Microwave Plasma-Assisted Heat Treating and Joining at Atmospheric Pressure
Heat-treating processes and metal joining applications can significantly benefit from the introduction of microwave heating technology that uses plasma at atmospheric pressure. The new technology can be a very efficient, cost-effective heat-treating method compared with other microwave and furnace-based systems currently available. Microwave atmospheric plasma systems can reduce cycle times and eliminate the need for costly vacuum equipment required by other low-pressure microwave technologies.
Metals are generally good conductors of electricity, but react negatively when exposed to direct microwave energy, which often causes arcing that can damage heating equipment and the materials undergoing treatment. Other factors, aside from material composition, impacting heat-treating include the size, and mass of the object [Ref. 1,2]. These factors present significant challenges for microwave heating technologies. Key challenges included the development of a process able to harness microwave energy, without damaging magnetrons while evenly heating materials. Susceptor-based microwave heating tests produced mixed results, and also consumed considerable electricity. Microwave susceptors (e.g., ceramics) possess "inside-out" heating characteristics, which can lead to extremely high temperatures inside, and eventually produce a type of thermal runaway situation.
The overall consensus in the industry was that microwaves were not practical for direct heating of metals and other materials. However, the potential benefits of rapid heating rates, reduced processing times, lower energy usage and lower capital expenses for equipment make microwave technology compelling for an industry beset by high competitive cost and productivity pressures. The introduction of plasma at atmospheric pressure proved to be crucial as it alleviated concerns of arcing and uneven heat, while reducing process complexity.
The role of plasma
Plasma is a proven medium to successfully harness microwave energy to heat metals and other materials such as ceramics. It consists of charged and neutral particles created by the partial ionization of atoms or molecules within the gas. Plasma can be an effective electrical conductor and a strong absorber of microwaves, and has the ability to alter normal chemical reaction pathways, offering the potential to produce materials with properties and specific phases not easily attainable by other heating methods.
Plasma also can rapidly reach extremely high temperature levels. Sustained plasma flow with exact composition control can be used to effectively synthesize materials and serve as an excellent tool for processes such as brazing. Plasma provides the heat-treating industry with an alternative method to indirectly heat metals with microwaves. Because plasma contains free electrons and ions, the gas is able to couple with the electromagnetic field present in microwave radiation to provide rapid energy absorption. Allowing plasma to form only where it is needed can focus energy toward a specific area on an object.
The traditional method for using plasma for various applications requires the use of reduced pressures to make the plasma easier to excite and control. Using plasma at low pressure requires costly vacuum systems, and has a number of practical limitations.
In the case of microwave plasma, creating a low-pressure environment makes it easier to initiate and sustain the plasma with reasonable power input. It also introduces several constraints when viewed from an industrial applications perspective including:
- The need for expensive vacuum pumps and systems
- Processing volume limitations depending on vacuum chamber size and pump efficiency
- Complex equipment not suited for the demands of continuous processing
- Vacuum break periods adding to overall processing time per batch and increased processing costs
Because vacuum conditions are necessary to make low-pressure plasma heating successful, the process is impractical to use in industries requiring high throughput rates, such as the automotive, aerospace and textile industries. Low-pressure microwave plasma processing applications have generally been limited to select production such as manufacturing semiconductors, magnetic media and other low-volume, high-tech applications. A plasma treatment process at atmospheric pressure, such as Dana's AtmoPlas™, is more compatible with continuous high-volume processing and benefits the heat-treating industry as a whole.
Dana scientists developed a simple, proprietary method to initiate and sustain plasma at atmospheric pressure. The microwave sources usually operate at 2.45 GHz or 915 MHz, but other frequencies can be used for efficient heat treating and other applications, such as brazing. Depending upon the nature of the gas and plasma volume, the microwave power level can be as low as 0.5 kW. From 90 to 100% of the microwave energy may be coupled with the plasma to enable rapid heating of metal samples to extremely high temperatures, with no known upper temperature limits. This enables controllable atmospheric plasma processing of materials without the restrictions of existing low-pressure vacuum systems and microwave processing techniques. Dana recently was awarded a U.S. patent for plasma-assisted joining of metal components using its atmospheric pressure microwave plasma heating technology, and 22 patent applications for a variety AtmoPlas based equipment uses are currently pending in the U.S.
The AtmoPlas process can be used not only with metals, but also with advanced ceramics and hard material coatings. It has already been used to successfully braze joints in several automotive parts; for example, copper brazing of 1008 steel "banjo block" and hose fittings to tubes (parts used in brake lines) and brazing of aluminum 6061-T4 block to 3003 tube ("P-nut block" used in HVAC systems). Plasma is created in a suitable gas by a proprietary ignition process and is confined to the vicinity of the joint, thereby eliminating thermal stress in other areas. Test results and processing comparison costs indicate that the process can be effectively used for mass industrial production. The initial benefits are generated from the low thermal mass of the heat source, the ability to deliver heat precisely where it is needed and the high efficiency of microwave absorption in the plasma.
