ADDITIVE MANUFACTURING A METALLURGICAL PERSPECTIVE. Julius Bonini

Transcription

1 ADDITIVE MANUFACTURING A METALLURGICAL PERSPECTIVE Julius Bonini V2 Dec 2016

2 INTRODUCTION Additive manufacturing (AM), also known as additive layer manufacturing (ALM) or 3D printing (3DP), is a manufacturing method with significant potential but a number of issues. It has been earmarked as the next industrial revolution and has already found some specific industry applications. AM involves three dimensional structures being built up one layer at a time. Each layer is melted to fuse powder particles together before the next layer is applied. This is repeated to continuously build a three dimensional product. In this white paper, we will take a look at AM from the metallurgical perspective. As with any other manufacturing process, different materials provide different benefits for particular applications. We will discuss the advantages and disadvantages of AM at each of the three main manufacturing stages: pre-processing, processing and post-processing. We will also discuss some typical component applications that this new manufacturing process is being used for, and all the metallurgical issues involved. MATERIALS Some of the common metals we see printed are: - Titanium - Nickel based alloys - Stainless steels series (316) PH - Ferrous alloys (tool steels) - Cobalt/chromium alloys - Aluminium alloys - Precious metals METHODS There are a number of different AM methods currently in use, some of the most common methods include: - Electron Beam Melting (EBM) - Direct Metal Laser Sintering (DMLS) - Laser Engineered Net Shaping (LENS) - Direct welding - Inkjet printing EBM and DMLS are the most common types used in metal printing. THE PROCESS There are two main approaches to metal 3D printing. LENS utilizes a powder spray method, where powder is sprayed down on a bed or substrate and then selectively melted by a laser beam. Another layer of powder is then applied and melted on top of the preceding layer once it has solidified. This method has found some application in the aerospace and biomedical industries. The more common approach for commercial application is the powder bed method that is used in EBM and DMLS. With this method a bed is created by a powder delivery system which moves the metal powder to the working platform where the laser or electron beam builds the structure. To build each layer of the component, the bed is filled with powder which is then melted with the E beam or laser at specific locations corresponding to the part. The bed then drops by the thickness of one layer and the process is repeated until the part is complete. The deposition of uniform layers onto the powder bed is thus critical to this method in order to create high quality parts. Additive manufacturing allows the creation of complex and novel structures which would be more difficult, or even impossible to make, via any other manufacturing technique. These novel structures might include internal porous structures, channels and chambers that can only be formed by AM methods. These complex structures would initially be designed using computer aided design (CAD) software, such as AutoCAD, and then reproduced within the 3D printer. The layer by layer approach allows any component designed in CAD to be printed within the limits of the printer. 2

3 In addition to the capability of manufacturing more complex internal structures, AM allows porous surfaces to be tailored into components for various purposes. The technology has found a niche in medical implants, where unique porous structures on implant surfaces are produced to optimize osseointegration. THE THREE STAGES OF PROCESSING Additive manufacturing has three key stages of processing to consider: - Pre-processing Besides a basic problem of powder flow, it has been found that these as-manufactured powder batches contain substantial levels of powder particles which are actually shells with voids in the center, as shown in Figure 1. The presence of these large voids affects the density of the powder, which can in turn create differences in flow properties, microstructure, and ultimately component performance. This phenomenon is even more likely to occur with gas atomized powder. Recycled powders can also have significant differences in their porosity, microstructure and other defects which can be detrimental in AM processes, as also shown in Figure 1. - Processing - Post-processing Each of these stages has different issues that need to be taken into consideration. PRE-PROCESSING Powder conditioning and handling is key for AM. In the pre-processing stage it is important that the powder being selected is of a high quality to provide the flow required during the processing stage. Usually, this implies the use of powder particles with smooth, spherical surfaces. Often, powders have to be treated by blasting or other methods of conditioning to remove satellites attached to the powder particle s surfaces to slightly roughen these surfaces to prevent a static cling effect. The ultimate effect is an improved and consistent flow. However, the flow behaviour of the powder deteriorates as powder batches are recycled; a common practice in AM. Continued assessment of the effects of recycling is certainly needed. Some of the techniques used for producing metal powders are: - Gas atomization - Rotatory electrode processing - Plasma rotatory electrode processing - Plasma spheroidization - Hydridization/dehydridization Figure 1. Photomicrographs of cross-sections of both virgin and recycled Ti-6Al-4V powder showing extensive void content and inconsistent microstructure and surface condition of recycled powder. PROCESSING While the flow and fill of the powder is still the biggest issue at the processing stage, tight control is needed over the various processing parameters involved with the AM method, including but not limited to; - beam parameters: power, speed and dwell - the build design, i.e. component layout - the hatch design: patterns, width, depth, overlap 3

