Joining Not Welding

Using powder formulations to develop specific characteristics.

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Metals and alloys all have their strengths, and best applications, and the growing ability to use these materials where they are needed, when they are needed, and alongside other materials with their own properties, makes this is an exciting time for welding as joining technologies are evolving.

One benefit of these evolutionary technologies is that it allows designers to join dissimilar materials and utilize these materials to their fullest. A component may need high-temperature resistance in one area, enhanced structural performance in another area, and long-lasting corrosion resistance in still a third area.

Another development is the move toward making components more multi-functional, adapting the materials they contain or their design or manufacturing processes to let them perform a wider range of functions. With these materials, metal goods can be made stronger yet lighter, something, yet something.

There are a variety of solid-state joining processes that are growing in application and use, says Narasimhan Sreenivasan, an application group manager for Trumpf. This is especially true when it comes to joining systems that utilize laser-based technology. One of those processes is Laser Metal Deposition. Sreenivasan recently gave a presentation on LMD, with an eye especially focused on the automotive field.

According to Sreenivasan, LMD is, in theory, a simple process. A laser beam is used to produce a melt pool on the surface of the metal component. Powder particles are injected into the melt pool and a metallurgical bonding between coating and substrate occurs.

In addition, functional coatings and structures can be deposited on 3D surfaces. Production of coating or parts combining materials in multilayer, gradient coatings are possible. The technology can be fast, with deposition rates of up to about 200 cm3/h shown to be possible.

While LMD can be used with wire filler, the use of powder as a deposit material offfers greater future opportunities, such as the possibility of Al-Al and Al-steel joining.

The powder formulation’s makeup should interface between one or two base metals, said Sreenivasan, who has focused recent research on aluminum to steel joining, meaning that one of the materials should be similar to one of the base metals.

Laser metal deposition offers a number of advantages. It allows for the joining of profiles with gaps in between, and the joining of aluminum to steel with a homogeneous welding seam, and the ability to tailor specific alloys to the job. The laser metal deposition process can be used not only to manufacture functional layer systems and to generate 3D structures, but also as joining technology.

Potential LMD applications include the repair or worn surfaces such as turbine and compressor blades, vanes, welding gap bridges, valve seats, tool and die steels, welding of gaps and other metallurgical bonding. In terms of industries, one of the main potential users of LMD are automakers who Sreenivasan says are looking to reduce weight in an effort to minimize gas consumption, and compensate for the addition of other technologies such as the weight of batteries for electric cars. Car manufacturers are alsolooking at upcoming regulations to improve fuel economy, and LMD might prove to be a promising way to join lightweight aluminum with traditional steel components.

“Powder metallurgy is the direction we are heading,” said Sreenivasan. “Making vehicles lighter is a big push.”

The U.S. Environmental Protection Agency and the Department of Transportation’s National Highway Traffic Safety Administration are issuing final rules to further reduce greenhouse gas emissions from automobiles and to improve fuel economy for model years 2017 through 2025 light-duty vehicles. EPA’s standards apply to passenger cars, light-duty trucks, and medium-duty passenger vehicles, in those model years. The final standards are projected to result in an average industry equivalent to 54.5 mpg if achieved exclusively through fuel economy improvements. One way to improve fuel economy is to reduce the amount of weight that has to be propelled.

Aluminum might be the answer in terms of weight, but it also offers manufacturers other benefits such as corrosion resistance, structural performance, ease of manufacturing, and material availability as compared to magnesium, composites, and other materials, said Sreenivasan.

At this time, it is not simply a matter of swapping out old parts for their aluminum counterparts. According to Sreenivasan, it will be some time before steel structures are eliminated. Until then hybrid structures might be the answer, using multiple materials. The problem is joining these. Generally, welding aluminum and steel is a challenging process.

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Joining challenges

There are a number of challenges when it comes to joining dissimilar materials such as aluminum to steel components. There are differences in physical properties such as the melting temperatures of the base metals and alloys, thermal conductivity, coefficient of thermal expansion and other properties. Some of the biggest challenges to overcome are brittle intermetallic phases at the join interface, and hot-cracking formation of the weld seam.

The LMD/powder theory is meant to work around these challenges of welding dissimilar materials by using similar materials in the process as well as other materials that address specific problems. For instance, 6000 Series of aluminum is very sensitive to weld cracking and it is generally not a good idea to weld it to dissimilar alloys. However, using LMD may be able to solve these issues. In testing, Sreenivasan found that welding with the laser metal deposition process, using powdered aluminum and a silicon content, avoided crack production.

In one experimental setup and parameter they used a powder makeup of copper, aluminum and iron to join aluminum to steel. Particle sizes ranged from -125 to +45um, the laser power was 2kw, spot diameter 2.5, traverse speed 1-1.8 m/min, shielding as 10 1/mm, and carrier gas 4 l/min.

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Sreenivasan found that welds with a reasonable tensile shear strength performance between 3 and 6kN and elongation up to roughly 7mm could be obtained. Within the present work, the LMD process was used to obtain the required joining and neither preheating nor ore-coated bonding layers had to be used.

Sreenivasan believes that powder melting can improve these other challenges especially when use powders of similar chemical makeup.

According to Sreenivasan, there is no golden standard formulation; the formula depends on what the user wants to join. One must, however, use compatible materials.

“If want to join copper to steel, then we would have to have those powders. We would try to have at least one material comparable for the process,” said Sreenivasan. “We want to increase the intermetallic zone, and we are working to add three or four materials to enhance the material’s properties.”

Research is ongoing. Sreenivasan said that future research will continue to look at inter-metallic phase analysis, improvement of welding speed and reproducibility, mechanical characterization, improved corrosion resistance and continued testing of additional powder materials.

Still, work has been promising, said Sreenivasan, and the future holds open the possibility for even more exciting applications that utilize the inherent capabilities of a slew of materials that will likely include plastics and other composites.

Trumpf

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