The Council for Scientific and Industrial Research’s (CSIR’s) Laser Materials Processing (LMP) application development facility has developed and proven the use of laser-based refurbishment processes that are able to extend the service life of industrial components that are currently being scrapped, says LMP operations manager Hardus Greyling.
The applications include the refurbish- ment of worn components by depositing new material onto the component, the selective hardening of the wear surfaces of a component and the manufacturing of new components with improved use of raw materials.
Laser cladding is done by aiming a laser beam onto a component and melting the material on the surface. Metal powder is then sprayed onto the molten weld pool, typi- cally of the same material but often of mate- rials with different characteristics, which solidifies as the laser beam moves away from the pool to form a new weld layer.
This layer of cladding is metallurgically bonded to the material to produce good fatigue resistance and impact resistance.
LMP research group leader Herman Burger adds that the laser refurbishment process can also be used to repair casting defects and to correct machining errors on parts that would otherwise have to be scrapped and remanufactured. For example, a cast aluminium gearbox housing, which was identified as having serious casting defects, was repaired using laser cladding and alu- minium powder to build up the material before machining the component to the required specifications.
Another example is rollers used in con- tinuous casting machines in steel mills. These rollers experience severe temperature variations, abrasive wear and corrosion, which limit their operational life. Again, a laser cladding process is used to repair the surface of the roll by cladding a specially developed low-carbon martensitic stainless steel, which has superior performance under these conditions, onto the surface of the roll. This improves the service life of the rolls beyond its original specifications, he says.
The high-specification cladding layer has increased resistance to pit corrosion, with the hardness of the steel being increased by additives other than carbon, which decreases its corrosion resistance through sensitisation. Molybdenum is added to reduce pit corrosion.
“There are two reasons why this process is applied and the first is to refurbish a compo- nent and the other is to improve the performance of the component, possibly even beyond its original specifications. Often these two objectives can be combined,” he explains.
Industries that have critical components, which would halt work if they failed, are the target of LMP’s laser refurbishment processes. Currently, the LMP does work for the electricity generation industry and the chemicals industry. However, the LMP is seeing increased interest from the mining industry and believes that there are many applications for the technology in that industry.
Building on its knowledge of laser metals deposition for refurbishment, the LMP has developed additive manufacturing processes using lasers. This involves building an entire component using the same method used in refurbishment, namely focusing a laser beam and a powder stream to create a functional component that requires little postprocessing, Burger says.
“There is significant interest in this method of manufacturing, especially from the aircraft indus- try and the medical industry,” he says.
The blade-integrated discs (blisks) used in turbines are often damaged on the leading edges of the blades. The damaged areas can be rebuilt using the additive method and then remachined. Alternatively, the entire blisk can be built in the same manner, but layer by layer, he explains.
“The machine that builds the components can also be used to repair them because the same computer-aided design data is used by the machine. Other benefits are that this production method does not need any so-called ‘hard tooling’, and that changing the design of a component does not mean changing your moulds or machines,” he notes.
Laser additive manufactur- ing is about 90% efficient in its use of materials. By contrast, it is common for 90% of raw materials to go to waste when complex parts for the aero- space industry are machined with conventional methods. This is especially useful if the materials are high-value metals such as titanium or super alloys, he adds.
Using lasers and powders to build a component also improves the material proper- ties, compared with castings through grain refinement of metals, which is a result of the very low heat input and rapid cooling. Further, the small weld pool means that large defects common in castings cannot form inside the material.
Laser additive manufacturing is an automated process, which means that there is consistent quality, irrespective of the opera- tor’s skill, he concludes.