in Vol. 20 - March Issue - Year 2019
Residual Stress and 3D Manufacturing Process Development
When a new design is proposed to replace an existing design but uses the same materials and similar processes, the first few of the new parts may receive a closer inspection than the last few of the old parts.
When a new design is proposed for a new application, and the part will be made by a process that has never been used to make similar parts before, from a material that has not been extensively tested and proven in that form … well, that proposal gets lots of attention!
Inspection techniques are designed to detect ‘defects’ that can weaken the part. Ultrasonic inspections detect internal discontinuities. Fluorescent Penetrant and Eddy Current inspections detect cracks and porosity close to the surface. Visual examination can reveal discontinuities at the surface due to local overheating, compositional anomalies, and areas of abnormal grain size. Inspection methods come with allowable limits, and limits of detection. Does this type of material need a new or improved inspection technique?
Quality Assurance measures may require adherence to a ‘frozen’ process to ensure that today’s product is the same as yesterday’s, and last year’s, and the initial batches which demonstrated that the design and product could survive in the service environment. In short, confidence in a product or a process is built on experience. What information will be needed to generate that confidence in this new generation of parts?
3-D printing has recently emerged as a powerful technique in mainstream manufacturing. Initially, industrial 3-D printing was used as a tool for production of prototype parts, usually in polymeric materials. The transition from a CAD file to a computer model consisting of slices is very direct, and making the part does not require the purchase of solid raw material or part-specific tooling, shortening lead time and reducing cost,
3-D printing has grown into a family of related techniques collectively known as Additive Manufacturing. The advantages of a rapid transition from design to part, and eliminating lead time for raw material are attractive in manufacturing, as well as in development. In addition, features impossible to produce by traditional methods can be included in designs. Most significantly, Additive Manufacturing methods can combine the individual pieces of a sub-assembly into a single piece. There are powerful motives to adopt Additive Manufacturing as a production process.
Three of the 7 categories of Additive Manufacturing processes defined by ‘ASTM group F42 –Additive Manufacturing’ are suitable for printing metals. They fall into two groups, processes that produce preforms, and those that produce solid parts.
Metallic preforms are fragile, and require significant processing to become fully dense. Details vary from alloy to alloy, but would typically include sintering. The preform is heated to a temperature (below the melting point) at which powder particles within the preform start to coalesce. The heating cycle and the atmosphere are controlled, to promote the removal of binding agents and to avoid oxidation.
Sintered parts have microstructures and crystal structures more like the raw powders with voids at the prior particle triple points, while parts whose surfaces have been melted during deposition will have recognisable grains and microstructures, more like those found in conventional welds. Beyond sintering, further processing may include Hot Isostatic Pressing (HIP), in which individual particles deform, and internal cracks and voids are closed. In both groups, additional processing (for example, heat treatment) will normally be required to optimize material properties.
The three categories also include two types of process those that utilize a powder bed in which the part is grown, and those that deposit material directly onto a surface to build the part.
Powder bed techniques use a processing chamber with a floor that can be lowered in increments corresponding to the slices in the computer model, usually 25 to 250 microns, (0.001 to 0.010 inches). The base plate (on which the part will be built) is initially raised to be flush with the surrounding horizontal surface. An adjacent chamber, with a floor that can be raised, contains the powder that will be used in the process. Initially, the surface of the powder in this reservoir is flat and flush with the surrounding surface. When other preparations have been made (some of the processes and media used with powder beds require specific atmospheres), the floor of the processing chamber is lowered by the thickness of the first layer and the floor of the reservoir is raised. The exposed powder is pushed, by a roller or a blade, to fill the processing chamber flush with the surface. With the powder in place, the printing process for that layer is performed. The cycle is repeated for successive layers until the part is complete. Unprocessed powder supports the part during its production.
This process uses a powder bed to produce a pre-form that will require additional processing to become fully dense (if that is the intended use). Each layer of the part is defined by bonding the powder together and to the previous layer using an adhesive, delivered by a device similar to an ink-jet printer head. The process can be carried out in air, at or near room temperature, and is suitable for ceramic, glass and polymeric powders as well as metal. The process is quick; it can accommodate layers significantly thicker than other methods.
In a related process, powders pre-coated with a binder sensitive to UV light can be used in a powder-bed process. Areas illuminated and cured by projected UV images of the sections or by a scanned UV beam create a preform to be processed as conventional powder metallurgy parts.
