By Rahul Patwa, Hans Herfurth and Jyoti Mazumder
Additive manufacturing (AM) processes (also commonly referred to as 3D printing) allows the layer-by-layer build-up of parts rather than through molding or subtractive techniques such as machining. The idea that AM machines can print 3D objects much the same way that inkjet printer creates 2D images on paper is being described as “the next industrial revolution”1. Currently there are a number of AM processes that use a variety of materials (plastic, metal, ceramics) in different forms (powders, liquids, wire or sheets) with different heating sources.
Laser additive manufacturing (LAM) process uses a laser beam as the heat source and is primarily divided into two processes: Laser metal deposition (LMD) and selective laser melting (SLM). In the LMD process, a laser is used to melt metal powder fed through the nozzle which is then deposited in layers thereby building a dense 3D part free of binder. In the SLM process, a layer of powder is deposited on a build platform and then a fast steered laser beam melts the powder layer according to programmed 2D geometry. The melted powder particles fuse together in the correct shape and multiple thin powder layers are deposited by lowering the build platform to create complex 3D parts. Fraunhofer Institute for Laser Technology (ILT) Aachen, Germany has successfully developed and implemented a number of LMD and SLM applications and has been a driving force in transferring this innovative technology to a variety of industries such as aerospace and medical. Typically, both of these processes are performed using a single laser beam.
A novel approach is the multi-beam laser additive manufacturing (MB-LAM) process which deploys several low power laser beams simultaneously. The single beams either work in parallel to scale productivity without sacrificing precision, or in close proximity creating desired heat profiles. This new approach is scalable in productivity through multiplication. MB-LAM shares similarities with both LMD and SLM processes and operates in a process domain between LMD and SLM.
Similar to the LMD process, MB-LAM uses a powder nozzle and one laser beam remains stationary with respect to powder focus and like the SLM process, the position and motion of the second beam is controlled by fast beam steering.
Fraunhofer USA, Center for Laser Technology (CLT) first presented the new MB-LAM processing system with two laser beams at ICALEO 20132. This system is comprised of two high brightness diode laser modules, beam combining and beam steering optics, and a compact coaxial powder nozzle. The laser modules contain a pair of diode arrays and each produce a single laser beam. The output power of both modules can be individually controlled. One beam is stationary while the beam path of the second beam includes a mirror that can be tilted in 2 axes. The controlled actuation of the mirror enables changing the beam position relative to the stationary beam. Both beams are optically combined and focused by the same lens. A coaxial powder nozzle is used to feed powder material into the laser spot using an inert gas jet. The MB-LAM system is a complete integrated processing unit with a size of 125 mm x 175 mm x 425 mm and an approximate weight of 10 kg. Its compact size and light weight makes it very adaptable to robotic motion systems.
The MB-LAM beam steering system can operate in three different modes. To establish conditions for pre- or post-heating of the substrate or the deposited material, the movable beam can be focused in either a leading or trailing position relative to the stationary beam. To achieve the desired track width, the movable beam can be placed parallel to the stationary beam or both beams can be superimposed to increase intensity. Ultimately the ability of the system to achieve continuously movable 2D beam oscillation in the processing plane provides the highest flexibility in creating specific temperature profiles.
To test the beam steering control, laser re-melting tests were conducted using the MB-LAM system without depositing powder. The re-melted track from combined beam (stationary + movable beam) is uniform across the width and illustrates the homogenous heat distribution.
Next, the MB-LAM system was applied for laser additive manufacturing using Inconel 738 powder and optimized process parameters were determined (laser power – 265 W, feed rate – 0.25 m/min, powder flow rate – 3 g/min @ 40 psi). It was found that the sinusoidal beam oscillation normal to the feed direction with a maximum amplitude of 150 µm and a frequency of 100 Hz or higher resulted in most favorable clad profile. Moreover, the clad profile could be precisely controlled by varying the oscillation parameters. Subsequently, area deposition was conducted which showed good overlapping tracks without defects. Multi-layer deposition with 2 and 3 layers resulted in up to 1 mm thick layers and exhibit a defect-free high quality deposition.
In summary, a novel laser additive manufacturing process using multiple beams has been developed. A compact integrated MB-LAM system has been built and tested and has demonstrated excellent results for single and multi-layer deposition. Ongoing development work is focused on processing of crack sensitive nickel super alloys and the integration of real time process monitoring techniques into the processing head.
Rahul Patwa is a project engineer at the Fraunhofer USA, Center for Laser Technology, Hans Herfurth is the Director of Engineering at Visotek Inc, and Jyoti Mazumder is the Robert H. Lurie Professor of Mechanical Engineering at the University of Michigan..
References:
1“A third industrial revolution”, The Economist, Apr 21st 2012.
2R. Patwa, H. Herfurth, J. Mazumder, J. Chae, “Multi-Beam Laser Additive Manufacturing”, ICALEO 32nd, Miami, FL, Oct 6-9, 2013.