Opportunities and Challenges in Additive Manufacturing to Redesign Metal Parts for Light-weighting and Performance Enhancement
Additive Manufacturing (AM) also known as 3D printing has been receiving a lot of attention lately from the industry, media, government agencies and other organizations. The market has been growing steadily (Ref. Wohlers Report 2017) for AM machine suppliers and producers of parts, patterns and tooling. The amount of functional parts that go into final products being produced by AM has more than doubled in the last 4-5 years. Users of AM already consider it, to be creating a competitive advantage. Majority of the AM activities are in automotive, aerospace, industrial and business machines, consumer goods and medical and dental sectors. Prototype and proof of concept creation are still the major applications. Production parts, education, marketing and demo samples, art and hobby come next. FDM, SLA, and SLS for plastics are the most widely used technologies. Plastics constitute more than 80 percent of 3D printed parts. However, AM for metals using powder bed fusion (PBF) technologies such as Direct Metal, Laser Sintering (DMLS) and Selective Laser Sintering (SLS) are the fastest growing segment.
Software, the major enabler for innovation in the AM machines
Metal AM based on PBF seems to have great potential to produce high value components for airplanes, automobiles, machine tools, and medical implants. Without the constraints of the conventional manufacturing processes such as closing and opening of dies and molds for stamping and casting, AM may provide unprecedented opportunities for innovation and consolidation of parts while reducing weight and improving product performance. Shown below (Courtesy APWorks) is the world’s first 3D printed prototype for Light Rider, an electric motorcycle capable of going up to 80 kilometers per hour with a total weight of just 35 kg. Topology optimization which starts with a package space and operational loads was used to design the 3D printed frame weighing just 6 kg. Another example of the design and manufacturing freedom offered by AM is shown (Courtesy Altair) for a robotic manipulator where the weight was reduced by 30 percent without compromising stiffness by applying topology optimization to the parts shown in yellow.
Complex shapes such as bionic structures, honeycombs, lattices and internal channels, difficult to manufacture with conventional processes are ideal candidates for AM. Bionic structures and lattices provide greater strength to weight ratio leading to light-weighting of structures while reducing costs associated with material consumption and build time. AM can also provide advantages for tooling used for conventional manufacturing processes. For example, conformal cooling channels can be 3D printed and inserted into metal molds for injection molding or die casting to significantly enhance production rate and part quality. AM has great potential to complement conventional manufacturing processes to create superior products. The wheel upright shown below (Courtesy Altair) needed a redesign to improve stiffness which necessitated a change from a milled aluminum billet to a combination of AM with investment casting. Topology optimization was used to generate the initial concept using a design space subject to load cases such as hard braking, cornering and driving over bumps. The initial concept was iteratively refined to create a pattern for investment casting and 3D printed with PMMA (polymethylmethacrylate) material in a Voxeljet printer. Casting simulation was used during the design phase to assess the part geometry for internal defects such as porosity and geometry modifications were made to eliminate defects. Casting simulation was also used later for the full mold filling and solidification simulation to create the most efficient manufacturing process. Although there was no significant reduction in weight, the optimized design is more than 3 times stiffer than the original part.
However, there are limitations with metal AM related to machine size, cost, print time, and usable materials. Process control, repeatability and part verification are difficult. Surface quality and dimensional tolerance are harder to achieve compared to machining. Distortion resulting from thermal stresses need to be minimized via proper orientation and geometry compensation. Support structures are often needed to dissipate heat and avoid war page and collapsing of the part on the build plate. It is time consuming to remove support structures followed by surface refinishing.
Traditionally design and manufacturing teams have very little interaction early in the product development phase. Although AM is subject to less limitations regarding geometric complexity, it is crucial to integrate manufacturability considerations with design very early. The underlying physics for 3D printing of metals is complex because of the coupled thermal and mechanical changes the material undergoes. Designing a part with tight tolerances would need allowances for distortions resulting from the thermal cycle and material phase change. Residual stresses from the rapid heating and cooling cycle may need to be relieved using additional annealing process. A physics based accurate process simulation software for AM would be invaluable to predict distortion and residual stresses to refine the design and process parameters.
The main priorities for using AM are accelerating product development and offering customized products (Ref. Sculpteo, The State of 3D Printing 2017). Contrary to conventional manufacturing, AM doesn’t require tooling such as dies and molds and eliminates the need for supply chain integration. Software is the major enabler for innovation with AM. A highly efficient and user-friendly design environment integrated with physics based analysis is essential where products that take full advantage of AM can be conceived or re-designed, assessed for structural performance and manufacturability, and optimized early in the product development cycle.