Additive metal manufacturing is extremely promising in terms of the production of components whose machining has so far been associated with high costs or was not possible at all with conventional manufacturing processes. In order to be able to produce components made of metal materials economically, industry and science are continuing to drive forward the development of metal additive processes. Innovative laser technology can help to improve the processes here. How can the enlargement of the laser field of view and optimum laser control support machine builders in achieving optimum additive manufacturing results? By William S. Land II, Business Development Manager at Aerotech, Inc.
In recent years, manufacturers of metal parts have purchased more and more additive manufacturing equipment to familiarize themselves with the technology and explore additive applications. Since many OEMs manufacture their own powder bed machines for DMLS (Direct Metal Laser Sintering), most designs follow similar regulations and nolens volens lead to similar products. Although there are minor design changes here and there, these are hardly worth mentioning, as they would really make the machines different.
In addition, some larger users have also designed and built their own OEM machines or have adapted the machines supplied to their specific process and production requirements on their own initiative. In order to be able to successfully produce high-quality parts for certain industries, comprehensive internal process knowledge must be available or acquired. Often the process must be adapted to the specific needs of the user. Large end users require versatile and flexible process control to successfully build and expand this internal manufacturing expertise. However, as the sintering process is extremely complex, these users look for the necessary tools and components to meet their specific requirements.
Technical challenges
The technical challenges are largely due to the fact that the powder bed processes are intended to produce parts that require little finishing, i.e. are of very high quality. Most of these requirements are interdependent, so compromises are often made. This ultimately leads to an additive machine with a mediocre resolution and too low throughput. Aerotech's technology and expertise helps users eliminate such compromises. They are given the ability to increase the laser scanner's field of view, limit variations in energy/power density, control laser pulses as a function of position, maximize yield, and eliminate thermal instability.
The tools help to accurately control some of the interdependent process parameters that are most important to the machine. By eliminating these factors, there is no longer any need to compromise on the machine to produce high-precision parts in a versatile way. If the critical process parameters can be controlled more precisely without affecting other performance ranges, this will have an impact on the quality of the additively manufactured parts. After all, in the increasingly tight market for additive manufacturing, only those who achieve the best quality will be able to compete.
In the meantime, a second generation of additive machines for the powder bed process is nearing completion. The changes made will help to realise the high-precision potential of this technology. Those who are at the forefront of this technology can gain enormous market shares.
Dependence on spot size and field of view
Depending on which F-Theta lens the machine builder chooses, the size of the field of view (the available machining space) and the laser spot diameter (tool diameter) is predefined. Field of view and spot size are interdependent: If you need a larger field of view for larger additive components, you will necessarily have to make do with a larger tool (spot size). However, it is then more difficult to produce finer parts with it. If really large parts are to be produced and the spot size is limited for process reasons, the only possibility is to use more than one scanner and try to overlap their fields of view. On the one hand, stitching several scanner fields is complicated and inevitably leads to quality losses when producing larger parts. However, this is often mistakenly seen as the only option.
Expand the field of vision
A so-called infinite field of view function (IFOV-Infinite Field of View) can be used to eliminate the mutual dependency between the size of the field of view and the spot size. Servo and scanner movements are seamlessly synchronized with one another in a controller environment. The machine designer can now select the lens exactly according to the desired point size in order to achieve both the tool diameter and the required energy density. By using the IFOV function, the production area can be variably expanded and the movement between positioning tables and scanners can be effortlessly coordinated. With the IFOV, the user simply programs the desired path of movement in 2D space, and the profile is automatically divided between the scanner and positioning tables. The scanners also eliminate the dynamic sequential errors caused by the positioning tables and generate scanner-based dynamic performance over the entire unlimited field of view. The user can simulate a simple two-axis assembly and, thanks to the scanner, achieve an incredibly fast and precise movement profile over the entire construction volume. In the future, IFOV technology could be used in the aerospace and automotive industries in particular, since processing large parts with the additive machines available on the market today is still difficult there.