Atmospheric microwave brazing
Figure 1 shows a sketch of the lab equipment used for brazing of automotive parts using microwave plasma at atmospheric pressure. It consists of a computer-controlled 2.45-GHz magnetron that continuously generates variable power from 0.5 to 5 kW. The power can be controlled as a function of time or temperature to achieve a suitable time-temperature profile. Microwave power is fed through waveguide sections into a 32 ¥ 50 in. (813 ¥ 1,270 mm) aluminum applicator chamber via a circulator, a dual directional coupler and a three-stub tuner, which allows better transfer of microwave energy. These components help protect the magnetron from reflected microwave power and measure the forward and reflected power into/from the chamber. A deionized water-cooling system helps cool the magnetron, flow circulator and 32 ¥ 22 in. (813 ¥ 559 mm) working platform inside the chamber.
The braze joint and surrounding areas of the sample part are kept inside a plasma confining cavity made of a ceramic material rated at 3000°F (1650°C). Figure 2 shows a close-up view of the plasma cavity with a sample part inside. Quartz cavities also can be used with one or more layers of high-temperature ceramic insulation to prevent significant heat loss from the cavity. Stainless steel tubes are used to provide an inert gas to the ceramic cavity and to exhaust the fumes generated in the brazing process. The temperature of the part is measured using a remote sensing dual-color optical pyrometer with an accuracy rating of ±0.33% and a response time of 500 ms. The pyrometer can accurately sense the temperature of the target surface directly through the plasma.
A small amount of braze paste is applied to the joint between the 1008 low-carbon steel block and the tube. A copper braze ring is placed over the joint and the part is placed in the ceramic cavity so the steel block and a small length of the tube are inside the cavity (Fig. 2). The remaining length of the tube protrudes from a slot in the sidewall. The opening in the slot is then blocked with a small ceramic piece, and the pyrometer can be focused at the surface of the part and braze ring through a small hole in the front of the ceramic cavity. A top cover (also made of ceramic, but shown as clear in Fig. 2) is placed over the open cavity to create an enclosed volume that keeps the plasma confined. Air in this area is flushed out with an inert gas, such as argon, and using a proprietary process, the microwave power ignites the plasma almost instantaneously.
A critical observation in this process is the detection of the braze signature generated when the copper braze ring melts. Brazing of 1008 low-carbon steel parts with copper generates a distinctive spike in the plasma emission intensity at the time when the copper melts. The braze signature is a sharp, transient spike in the time-temperature profile as recorded in the optical pyrometer (Fig. 3). Presence of this signature confirms the successful melting of the copper braze ring. Melting of the braze material in microwave plasma can generate a distinctive braze signature that can be used for quality verification during the manufacturing process.
Results from tensile tests for the brazed joints using this process show that the connecting tube breaks at a load exceeding 2,600 lbf (11.5 kN), but the microwave brazed joints are still intact. No signs of corrosion at the brazed joints are detectable even after 1,224 hours of salt spray tests. Standard pressure burst tests and thermal cycling tests also exceed accepted criteria standards.
Dana's AtmoPlas technology efficiently harnesses microwave energy to join metal components. It can quickly generate plasma temperatures exceeding 2190°F (1200°C) within seconds, and has no known upper temperature limit. Heating rates over the part surface are also uniform as plasma evenly surrounds the entire part.
Microwave-absorbing plasma uses 90 to 100% of the energy in microwaves to rapidly and reliably join metal parts through brazing, welding, bonding and soldering. By effectively harnessing the microwave energy, the plasma significantly reduces the possibility of arcing while evenly distributing the heat, lowering the potential of damaging the unit's magnetrons. Tests show equivalent or better metallurgical properties than those components treated inside traditional furnaces.
The process is flexible and safe. Because the plasma (and not the microwaves directly) heats the metal, the heat flux around the part can be shaped to exactly match the part by adjusting the thickness of the plasma layer. This level of flexibility is not available in conventional furnaces. Strategically placed radiation monitors automatically shut down the system if leakage is detected above the specified safety limits set forth by FCC/OSHA. Safety interlocks are also present to prevent accidental starting of the system.
Other advantages of the AtmoPlas compared with traditional heat-treating furnaces and conventional low-pressure plasma microwave processing technologies include:
- Eliminating the need for expensive vacuum chamber and associated pumping systems and traditional costly heat-treating equipment
- No processing volume constraints related to the chamber size and pump efficiency
- Accommodating batch and continuous processing suitable for conveyer systems, reducing processing limitations resulting from restricted throughput
- Generating dramatically shorter cycle times
- Lowering operating and maintenance costs, with up to 30% lower capital investment costs than for traditional systems
- Providing significant energy savings
AtmoPlas technology is capable of a variety of applications that not only include heat related processes like brazing, sintering, and tempering, but also plasma-chemistry assisted processes such as carburizing, coating, and nitriding. In addition, future applications for atmospheric microwave plasma technology may include using microwave energy to improve engine combustion, treat vehicle-exhaust gases, generate hydrogen and create carbon nanostructures for use in the electronics and medical fields. IH