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TDTng, the electrical power was turned on and the temperature control adjusted to a specific temperature. In this experiment, the temperature was 1,832F (1,000C), represent- ing cast iron/sand mold interfacial temperature acting on a core or mold wall (head pressure). Te computer and


Table 2. PUCB Specimens Before and After TDTng 0.9% PUCB disc


1.4% PUCB disc


Before TDTng


After TDTng


data acquisition system was switched on for controlling, monitor- ing and plotting graphs of temperature/time versus distortion (Fig. 2). Te temperature was controlled using a K-type thermocouple at the hot surface and a non-contact digital infrared temperature sensor monitored the temperature at the back of the test specimen. Te temperature monitoring system allowed a thermal gradient to be calculated across the thickness of the disc-shaped specimens. Te test specimen was inserted into


After TDTng (0.07 MPa air)


scanning sensor to measure the distortion longitu- dinally and radially. Te data acquisition system automatically logged and plotted the distortion/ temperature versus time curves. Te duration of the TDTng was 90 seconds, although this could be varied. During the test, the programmed force was set to represent the force of molten metal pressing against the mold/core wall. Te force loads the cir- cumference of the bottom of the specimen against the hot surface diameter. Any downward movement of the gimbal is recorded as expansion and upward movement is recorded as plastic deformation.


a ceramic tray placed into a pivoting holder (gimbal). Te test specimen was automatically raised until direct sym- metrical contact was made with the 0.8-in. (2-cm) diameter hot surface. Tis simultaneously engaged the actu- ator and a high speed laser micrometer


INSTRUMENTATION IN THE NEW TDT HILL


The next generation thermal distortion tester’s functional- ity is accomplished through the use of several instruments, controllers and mechanical devices: Load calculators—A disc shaped specimen receives a load about the circumference of one side as the other side is pressed onto a heated metal surface. Dividing the total load by the area of the heated surface approximates pressure. Vary- ing the load emulates metallostatic pressure while controlling the metal surface temperature. Loading mechanism—This allows for the approximation of metallostatic pressures during mold filling and solidifica- tion of a casting. For better control and quantification of the resulting distortion, a uniaxial pressure load is applied with a free floating linear bearing slide coupled with an electronic actuator. The slide ensures the specimen’s center axis comes into contact with the heated surface’s center axis. The speci- men is loaded into a ceramic tray, which locates the speci- men against two minds. A two axis gimbal prevents any short forces from acting on the specimen surface in contact with the heater. The gimbal used three separate rings, one ring fixed to the linear slide while the remaining two allowed to rotate on two axes oriented 90 degrees from each other and


34 | MODERN CASTING July 2013 90 degrees to the heated surface axis.


The axis for the gimbal system are centered at the face of the specimen, preventing scuffing that might occur when the specimen experiences uneven distortion. Heat source—To provide temperature that simulates molten metal, a direct current supply was wired in series with resistive heating elements, which pass through the heater mass/tip made out of a superalloy. Instrumentation—The data acquired during each test is radial and longitudinal deflection, temperature at the hot surface and backside of the specimen, and time. Longitudinal deflection is tracked using a real-time feedback loop within a commercial controller. The controller software uses the load as a reference and maintains the set value by changing the actuator’s position. To track radial movement, the TDTng uses a green light camera system, which uses a green light to create a shadow of the specimen to measure its diameter. The hot surface temperature is sensed by a K-type thermocouple. A non-contact infrared device measures the temperature of the backside of the speci- men. All the temperature and movement signals are fed back to a data acquisition system attached to a personal computer. Data is analyzed, stored and displayed for each test.


Prior to TDTng, each specimen


was weighed, blown with air pressure to remove loose sand, and weighed again. Te percent change in mass was recorded. Next, the specimens were visually examined for signs of ther- mally induced surface cracking, loss


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