Sintering variability
Another challenge is the complexity of the sintering process - its control directly influences the quality of the manufactured part, both geometrically and metal-morphologically. The use of time-triggered laser pulses leads to variable energy and power input into the powder surface when the speed of the laser spot changes. However, variable energy / power input is not desirable, unless there are explicit reasons for this. In order to minimize the variability during sintering, the motion programmer must specify a constant speed. In the case of highly dynamic movements, however, this can lead to tracking errors, negatively affecting cycle times and leading to more complex movement profiles in order to compensate for tracking errors.
This is why the machine builders try to control the sintering process in a closed control loop with various types of sensors. However, this requires laser control functions that close the control loop between sensor feedback and the sintering result. Laser control is usually the domain of laser providers, who in turn know little about positioning systems. In order for this technology to be advanced, however, the providers of positioning systems have to take a closer look at the motion-coordinated laser control functions.
Optimally controlled with PSO and analog set
A so-called Position Synchronized Output (PSO) allows the motion programmer to select the desired energy density on the workpiece in such a way that the laser pulses are understood as a position function. In other words, the laser is no longer triggered time-wise, but depending on the location. The positioning system can now slow down in sharp curves in order to maintain the dynamic accuracy without laser pulses in bundled form resulting in poorer sintering quality in these edge areas. The PSO enables the programming of a completely asynchronous, position-controlled pulse setting, which means for the user that he can determine positions in advance at which a laser pulse is emitted. This can be achieved by using a position array for laser pulses. The most important feature of the PSO, however, is that the control loop of the entire positioning system depends on the feedback and thus offers a real position-based vector laser control, even when combined movements are used, e.g. with the IFOV function.
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With the help of the "Analog Set" control function, the laser power can be regulated as a function of the speed. With the "Analog Set", an analog output voltage is output depending on the vector speed of the coordinated system movement. Similar to the PSO, the user can use the analog set to vary the average output power of a laser when the laser spot accelerates or decelerates. It can be used to control the laser power density over the entire trajectory and serves as an integrated tool for closed-loop laser sintering.
In order to be able to work economically, machine users try to make optimal use of the available processing area for each sintering process. This means that there are often several similar components next to each other. However, since the field of view of the F-Theta lens is limited, the laser spot is sometimes considerably distorted in the edge areas. This leads to a fluctuating energy density and inevitably to quality losses on the component from one section of the processing area to the other. Either you limit yourself to the available processing area with minimal distortion and reduce the throughput of the machine, or you try to use the full capacity at the risk of a lower quality yield.
Power correction mapping for consistent quality
Power correction mapping as an integrated controller function can help here. The user has the option of scaling the power of the laser via an analog output as a function of the position within the scanner's field of view. The effects of the laser spot distortion and the resulting changing power density caused by the F-theta lens are largely canceled out by changing the output power of the laser. The use of the power correction mapping leads to uniform energy densities that are applied to the powder, regardless of where you are in the processing area. The part yield increases and makes it possible to safely use the entire production area - the parts sintered in the middle of the area are of the same high quality as the parts at the edges of the production area.
Thermally stable galvo scanner against thermal instability
With additive sintering, the layer thickness is normally in the order of 20-100 µm. The construction of even medium-sized parts can therefore take a long time. As a result, each setup is a significant investment of time and resources. In addition, the powder bed is usually heated, which slowly heats the surrounding structure. It should not be forgotten that there is practically no thermally stable environment. Due to the long process time, thermal drift can be a problem in all components including the galvo scanner.
Therefore, a thermally stable scanner is required for the additive manufacturing of precise parts. Every deviation of the galvo scanner over the process time has a direct effect on the geometric accuracy of the manufactured part. An AGV with <10 µrad / ° C drift is currently the most thermally stable galvo scanner on the market. It is also available with water cooling to ensure high stability in temperature-inconsistent environments.
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