Manufacturers of automotive and aerospace parts, medical devices, robots and machine components depend on advances in metals processing to address a range of challenges, often simultaneously. This includes the need for lighter-weight and higher-strength components, performance at temperature extremes, rapid prototyping and manufacturing, superior surface quality and durability and, of course, controlling production costs.
Powder-Metal Sintering: Tightening Carbon Control, Extending Furnace Life
The powder-metal industry has examined many aspects of carbon control as a means to improve the output of functional, high-quality sintered parts. In the past, the industry has often dealt with carbon control issues on an ad hoc basis, by trial-and-error or practical estimations. However, the automotive industry demands strict quality control and repeatability of the powder-metallurgy (PM) sintering process, backed by Total Process Control and FMEA procedures. Therefore, inline closed-loop control is the future of PM so that parts quality can be documented and issues detected in real-time – not after finished parts are tested in the lab.
Carbon steel is generally considered the most important commercial steel alloy. Increasing the carbon content increases hardness and strength and improves hardenability. But carbon (C) also increases brittleness and reduces weldability because of its tendency to form martensite. This means carbon content can be both a blessing and a curse for commercial steel.
Linde developed a simple but highly accurate method of measuring the concentration of free oxygen (½ O2) in the furnace atmosphere that uses a zirconia oxygen probe. The probe is heated to the same temperature as the furnace and then calculates the reaction of CO=C+½O2. Since the restrictions to this method are small in terms of measuring sensitivity and reliability, Linde selected this atmosphere measurement system for further industrial application. What emerged was the SINTERFLEX® control system for sintering atmospheres (Fig. 7).
The sintering for the press-and-sinter process requires the lubricant to be removed before entering the high-heat zone, and the same principle applies for the MIM sintering process. The atmosphere-control system (ACS) starts by completely removing the binders in the preheat (debinding) zone of the furnace (red line in Fig. 8). A very decarburizing atmosphere is created to make sure the binder is removed at this zone, so no binders are present when the brown bodies enter the high-heat sintering zone.
The second process step using the SINTERFLEX system is to start to create a neutral atmosphere as with most industrial hardening furnaces regardless of the final part requirement. The neutral atmosphere protects the carbon remaining in the matrix after the debinding.
Parts then enter the cooling zone, where there is still a slight carbon-containing atmosphere. This provides final protection of the carbon from the surface. In this way, a high percentage of in-specification parts are expected to exit the furnace within the final carbon-content requirement and with the lowest possible variation in carbon concentration from batch to batch.
Figure 9 shows the rejection statistics before and after implementation of the ACS for carbon control in a MIM sintering furnace. After the change, the percentage of parts “in spec” almost doubled, and the number of unusable parts decreased 83%, resulting in significant operational cost savings.
Furnace Life Extension
Significant maintenance costs are not uncommon when operating sintering furnaces due to premature wear of conveyors and muffles. Beyond the cost of replacement parts, unscheduled downtime of furnaces means lost production and higher operating costs.
Premature wear of these parts is often the result of a form of high-temperature corrosion in furnace atmospheres whose constituents are carbon carried by hydrocarbons. (In addition, carbonaceous residues of the binder or lubricant that reach into the high-temperature range of the system can influence these mechanisms.)
The carbon in the atmosphere can carburize furnace components and cause high-temperature corrosion where the atmosphere temperature rises or cools down. Linde has applied SINTERFLEX technology to improve the life of furnace components and to optimize the furnace atmosphere and the sintering process. Industrial implementation has shown that sintering furnace belt and muffle service life could be extended by 50%. See a comparison of belt condition before and after control in Fig 10.
Additive Manufacturing: Oxygen Monitoring System Boosts Process Quality from Inside the 3D-Printer Chamber
3D printing, or additive manufacturing, has rapidly gained popularity because it provides compelling benefits including rapid prototyping, shorter lead times and reduced waste. Most notably, additive manufacturing can produce new designs and complex shapes that are not possible with conventional techniques such as casting and machining (Fig. 11).
With no need for tooling, parts can be manufactured faster and on demand, reducing the need for holding stock and helping to save expensive materials, like titanium, by using only the required amount of material. Additionally, greater design and production flexibility allows for the creation of highly articulated forms (e.g., dental devices and prostheses) that pave the way for mass customization.
As additive manufacturing has advanced, those involved in laser sintering and laser melting have primarily focused on efficiency improvements around the lasers rather than the environment of the 3D-printing chamber. However, atmospheric gas levels within the chamber have a critical effect on the process. For example, the presence of too much oxygen or humidity can negatively affect the quality and performance of the printed part.
Following rigorous testing and development, Linde engineers developed the ADDvance™ oxygen-monitoring system that can be a game-changer for quality control (Fig. 12). Used alongside 3D printers, the independent monitoring unit can analyze and more precisely control the level of oxygen and humidity inside the 3D-printer chamber. It detects O2 levels down to 10 parts per million (ppm) and then modifies the gas atmosphere by adjusting the level of argon or nitrogen. In addition to delivering a more controlled printer chamber environment, ADDvance does so without cross-sensitivity effects and ensures a constant level of oxygen during the printing process.
Finishing Treatments for Perfect Parts
In addition to the five core areas discussed, industrial gases also play a role in other processes and surface treatments after manufacturing.
For example, hot isostatic pressing (HIP) is an advanced material heat-treatment process that utilizes high temperature and pressure to eliminate internal porosity and voids within cast-metal materials and components. This helps ensure the integrity of manufactured parts by improving mechanical properties and fatigue performance for high-performance products. These include gas-turbine components, automotive engine parts, turbo charter wheels, aerospace structural parts, medical implants and prostheses. High-purity argon is typically used to provide the inert atmosphere necessary to prevent chemical reactions that might adversely affect the materials being treated.
Finally, once any component is produced, it must go through a final cleaning step. 3D-printed parts typically have some rough surfaces and flashing that require smoothing. Technology to efficiently solve these final quality issues is integral to the entire AM process. Linde developed an innovative, waterless and solvent-free cleaning technology specifically for industrial surface finishing. The CRYOCLEAN® Snow unit produces dry-ice particles on demand. Liquid CO2 is fed into a specially designed snow chamber, and solid dry ice particles are then sprayed onto the component using compressed air to effectively clean surfaces and smooth edges of a finished piece.
Industrial gases and related process-control technology play a critical role in forming, treating and finishing advanced metal parts. Responding to challenges is an ongoing process that can yield significant cost, quality and performance advantages. For advanced metal manufacturing, solutions can literally shape the future.
For more information: Contact Linde LLC, 200 Somerset Corporate Blvd., Suite 7000, Bridgewater, NJ 08807; tel: 800-755-9277; web: www.lindeus.com. Author Grzegorz Moroz is program manager, metals; Akin Malas is head of applications technology, metals; and Johannes Lodin is sr. expert combustion technology, metals.