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Precision Manufacturing, White Paper
White Paper

Metal Additive Manufacturing

William Land II
Business Development Manager

The State of the Industry

Metal additive manufacturing holds tremendous promise in how it will enable the production of components that were previously cost prohibitive or could not be produced with conventional manufacturing processes. As a result, industry and academia are making a concerted effort to further the development of metal additive processes. An increasing number of finished machines from OEM manufacturers are being purchased each year in order for end-users to become familiar with the technology and find valuable uses for it. While many OEMs are making their own powder-bed manufacturing machines, most of the designs follow very similar prescriptions and result in similar products. There are minor design changes here and there, but little in terms of drastic differences that would help an OEM really differentiate their product from the pack.

Figure 1. Rocket engine prototype produced by Lawrence Livermore National Laboratories for the NX-01 Nanosat Launch vehicle. Image courtesy of Lawrence Livermore National Laboratories.

In addition, some large end-users are building their own or customizing OEM machines to suit their specific process and production needs. To successfully make high-quality parts for particular industries, in-depth internal knowledge of the process must be acquired and mastered, and often the process must be fine-tuned for a user’s specific needs. Large end-users are in need of more versatile and flexible process control in order to build the in-house manufacturing expertise required to be successful. The sintering process is very complex and end-users are looking for components that give them the tools to satisfy their own specific needs.

Figure 2. Freeform metal shape designs are made possible by modern DMLS powder-bed manufacturing machines.

Technical Challenges

Technical challenges abound in the pursuit of quality parts from powder-bed processes. Many of these challenges are interdependent and therefore compromises are frequently made that result in an additive machine with mediocre resolution and less than desirable build volume. Aerotech’s technology and expertise eliminate these compromises, giving OEM and end-user machine builders the ability to increase field of view, limit variations in energy/power density, control laser pulses as a function of position, maximize yield, and eliminate thermal instability.

Figure 3. Femur implants made via powder-bed additive manufacturing allow for better biological ingress, denser bone growth, and an overall better outcome for the patient.

The tools exist to alleviate some of the interdependencies of the process parameters that matter most in your machine. By eliminating these, the compromises made that affect the machine’s ability to make high-precision parts in a versatile fashion can also be eliminated. Being able to more finely control the critical process parameters without compromising other performance areas will allow you to create a better product for your customers (OEM) or for yourself (End-User). This will differentiate you from the ever denser field of competition.

Figure 4. Process parameter control is key to producing high-quality, precise parts in metal additive manufacturing. Institutions like Lawrence Livermore National Laboratories are identifying the relationships that connect process to part output, but your motion system requires the controller tools to bridge the connection. Image courtesy of Lawrence Livermore National Laboratories.

A second generation of powder-bed additive machines is right around the corner. The changes made will help realize the high-precision potential of the technology. Being on the leading edge of this change can mean a large gain of market share; being on the lagging end means being left behind.

The Challenge: Spot Size Vs. Field of View

The choice of F-Theta lens made by a machine builder dictates the size of the field-of-view (the available build area) and the spot size (tool diameter). These two things counter each other – if you want a big field-of-view to make bigger parts you also get a bigger tool (spot size), which can make it harder to make fine features. If you want to make really big parts, but have a limit on spot size for process reasons, the only option is to have more than one scanner and try to align/stitch their fields of view. The stitching of multiple scanner fields of view has many of its own complications, and is an undesirable way to produce a very large, flawless part; however, it is erroneously thought to be the only option.

The Aerotech Solution: Infinite Field of View

Aerotech’s Infinite Field of View (IFOV) feature can be used to eliminate the interdependency between field size and spot size by seamlessly synchronizing servo and scanner motion under one controller environment. A machine designer can now choose the lens they need to achieve the desired spot size for both tool diameter and energy density reasons. The build area can be extended as large as desired by using the IFOV feature to effortlessly coordinate motion between positioning stages carrying the scanner and moving the scanner itself. With IFOV, the user simply programs the desired motion path in 2D space, and the profile is automatically divvied up between the scanner and servo stages. The scanners also feed off of any dynamic tracking errors made by the servos, producing scanner based dynamic performance over the entire Infinite Field. This all allows the user to act as though the system is a simple two axis assembly, yet get blazingly fast and accurate motion performance over the entire build volume thanks to the scanner. This is valuable in industries like aerospace and automotive where large part processing is difficult using today’s commercially available machines.

Figure 5. Demonstration of Aerotech’s Infinite Field of View software for scanner and servo motion combination.

The Challenge: Sintering Variability

The sintering process is complex and controlling it directly affects the quality of the part produced, both geometrically and metal morphologically. Relying on a time-based laser pulsing system leads to variable energy and power densities applied to the powder surface as the velocity of the laser spot changes. Variable energy/power is undesirable unless for explicit reasons. To minimize variability in sintering, the motion programmer is constrained to command constant velocity from their equipment. This can increase tracking errors in high dynamic moves, affect cycle times, and lead to more complex profiles as a means of compensation.