Powder Bed Fusion and Powder Bed Sintering:
These two techniques differ in the amount of heat applied to the successive powder layers. The powder is heated using the energy of a high-powered laser, an electron beam or a plasma torch. The heat source is traversed over the area to be fused or sintered in each slice. This procedure is repeated until all the intended layers have been processed.
Each heat source imposes its own restrictions on the atmosphere in the chamber.
Lasers are unaffected by atmospheres, but the hot metal being processed will be affected by oxidizing atmospheres, and by moisture in the atmosphere. Having an inert atmosphere in the chamber will also help with heat transfer immediately after processing.
Electron beams require a high vacuum, which protects the hot metal and components of the electron source.
Plasma torches are designed to operate in air. They generate extremely high temperatures by ionizing a gas stream. Two gas supplies are needed. The first participates in the plasma, the other shields the hot metal from oxidation. The plasma gas is typically Argon, with additions depending on the material being processed. The most common shielding gas is also Argon, at a higher flow rate than the plasma gas.
Directed Energy Deposition:
This family of processes is characterised by simultaneous delivery of energy and the material to be deposited at the substrate. At least two of the processes predate common use of the term ‘3-D printing’, although they clearly belong to the Additive Manufacturing family.
Directed Energy Deposition processes are used in the repair of Nickel and Cobalt alloy components of gas turbine engines. Damaged airfoil surfaces are rebuilt with alloy powder blown onto the surface and melted in place by laser beams, electron beams or plasma torches.
Electron Beam Free Form Fabrication (EBF3) was initially developed with space exploration in mind. An electron beam is used to create a molten pool on the surface of an Aluminium build plate, and a wire of similar composition is fed into the pool as the plate is moved, building a 3-dimensional shape. The vacuum environment required for this process is readily available in space, and it is obviously easier to direct a wire into the melt pool than a powder in the absence of gravity. The patent on this process was abandoned in 2009. Interest in the technology, for terrestrial use, has recently increased with an updated process, Electron Beam Additive Manufacturing.
Residual Stress Measurement and Process Evaluation:
Problematic residual stresses are often associated with processes which generate severe thermal gradients. The most prevalent of these are welding, quenching, and abusive grinding.
Residual stresses appear to be important in the context of Additive Manufacturing in a number of areas:
In Powder Bed Fusion processes it is not clear whether the powder surrounding the growing part acts as a heat sink or as a blanket. Thermal strains due to non-uniform temperatures in the previously processed material could cause parts to distort during manufacturing. When returned to a uniform temperature, the shape may change again, and the part would contain a residual stress pattern. In powder bed processes, distortion may not be seen until the part is removed from the bed. Parts with complex geometry may be more affected than simple symmetrical shapes. Processes with high instantaneous heat input (higher melting point materials, larger areas illuminated by lasers and electron beams and thicker layers to melt) may be more affected. Conversely, processes with lower instantaneous heat input may be less affected.
Powder Bed Sintered parts will be affected by shrinkage during the process, as the density increases. Rather than attempting to complete the sintering process progressively in the powder bed, components may be partially sintered, and the process completed in a conventional facility.
Preforms produced by Binder Jetting and similar processes also need to be sintered in a conventional facility.
Local conditions similar to both welding and quenching are present in Additive Manufacturing processes involving melting of both the newly deposited layer and the recently solidified surface beneath it. The thermal gradients beneath the molten layer will be steep during processing, suggesting that thermal strains will also be large at that point in the process, leading to local residual stresses parallel to the surface. However, the thermal process is cyclic, and the subsequent passes may provide enough heat to relieve those stresses. This is an issue to be explored experimentally, by profiling the residual stresses through the last few layers to be deposited. The X-ray diffraction technique offers enough depth resolution to reveal a periodic pattern coinciding with the slice thickness.
While there is concern about the presence of residual stresses in parts made by some Additive Manufacturing processes, the as-manufactured parts will need to be heat-treated to develop their mechanical properties, and such heat treatments will effectively relieve any significant residual stresses. Also, it may be the case that some additive processes such as the EBF3 process can apply rather thick overlapping weld beads to an Aluminium base plate. Such a process is likely to leave the weld beads with very coarse grains. Residual stress measurements by X-ray diffraction requires a fine-grained microstructure. If the microstructures of the modern derivative of that process and the Powder Bed Fusion process discussed earlier are similarly coarse, then characterizing the residual stress may even be difficult or impossible to characterize.