What Research is Being Done to Make Additive Manufacturing an Industrial Standard?

Additive manufacturing is the process of building a part layer by layer from the ground up. The consumer market already has printers using this process to make plastic prototypes. A computer CAD model is sliced into horizontal layers stacked ontop of each other. Each of these layers is a 2-D cross section of the part being manufactured. The printer starts by extruding melted plastic in the shape of the first 2-D cross section. It then moves up to repeat the same process with the second 2-D cross section on top of the first. Since the printhead is extruding melted plastic, the second cross section bonds with the first. The process is repeated until all the layers of the CAD model are printed.

 Basics of Additive Manufacturing

Consumer 3D Printer in Action [15]

Additive manufacturing, unlike traditional subtractive manufacturing, can build internal structures. Subtractive manufacturing, as the name suggests, is the process of starting with a block of stock material and shaving away material until you end up with the final part. Because the manufacturing method starts with a solid block of material, the inside of the part is solid. This is a serious problem for parts where strength to weight is a critical design parameter. All that internal material adds a lot of weight for not much of a gain in strength. Honeycomb structures have incredible strength to weight properties. [1] By building from the ground up, these honeycomb structures can be built where there would be solid material in a subtractive process. Additive manufacturing can produce complex unique 3-D structures, however, the time it takes to produce and the consistency of a part both need to improve for the technology to be adopted into a widespread industrial manufacturing method. 

Internal Honeycomb Structures [14]

Additive Manufacture of Metal

Plastic is a widely applicable material. It’s cheap, easy to work with, and lightweight. However, the plastics used to 3-D print have very poor mechanical properties, when compared to metal. Individual layers of the plastic maintain the material properties of the original stock. However, the layers don’t bond as strongly with each other. Parallel tensile force isn’t much of an issue, as the tensile force moves through individual layers. However, the tensile strength is heavily compromised in the perpendicular direction as the tensile force is pulling the already poorly bonded layers apart. [2] This issue is compounded by the poor mechanical properties of plastic making these 3-D printed parts unsuitable for industrial applications.

Today it is possible to 3-D print with metal. Additively manufactured Titanium hip implants are already being tested in patients. [3] The technology used to 3-D print metals is called Laser Additive Manufacturing (LAM). There are currently two technologies used to 3-D print metal parts: Selective Laser Melting (SLM) and Laser Cladding Deposition (LCD). [4] Both these technologies use the same basic idea of building the part by layers, but they differ in how these layers are built. For LCD extra material is added, in the form of a wire or powder, as a laser melts the new material, bonding it with the old. SLM uses a powder bed in which a laser melts the first layer into the bed. The bed then drops with a new layer of powder on top, repeating the process until the last layer is printed. Finally the excess powder is blown away to reveal the final part. 

Unlike plastic prints, metal prints can maintain their material properties. This is important, because weaker metal means more metal. The entire goal of additive manufacturing is to produce parts that are lighter with the same strength. But if more material is needed to maintain that strength, the advantages are negated.

A360 is an aluminum alloy that is commonly used for die casting. Die casting is the process of forcing liquid metal into a negative mold of your part. The liquid metal is left to harden. After hardening, the cast is removed, revealing your part. This process creates strong parts quickly and cheaply. AlSi10Mg is an aluminum alloy that can be additively manufactured. A thin wall of this alloy, additively manufactured using the SLM process, has comparable material properties to the widely used A360 alloy. [5] This shows that SLM can produce parts with the same strength as die cast parts. 

Low carbon 1008 steel is commonly used in automotive parts and home appliances. It’s material properties are comparable to other low carbon steels, but has much better corrosion resistance. Welding electrode ER70S-6 is a steel alloy that can be additively manufactured using the LCD process. The chemical composition of both these alloys are very similar, with a few extra elements in the electrode to improve its welding properties. When additively manufactured, the ER70S-6 wire had a minimum strength comparable to 1008 low carbon steel, and in some areas greater strength. [6] Furthermore, the fatigue life of an LCD produced part is equivalent to that of the stock material. [7] This shows that the LCD process can produce parts that maintain the material properties of their stock. 

While these studies show that it is possible to 3-D print metals and maintain their material properties, this doesn’t mean it’s true across the board for all metals and alloys. Melting temperatures vary wildly across metals. Aluminum and Titanium, two of the most commonly used aerospace metals, have an over thousand degree centigrade difference in melting point. The rate of cooling, which is a function of temperature difference, is an important factor for the formation of the metal’s lattice structure. 

A consistent melt pool is the key to having a consistent cooling rate, and hence a consistent lattice structure. A melt pool is exactly what it sounds like. It’s the pool, created by the laser, of melted old and new material. A better understanding of this melt pool behavior, and the parameters that affect the melt pool, are key to additively manufactured parts quicker and more consistently.

Next Steps

The method of finite element analysis is being used to better understand the cooling behavior of the melt pool. This works by creating a computational mesh, mathematically just a matrix, that represents the melt pool. Traditionally the double ellipsoid volumetric heat model is used to analyze the pool. However, using the well-distributed volumetric heat source model proved to be a computationally more efficient and just as accurate model of a LCD melt pool. [8]

A similar method of finite element analysis was used to understand the SLM process. However, the model was of the entire print, rather than just the melt pool. It was concluded that the model accurately described the behavior of the melt pool and could be extended to describe the mechanical properties of the print. [9] These models are used to optimize the printing parameters. The balling phenomenon in the SLM process is highly undesirable, and was significantly minimized using finite element analysis to gain a better understanding of the process. [10]

What is Possible

Composite parts, like those made of carbon fiber and fiberglass, use different materials in different areas of the part. Material that has very good compressive properties is put in areas of the part that are under compression, like the horn used in a composite bow. The same is done with tension, like the sinew in a composite bow. This means less material can be used, making the part lighter. A metal part produced through traditional subtractive manufacturing is homogeneous throughout. This is because it is carved from a homogeneous material. This means you have to choose a material that can withstand any force that may exist in the part. However, with additive manufacturing, a fabricator could theoretically use different metals in different areas of the part. Progress has been made in bonding these different metals during the LAM process. [11] However, more research is needed to fully understand how these different metals bond during a print. [12]

With a greater understanding of the melt pool, LAM can be faster and more consistent. With greater speed and consistency LAM will be adopted as an industrial manufacturing method. Boeing has already released a design they created using a supercomputer for a 777 wing. The computer was able to design a new wing that was 5 percent lighter while maintaining the same strength. This reduction in weight could potentially save 200 metric tons of fuel a year. [13] The design can only be implemented with additive manufacturing. If additive manufacturing can effectively put different metals in different areas of the design, the computer now has more parameters it can work with to optimize the design to be even lighter. Additive manufacturing will allow for a revolution in mechanical design that dwarfs what was made possible with the advent of CNC milling. 

Supercomputer Designed Boeing 777 Wing [13]

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