How to improve metal additive manufacturing processes

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This article from Aerotech discusses the current state of metal additive manufacturing technology and presents ways to improve the process.

How to improve metal additive manufacturing processesMetal 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.

In addition, some large end-users are building their own or customising OEM machines to suit their specific process and production needs. To make high-quality parts successfully 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.

Overcoming mediocre resolution and restricted build volumes

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, maximise yield, and eliminate thermal instability.

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 enables users to create a better product for customers (OEM) or for the end-user. This will differentiate an OEM from the ever-denser field of competition.

A second generation of powder-bed additive machines is right around the corner. The changes made will help realise 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.

Spot size versus field of view

The choice of F-Theta lens 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 very large, flawless parts; however, it is erroneously thought to be the only option.

The alternative option: 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 synchronising servo and scanner motion under one controller environment. A machine designer can now choose a lens 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 co-ordinate 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 divided 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 enables 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.

How to overcome 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 minimise variability in sintering, the motion programmer is constrained to command constant velocity from the 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.

Watch this short video to see how Aerotech eliminates sticking errors.

Position Synchronized Output

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.

Power = fn(Velocity) can be achieved using Aerotech's 'Analog Set' control feature. This gives the user the ability to scale an analogue output voltage as a function of the vector velocity of co-ordinated system motion. Similar to PSO, Analog Set enables 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.

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 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, and thereby reduce the efficiency of the machine by reducing its capacity, or you try to utilise its full capacity with the added risk of poor yield.

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 analogue 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.

Thermal instability

Layer thickness is typically in the order of 20-100um, 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.

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 <10urad/degC drift. It is also available with water cooling to ensure stability while in variable environments.

Technological evolution

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 AGV galvo scanner and Automation 3200 (A3200) controller offer the best available combination of both open data architecture and dynamic ability to perform precision moves. The A3200 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.

Follow the link for more information about Aerotech products and systems for metal additive manufacturing systems.

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