Many machine builders are working toward closed-loop control of the sintering process using various sensors. However, they need laser control features to act as the bridge between sensor feedback and sintering output. Laser control features are typically the province of laser vendors, which make them difficult to coordinate with the motion hardware. In order for this technology to move forward, motion suppliers need to take a more in-depth look at motion-coordinated laser control features.

The Aerotech Solution: PSO and Analog Set

Figure 6. Example of maintained pulse spacing while decelerating in a corner with Position Synchronized Output (PSO).

Aerotech’s Position Synchronized Output (PSO) gives the motion programmer the ability to select the desired energy density across their part and maintain that setting through commanding laser pulses as a function of position. Now the motion equipment can slow in sharp corners to maintain dynamic accuracy without the concern of pulses bunching up and giving poor sintering quality in those areas. PSO even allows for the programming of completely asynchronous position-driven pulse placement, meaning the user can preset positions they wish a laser pulse to fall. This can be achieved through the use of a position array for firing events. Most importantly, PSO functions off of the combined feedback of the entire motion system, producing true vector-position-based laser control even when combined motion is used such as with the IFOV feature.

Figure 7. Aerotech’s Analog Set function allows the user to automatically vary the average power output of the laser system as a function of the vector velocity of laser spot as it traverses the part, enabling the fine power and energy density control required to make intricate quality parts.

Power = fn(Velocity) can be achieved using Aerotech’s “Analog Set” control feature. This feature gives the user the ability to scale an analog output voltage as a function of the vector velocity of coordinated system motion. Similar to PSO, Analog Set allows the user to vary the average power output of a laser as the laser spot speeds up and slows down. This can be used to control power density to the powder over the path. It is also another versatile integrated laser control tool that can be used for closed-loop sintering control.

The Challenge: Efficiency and Yield

In order to be economically efficient, machine users try to fill the available build area as much as possible any time the machine is running. Often times this means making many of the same part side-by-side. However, because current machines rely on the field-of-view of the F-Theta lens to produce the build area, the laser spot gets significantly distorted in different parts of the build area. This causes variable energy density and inevitably variable quality parts from one section of the build area to another. Either you self-limit the available build area to mitigate this problem, reducing the efficiency of the machine by reducing its capacity, or you try to utilize its full capacity with the added risk of poor yield.

The Aerotech Solution: Power Correction Mapping

Figure 8. Aerotech’s Power Correction Mapping function implements a calibration file to scale laser power output across the galvo’s field of view to account for changes and distortion in the spot diameter. Like Analog Set, this helps stabilize energy density (fluence) at the part, improving yield and consistency through the build area.

Aerotech’s Power Correction Mapping feature is an integrated controller function that gives the user the ability to scale power output of the laser via an analog output as a function of position within the scanner’s field of view. The effects of spot size distortion by the F-Theta lens can be largely nullified by changing the power output of the laser to account for diameter changes in the laser spot. Using this power correction map will result in very even energy densities applied to the powder, regardless of where you are in the build area. Part yield will go up and allow the user to confidently fill the entire build area knowing parts sintered in the middle of the build area will come out the same as parts sintered on the edges of the build area.

The Challenge: Thermal Instability

Layer thickness is typically on the order of 20-100 µm, meaning builds of even medium-size parts can take a long time. As a result, every build represents a substantial investment in time as well as resources. Additionally, build platforms are normally heated to an elevated temperature which slowly heats the surrounding structure. Needless to say, it is not a thermally stable environment, and since builds can be very long, thermal drift can be an issue in all components including the galvo scanner.

The Aerotech Solution: AGV Thermally-Stable Galvo Scanner

Figure 9. The AGV-HP is designed to be extremely stable over long operation periods. The above plot shows consistent accuracy with minimal drift over an 80-hour period, even while the galvos are performing extremely vigorous full travel oscillations in between measurements. This galvo is made for high-precision processes in a 24/7 production environment.

A thermally stable scanner is required for more accurate parts. Any drift in the galvo scanner over the build time directly affects the geometrical accuracy of the part produced. Aerotech’s AGV is the most thermally stable galvo on the market with <10 µrad/°C drift. It also is available with water cooling to ensure stability while in variable environments.

The Challenge: Functionality in a Fast-Paced Environment

As the metal additive industry is still relatively young, process development and machine design R&D are continual. All other galvo systems offer little in the way of data collection abilities, real-time access to position feedback and controller triggers, and most operate in a black-box fashion. Typical galvo scanners use an antiquated and over-simplified motion controller and trajectory generator that hinder their performance in high dynamic, precision tasks.

The Aerotech Solution: AGV and Automation1

The Aerotech AGV galvo scanner and Automation1 Software-Based Machine Controller offer the best available combination of both open data architecture and dynamic ability to perform precision moves. Automation1 provides the ability to monitor and capture hundreds of different data items, including the scanner’s actual position in coordination with laser firing and other processes. Aerotech’s state-of-the-art controller and trajectory generator, in combination with our precision design expertise, make the AGV the most accurate and dynamically capable galvo on the market today.