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The ignition phenomena that can occur during the reaction-bonding of aluminum oxide is investigated. It is experimentally characterized through measurements of the combustion temperature and the velocity of the ignition front. One- and two-dimensional, transient, simultaneous material and energy balances are used to model the phenomena in a cylindrical geometry. The models are used to predict the aluminum concentration, oxygen concentration, and temperature distributions as a function of time during ignition. Thermal explosion analysis is utilized to predict the conditions under which samples will ignite, and to calculate furnace temperature programs that will avoid ignition. The aluminum concentration and temperature distributions are used to estimate the elastic stresses that are developed during ignition. Feedback control is applied to the reaction-bonded aluminum oxide system with great success. The heating rate is adjusted automatically based on the reaction rate, measured through differential thermogravimetry. Adjusting the furnace temperature in this way is more reliable and allows the sample to react in a slow and controlled manner, avoiding ignition and cracking altogether. Criteria based on heat transfer and oxygen diffusion considerations are developed to determine the reaction rate set-point, and simplify the process of reaction-bonding considerably.
An experimental study was undertaken to investigate the ignition phenomena of 6061 aluminum alloy as a function of oxygen pressure. Cylindrical aluminum alloy specimens were ignited in a pure oxygen environment by a focused cw CO2 laser beam. To study the effect of oxygen pressure on the surface temperature at ignition of 6061 aluminum alloy, the experiments were conducted at oxygen pressures ranging from 0.084 to 2.413 MPa. The temperature history of the entire upper surface of the specimen and of a 0.5 mm diameter spot located initially at the center of the specimen top surface was recorded by using a commercial two-color ratio pyrometer and a fast-response, narrow-band, two-color pyrometer. Mass, brightness, and interior temperatures, for certain experiments were also recorded throughout the experiment. The results show that the surface temperatures at ignition of the alloy obtained from the temperature curves are below the melting temperature of the aluminum oxide and are slightly dependent on oxygen pressure. The data indicate that the ignition mechanism is complex and probably composed of several phenomena acting both separately and in conjunction with each other.
Transient material and energy balances have been utilized to model the reaction-bonded aluminum oxide (RBAO) and alumina-aluminide alloys (3A) processes. The model for the RBAO process considers the diffusion of a gas-phase reactant into a porous solid followed by a solid-gas reaction, while the 3A model considers a solid-solid reaction taking place within a porous solid. The modeling work on the RBAO process reveals that the process may proceed via an ignition/extinguishment phenomenon with thermal runaway. It is believed that this type of behavior can lead to stress development, and subsequent sample cracking. Thus, the model is used to determine conditions under which RBAO bodies may be fired in a controlled manner (i.e., avoiding the runaway reaction). A complimentary experimental study, utilizing simultaneous thermogravimetry (TG) and differential thermal analysis (DTA), in-situ temperature measurements, and analysis of samples fired in a box furnace, verifies the predicted reaction behavior and shows that by controlling the reaction, high Al content powders can be used to produce crack-free RBAO samples. The modeling work on the 3A process demonstrates the effects of various processing parameters on the general reaction behavior. After considering the general behavior, the model is used to predict the reaction behavior of the $TiO\sb2/Al$ system. A reaction sequence for the $TiO\sb2/Al$ system (based on XRD data) is proposed and used to model the system. The effects of the heating rate, the convective heat transfer coefficient, and sample size are investigated.
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