4 - cleanliness of environment, quality of vacuum or protective gas - and handling procedures Electron beam and laser melting approaches vary in power, intricacy, speed and cost. Selecting the best method for an application is important to obtain the best results in a cost effective and timely manner. Widmenstatten structure. Acicular alpha with prior beta grain boundary microstructure is shown on the right of Figure 4, and is the result of the slower cooling rates associated with heat treatment or Hot Isostatic Pressing (HIP) as part of the post-processing stage. This acicular alpha with prior beta grain boundary microstructure is considered more desirable for typical of Ti-6Al- 4V applications. Additive manufacturing is a melting process, much like welding or casting. Both electron beam and laser techniques tend to form columnar grains as each layer melts into the one below it. The direction of the columnar grain can identify the build direction. AM takes the material above the liquidus temperature, where the material is entirely in a liquid state, then the material is quickly cooled back to a solid. Welding has a slower cooling time, and casting is even slower, but both heat to the liquidus temperature only once. For Ti-6Al-4V, the liquidus temperature is approximately 1650 C, as shown in Figure 2. It is unknown how many times one part of an AM component may exceed the liquidus temperature during manufacture, but this could have an effect on the final structural properties of the product. Figure 3. Critical continuous cooling transformation diagram for welded or cast (green) and AM (red) Ti-6Al-4V. Figure 2. Critical phase diagram section for Ti- 6Al-4V, with the red line indicating approximately 4% vanadium. Figure 4. Photomicrographs of AM Ti-6Al-4V in the as-fabricated (left) and Hot Isostatic Pressed (right) conditions. AM Ti cools more rapidly during the processing phase compared to a typical casting or welding method, as shown in Figure 3. This fast cooling forms a martensitic microstructure in a metastable phase, which can be seen on the left in Figure 4, and is commonly known as a POST-PROCESSING As mentioned above, AM metal products generally require heat treatment following processing to stabilize microstructure, and close 4

5 internal porosity. AM products might also require surface finishing and cleaning. This is especially important for medical implant and device applications where components need to meet extremely high levels of cleanliness. Particles trapped inside an internal matrix of cavities and pores are a serious issue for AM medical devices and implants, as they are extremely difficult to eliminate and can migrate into the body in-vivo. ADVANTAGES AND DISADVANTAGES OF AM ADVANTAGES One of the major advantages of AM is that intricate, complex geometries which cannot be manufactured by traditional methods can be designed and built with AM technology. Complex internal and external structures can be produced in one step due to the layer by layer nature of the construction. While other manufacturing methods may require several steps to produce complex products, AM can significantly reduce manufacturing procedures and steps and thereby reduce overall fabrication costs. A popular example of this is the manufacture of fuel nozzles which can reduce the traditional manufacturing processes from many steps down to just one with 3D printing. AM also allows a product to be designed with a specific porous surface pattern to help it interact with other materials and its environment. As already mentioned, the porous surface structure on a medical implant can encourage bone ingrowth and integration. Due to the limitations of AM its early adoption has primarily been within market niches that take advantage of the following attributes of AM: - Rapid prototyping - Unique geometries - Custom manufacturing - Unique surface features - Reducing production operation steps - Cost-saving practices CHALLANGES On the other hand, AM, as a developing technology, suffers from many drawbacks. Materials made with this technology exhibit many types of defects: porosity, disbonds between layers, contamination and other flaws. The general perception is that AM products are of inferior quality to those made from other conventional processing methods. While some of the defects, such as voids and undesirable porosity, can be resolved by HIPing during postprocessing, the general quality of the material remains in question. Our experience is that this perception of inferior quality may be justified, and extensive AM component testing and evaluation is highly recommended. A further disadvantage for the AM industry as a whole is the pace at which this technology is evolving. The rapid expansion of AM technology is leading to the continued production of new machines, new methods and new techniques at all stages of processing. Standards and regulatory agencies are struggling to keep up with the pace of this technology development. ASTM, the FDA and ISO all have groups or committees that are trying to issue new standards and regulations for AM. But this process can take up to two years or more to complete, and by then the technology and methods will have progressed substantially resulting in the standards or regulations potentially being out of date when issued. Another growing challenge to the AM industry relates to how products need to be tested differently compared to traditionally manufactured materials. It is likely that a new paradigm needs to be adopted for the entities involved to be able to produce standards and regulations to match the unique aspects of AM technology. The difference between the validation of builds and the validation of the components manufactured from those builds needs to be understood. The use of witness coupons for test and evaluation may not be acceptable for critical design components. An additional disadvantage which the industry is still addressing for more widespread adoption of AM, is the associated high costs. Industry standard AM machines are expensive to purchase and the fast pace of development causes machines to become quickly outdated. The metal powders used in 3D printing are also expensive and there are the added costs of pre and post-processing to consider as well. 5

