constantly filled with protective gases.

New machining heads with rotary functions are used to refurbish cylinder holes in diesel engines. The integration of a rotary driver into the machining head allows the repair of eccentric holes in static mounting parts. Even when inserted horizontally, the machining head ensures uniform powder application without changing its rotation Angle. In this particular application, however, the rotating head is inserted vertically into the cylinder hole via a five-axis linkage system. One of the main points of contention with the rotary head is that, given the weight of the machining head itself, it is impossible to rotate a diesel engine weighing up to 700 kg through a fixed machining head. Eccentric holes create a large amount of unbalance, making it very difficult to guarantee the required machining accuracy. Centring each cylinder individually is a less feasible option because of the large cost factors involved.

The bore diameter of the cylinder inner diameter is 178 mm; The lower area that needs to be coated is about 340 mm deep and consists of a ring of about 60 mm high. Conventional machining heads are only suitable for angles close to the hole because the nozzle is located at the tip of the machining head. Therefore, although the diameter of the hole reaches 180 mm, these machining heads can only be processed to a maximum depth of about 40 mm. In contrast, the rotating iClad head can be inserted vertically into the cylinder bore and can handle the entire internal surface at 90 angles. The decisive parameters of the rotating iClad model can be summarized in three aspects: in addition to its rotating ability and narrow machining head, the maximum machining depth of 500 mm is also a key factor.

The inner layer of the cylinder between the upper and lower failure points is worn and covered with a coating greater than 1 mm thick. This is called stainless steel alloy cladding to ensure not only the necessary wear resistance, but also good corrosion resistance in acidic and alkaline environments. The toughness of the selected alloy material prevents the formation of cracks, but also has good machinability and minimal surface coating porosity. First, LaserCo DT customers' wear holes are machined to size to remove wear and cracks, creating a smooth surface for subsequent laser cladding.

At the Fraunhofer Institute of Laser Technology, engine blocks are placed on steel supports, then aligned and properly adjusted. Dr. Andreas Weisheit and his team, together with Stephan Kalawrytinos from Pallas, developed the internal rotary machining head). In the lower position, the surface of the cylinder hole can be machined in the damaged ring area. With a five-axis linkage system, two coats are applied using two overlapping rotating motions to achieve the desired coating thickness of 1 mm. A numerical control program was developed for this process. Starting at the bottom edge at a depth of around 300 mm, the coating covers the surface in a spiral form.

Mr. Weisheit was very pleased with the test results. "This proves beyond a shadow of a doubt that laser cladding is technically feasible and cost-effective in the repair of cylinder holes without the need to buy replacements," he said. Using the rotating internal profile machining head iClad, the resulting coating has excellent surface quality." In his opinion, the running engine has proven its capability for industrial applications. Even after several hours of continuous use, its ability to continue working remains absolutely reliable. The complete success of this test provides ground-breaking evidence for the repair or repair of machine parts, which were previously unattainable.