SIEMENS

Materials make a fundamental difference in terms of the ultimate cost, environmental impact, and competitiveness of products. Take gas turbines, for example. To improve their efficiency, one must increase their combustion temperatures, and that requires new, more heat-resistant materials. Similar examples can be found in nearly all of Siemens’ businesses.

But new product properties do not always call for new materials. In many cases, old materials would be adequate if they could be joined together in new combinations. The search for ways of accomplishing this is being conducted by several teams at Corporate Technology (CT), Siemens’ global research arm. This is their thinking: Where presently only a single type of metal or ceramic is used in a product, the future will see the use of combinations of materials, each tailor-made for a specific application. The advantages of this strategy are clear. They include significant potential for enhanced performance, reduced weight and demand for raw-materials, improved efficiency, and cost advantages.

Friedrich Lupp, a specialist in laser joining and manufacturing technologies at CT, explains where this journey will lead by using the example of “narrow-gap welding.” In manual welding, he points out, joints are prepared by first beveling the edges of the metal parts sufficiently to create a V-shaped notch at their junction. This process removes material and produces waste. The notch is subsequently filled with molten metal to create a weld. Since the thicknesses of the metal parts are well over 250 millimeters, the resulting crevices must be quite large. As a result, a large amount of welding wire as well as energy is required to fill them. This problem is solved by a method in whose development CT was a major participant. With this method, the gap is only 12 millimeters. A special welding head travels along this crevice and automatically fills it without requiring any manual work.

This is not a new method. The development of these welding heads began 30 years ago. But with today’s computing power, even varying gap widths can be automatically detected and compensated for. The welding head itself is part of the sensing system. The welding arc that melts the wire and the base material also functions as a sensor. Its voltage and current are a function of the distance to the walls and to ground. Also new is the fact that the researchers achieve quality control by using a temperature-independent X-ray sensor that detects defects in the part when it’s still hot, immediately after welding.
Siemens’ teams are working on other methods as well. One of those involves welding with high-performance lasers that create such small seams that the metallurgical properties of materials remain almost unchanged while energy use is substantially reduced.

Even more exotic is pressure welding. In this process, two massive workpieces are rubbed against each other with great force, so that the surfaces of both parts become bonded without actually melting the workpieces. This makes it possible to weld together metals that have traditionally been difficult to join, such as aluminum with copper or aluminum with steel. Much of the work being done is still basic research. But if the tests at CT’s labs are successful, such new material combinations would provide cost savings. The use of costly metals such as molybdenum and indium, as well as chrome, copper and silver, could then be limited to particularly sensitive locations in a product.

In a combined cycle power plant, the turbine rotor, for example, has to meet very dissimilar temperature and materials requirements. Rather than using a high-cost material throughout, the narrow-gap welding of sections optimizes the use of raw materials. “My dream is a software package that will tell design engineers what materials to use at which locations, with which manufacturing processes, in order to optimize the efficient use of raw-materials and reduce costs,” says Lupp. At the same time, the subsequent recycling capability of the materials mix should also be taken into account. However, such a project could only be carried out with Siemens PLM Software.

Reuse instead of Recycling. Over time, the surfaces of turbine blades can develop micro defects. CT has therefore developed a repair method in which image processing software detects and records the specific locations of defects. The resulting information is used to guide a robotic arm equipped with a welding nozzle. The nozzle sprays metal powder onto the specific points where it is needed. As the powder lands, it is melted by a laser, bonding it to the blade surface. This process allows semiautomatic repairs of components at the customer’s site. Specialists at a Siemens turbine plant in Berlin have already successfully tested this method. “Reuse instead of recycling is our goal here, because it saves valuable raw materials as well as energy,” says Lupp.

Recycling instead of rebuilding is also preferred by Dr. Ursus Krüger, head of the Coatings Research Group in Berlin, where he is pursuing advances in cold-gas spraying. In this process a supersonic nozzle bombards the surface of a component with powder particles at up to 1,000 meters per second. On impact, the particles release so much energy that they become welded. Due to low gas and powder temperatures, components treated with this process do not heat up and therefore do not deform, harden or soften. As a result, this process is ideal for the repair of parts such as cast-iron housings or lightweight construction components that have been damaged or were delivered with faulty measurements. Cold-gas spraying can also be used to repair larger defects with the same material, or to create entirely new shapes that approximate the final contours. In some cases, this can even be done on-site and without the need to dismantle parts.

Unlike flame or plasma spraying, the process gas used in cold-gas spraying does not react chemically, so the composition and structure of the sprayed material remains unchanged during its application. Cold-gas spraying is therefore suited for the coating of metals, ceramics, glasses, and plastic materials with top-quality metal layers of practically unlimited thickness. It has a cleaning effect like sandblasting and a strengthening effect like shot peening, making pretreatments largely unnecessary. Sprayed-on hard coatings can be even harder than the original material. Cold-gas spraying simplifies some production steps, says Project Manager Dr. Oliver Stier. That’s why Krüger intends to further augment the strong patents and technology position Siemens has in this field. His team has already patented a combination of cold-gas spraying and powder deposition welding, as well as cold-gas spraying of suspensions of nanoparticles. Stier can even calculate the process costs for a new application before the process has been tested. This ensures that only processes that will be profitable are developed.

New combinations. At CT in Munich, another team, this one headed by Dr. Wolfgang Rossner, is working to develop extraordinary methods of combining radically different materials. One of these is ultrafast sintering, an alternative to conventional joining techniques such as welding. In this process materials are pressed together under extremely high pressure and at high heat until they bond. This method has been used for quite a while, but was time-consuming. However, when the materials are not heated by an external source, but are instead heated internally by an electrical current, it only takes 20 minutes. The method, which is known as spark-plasma sintering, has recently come to be used in production for common ceramic and metallic materials. Rossner’s team is particularly interested in new combinations, such as composites of metals and ceramics.

Rossner displays a test piece the size of a coin. One side consists of high-temperature steel, while the other side is a metal-oxide ceramic. “You cannot separate these two materials,” he says. The boundary layer is only a few micrometers thick and contains a continuous transition between metal and ceramic. The team is now working to improve the adhesion between ceramics and metals, a step that would benefit all materials that have to withstand extremely high temperatures.

The researchers are already thinking one step further. They want to press together mixtures of fine metal and ceramic powders to form extremely refractory materials. This would be feasible in principle, says Rossner, whose team is exploring applications for gas turbines and high-voltage switchgear. “Such materials would make possible entirely new combinations of properties, such as the electric insulation of ceramics along with the plastic formability of metals,” he says.