6 Additionally, there are health and safety considerations to address when working with fine metal powders that add another layer of cost. Costs for the expected extensive testing and validation that will be required to validate every build and component also need to be taken into consideration. METHOD COMPARISON AM processes create very different microstructures than conventional processing methods. EBM Ti-6Al-4V produces fine, columnar grains with some porosity, while cast Ti-6Al-4V has large generally equiaxed grains as shown in Figure 5. Compared to a cast material, EBM creates a somewhat finer, more oriented microstructure, which may provide better performance with respect to tensile properties in the build direction. In Figure 6, EBM Ti-6Al-4V is compared to wrought Ti-6Al-4V. The wrought Ti-6Al-4V material possesses the fine equiaxed alpha grain with grain boundary beta microstructure, which has been classically preferred and has been known to exhibit the best overall performance characteristics. By the very nature of the process being akin to welding and casting, AM technology will likely never be able to achieve this ideal, preferred microstructural condition. Figure 6. Photomicrographs showing microstructure of EBM Ti-6Al-4V (left) and wrought Ti-6Al-4V (right). OTHER MICROSTRUCTURAL ISSUES While the structured surface porosity that can be designed into AM products can be beneficial for osseointegration, the unwanted internal porosity can affect the ultimate performance of AM products. HIPing has historically been used in the casting and powder metallurgy industries to resolve and heal internal porosity. Figure 7 shows a 3D printed Ti-6Al-4V alloy product before and after HIPing. HIPing has noticeably healed the internal porosity and can certainly minimize the negative effects of these latent defects. HIPing is therefore highly recommended for any components being used in critical applications. Figure 5. Photomicrographs showing microstructure of EBM Ti-6Al-4V (left) and cast Ti- 6Al-4V (right). Figure 7. Photomicrographs showing internal porosity of an as-manufactured AM part (left) and reduction of porosity of a similar part in the HIPed condition (right). Unfused particles within the structure are another microstructural feature unique to additive manufacturing. These defects will ultimately act as an internal stress concentration, as shown in Figure 8, and are virtually undetectable by any standard non-destructive testing (NDT) method. These discontinuities will impact fatigue and fracture performance. The origins of this type of defect need to be better understood and steps taken to minimize or eliminate their presence in AM materials. 6

7 Additionally, incomplete fusion between the AM layers has also been found and can lead to premature fractures, as shown in Figure 9. Figure 8. Photomicrographs of incomplete fusion of powder particles under no load (left) and under tensile load (right). SURFACE ISSUES While HIPing is recommended as a postprocessing operation for AM products to help heal internal porosity and fuse loose particles, problems with the applications of this additional operation are known. If the HIPing environment is not tightly controlled, contamination can occur. Discoloration of AM acetabular cups caused by the HIPing process as shown in Figure 10, have been seen. This discoloration is typical of environmental contamination in titanium materials. The phenomenon is probably due to oxygen pickup on the surfaces of the parts during the HIP cycle and could potentially also form a deleterious alpha case. Keeping AM parts clean and free of surface contamination during post-processing treatments such as HIPing will be crucial to the production of high quality parts. Figure 10. Optical photograph of AM acetabular cups in the HIPed (left) and as-manufactured (right) conditions. Figure 9. Photomicrographs of premature fracture and linear defects caused by internal porosity on AM tensile test samples. Another microstructural feature known as striations has also been seen in EBM Ti-6Al-4V, partially seen in Figure 9. This unusual feature appears to be tied to a slight variation in alloy content and is usually seen transverse to the build direction. The cause of this alloy variation and the overall effects of these striations in AM Ti-6Al-4V are not fully understood, but they are clearly related to the layer by layer fabrication method. TESTING In general, all AM products will need to be tested extensively to gain regulatory and standard approval. Medical implants will also need to be tested for cleanliness and particle shedding issues mentioned previously. General mechanical tests such as tensile, compression, and hardness tests are typically performed. Depending on the application of the product, charpy impact and fatigue tests may also be recommended. Along with mechanical testing, it is highly recommended that the microstructures of products be extensively checked in various regions of the build. Proper sectioning in the appropriate orientations with respect to the build 7

8 direction and proper metallographic preparation can provide extensive insight into the quality of the build and the associated components. Chemical analysis on the powder used in the AM process and the resulting components is also useful to determine the potential processing effects. Accurate identification of chemical composition is important to identify any alloy variations or contamination pick-up as a result of processing on formed parts. Surface analysis and cleanliness evaluations are relevant to certain industries and high temperature or environmental testing is relevant in others. As built, in-situ and custom testing is also suggested to ensure that the build meets the design criteria. THE FUTURE OF AM Although there are many hurdles still to overcome, the future of AM looks bright. We are already seeing AM products being adopted in various industries for many different applications. In the future, AM could be commonly used to produce custom medical implants, offering greater comfort and function. Medical modelling is also becoming increasingly useful with advances in printing machines that can print multiple polymer materials to better mimic living tissue and bone. In the future, the use of presurgical models will save time during surgery thus minimizing the impact of sedation on patients. Printing machines will get both bigger and smaller, allowing a wider range of components to be printed. Smaller machines being developed will allow micro devices to be printed with increased precision, which could open up a range of possibilities including nanorobotics. While AM processing is likely to become faster, it will still likely require multi-day manufacturing cycles for the foreseeable future. Hopefully, as techniques become more refined we will see the minimization or even elimination of the defects that are currently involved with AM. Manufacturing costs are also likely to reduce in the future through process optimization and a general increase in competition in each area of processing. LUCIDEON AND AM At Lucideon we work with component owners, print shops, material suppliers and machine producers to optimize AM products and processes. With expertise in many different materials and across many different industries, we are able to assist with materials selection, component design and validation, process optimization, quality assurance and failure analysis. A key area of focus for us is metal powder additive manufacturing, where we aim to educate people on both the benefits and issues currently surrounding metallurgical AM. Through detailed analysis of issues, we solve problems and break down the barriers at any stage in development. Our knowledge and understanding goes beyond the measurable physical and mechanical properties; we identify the metallurgical issues involved and provide solutions. CONCLUSION Additive manufacturing is a rapidly advancing technology fraught with technological challenges as outlined herein. Every aspect of the technology is being developed for improved performances and reduced costs. Metallurgical issues continue to emerge at every stage of processing and development, such as design, prototyping, validation and production. Most of these issues can be assessed and addressed metallurgically. It is critical that the designers and component owners know and understand AM limitations and implications. The potential benefits are so abundant that the drive to improve additive manufacturing continues. 8

9 ABOUT LUCIDEON Lucideon is a leading international provider of materials development, testing and assurance. The company aims to improve the competitive advantage and profitability of its clients by providing them with the expertise, accurate results and objective, innovative thinking that they need to optimize their materials, products, processes, systems and businesses. Through its offices and laboratories in the UK, US and the Far East, Lucideon provides materials and assurance expertise to clients in a wide range of sectors, including healthcare, construction, ceramics and power generation. ABOUT THE AUTHOR JULIUS BONINI Julius is a licensed Professional Engineer with a specialty in metallurgical failure analysis. He joined Lucideon in early 2010 when Lucideon M+P (Lucideon s largest US laboratory) acquired his metallurgical consulting firm MPSI. He is responsible for On-Site Materials & Process Troubleshooting as well as supporting a diverse roster of clients. His expertise encompasses analysis of corrosion, wear, fatigue, heat treatment, castings, metal coating and surface treatment, powder metallurgy, welding, component design, and the analysis of contamination. He has worked with many materials, including steels (stainless, alloy, tool) and alloys (Aluminum, Copper, Titanium, Nickel), precious metals, soldering alloys, specialty electronic and magnet materials, polymers, ceramics and concrete materials. Julius began his career as a Metallurgical Engineer in the Materials and Process Department at McDonnell Douglas and has amassed over 30 years of industry experience. He has contributed papers for symposia and publications of MPIF, APMI, AIME, SAMPE, and others and taught a variety of Mathematics courses for Purdue University. He holds multiple patents. julius.bonini@lucideon.com Tel. (US) Tel. (UK)