4.1 Casting and Powder Metallurgy 4.1.1 Casting H. D. Haferkamp, M. Niemeyer and J. Weber, Universität Hannover, Hannover, Germany 4.1.1.1 Introduction The casting process represents the shortest route from the basic material, the al- loyed melt, to the casting ready to be installed with optimized multiple functions. In contrast to this unique advantage exists the problem of the difficult control and diagnosis of the casting parameters which are responsible for the quality and the functionality of the casting. Only the melting parameters, chemical alloy composi- tion and pouring temperature which can be set by inoculant or alloy wires and the heating capacity of the furnace before the casting are exceptions. The other pa- rameters are subjected during the extremely short period of production in the casting process and solidification to dynamic and for the most part also reciprocal influences which are difficult to control. It is still said that for difficult and costly casting processes, eg, bell founding, you have to take off your hat before praying before the casting starts [1, 2]. With modern automated casting methods and the increasing use of computer- integrated manufacturing (CIM) systems in foundries, inaccuracy of the parame- ters must be avoided to guarantee a high quality of the casting products and to avoid a cost intensive interruption of production. The aims of perfect production and total quality management (TQM) require sensors which also control the mold filling and the solidification processes and thereby permit efficient process control and process control engineering [3, 4]. This demanding process control can only be realized with sensors which are ad- justed to the severe conditions in a foundry such as high temperatures, difficult accessibility of the measuring point and the chemically aggressive effect of the melts. Because of the operating conditions, the sensors for casting process con- trol, shown in Figure 4.1-1, can be divided into ‘sensors without melt contact’ and 143 4 Sensors for Process Monitoring Sensors in Manufacturing. Edited by H.K. Tönshoff, I. Inasaki Copyright © 2001 Wiley-VCH Verlag GmbH ISBNs: 3-527-29558-5 (Hardcover); 3-527-60002-7 (Electronic) 4 Sensors for Process Monitoring144 Fig. 4.1-1 Classification of sensors ‘sensors with melt contact’. Further subgroups are distinguished by the particular control task, the control of the alloy composition, the temperature, the dosage and current of the melt and solidification. This division deliberately does not distin- guish with regard to the separate casting processes as many of them do not allow a general summary without double naming of the sensors and also lack clarity. In this classification, the physical measuring principle will be a final character- istic. The control and regulation of these casting parameters determine the quality of the casting products and the productivity of the foundries. 4.1.1.2 Sensors with Melt Contact The functional groups of this type of sensor come directly into contact with the melt or the mold or are separated from the melt by protecting tubes. Normally the protecting tubes consist of thermodynamic permanent ceramics with high temperature stability as aluminium melts, for example, have a corrosive effect on the sensor material. Sensors with melt contact can be divided into types for the control of the chemical composition, types for the control of the temperature, and types for the control of the dosage or the level. 4.1.1.2.1 Sensors for Controlling Chemical Characteristics The gas content, the chemical composition, and the purity of the melt are of deci- sive significance for the quality of the component. The chemical composition of the melt determines, in addition to the solidification characteristics of the casting which are influenced by the grain refining agent above all through the element content of the alloy, the mechanical properties of the component. The solvent power of metal melts for gases decreases with decrease in temperatures. Because of this, evolution of gaseous hydrogen and oxygen which are absorbed from the atmosphere and dissolved in the metal melts takes place and pores are formed in the casting. To guarantee a perfect component, the gas content must be controlled frequently before and during serial casting [5–7]. Partial Pressure Measurement As hydrogen is the only gas which dissolves in aluminium melts, the hydrogen content can be simply controlled with the Chapel (continuous hydrogen analysis by pressure evaluation in liquids) and the Telegas or Alscan process. With the cha- pel process a porous graphite punch which is connected through a gas-tight cera- mic tube to a pressure gage will be immersed in the melt and evacuated for a short time. The graphite punch reacts like a bubble into which the hydrogen dif- fuses out of the melt until the pressure in the probe and the hydrogen partial pressure in the melt are the same. If the state of equilibrium is reached the hy- drogen content of the melt at a constant temperature can be calculated by using the Sievert laws [6–9]: log C H  0:5 log p H 2 À A=T  B 4:1-1 4.1 Casting and Powder Metallurgy 145 where C H = concentration of hydrogen dissolved in aluminium, p H 2 = partial pres- sure of segregated hydrogen, T=temperature, and A, B=Sievert constants, de- pending on the alloy composition. The chapel process is easy to handle, reliable, fast and has been proved espe- cially in Europe. Thermal Conductivity Measurement The Telegas or Alscan method has a ceramic probe below the melt level, out of which pure inert gas or nitrogen flows continuously into the melt and is then col- lected in a hood. While the blowholes are rising the dissolved hydrogen diffuses out of the melt until equilibrium of the gas circulation is reached. The hydrogen partial pressure is measured with a thermal conductivity-measuring cell [10–12]. The telegas or Al scan method is especially used in the USA. In contrast to the chapel process, the measurements must be carried out over a longer period, at least 15 min. Electromotive Force Measurement In the steel and copper industry, an electrochemical cell made of ceramic (Figure 4.1-2) is used for determining the oxygen content in the melt [13–16]. The gage heads contain a thermoelectric couple (see next section) and a voltaic cell which has a mixture of a metal and an oxide, eg, Cr/CrO, inside with a known oxygen partial pressure as a reference material. 4 Sensors for Process Monitoring146 Fig. 4.1-2 Electromotive force cell On immersing the gage head in the melt, an electromotive force between the reference material and the melt arises because of the oxygen ion conductivity of the partially stabilized ZrO 2 . The relationship obeys the Nernst law: E ÀRT=4F ln p O 2 =p H O 2 4:1-2 where E = energy, R=gas constant, T = temperature, F = Faraday constant, and p O 2 , p H O 2 = partial pressure of oxygen at the two electrodes. The potential difference as a measure to calculate the oxygen activity of the melt can be used here. The temperature of the cell is an important factor in the measurement. A voltaic cell can be used at higher temperatures for the measure- ment of the oxygen content of solid or liquid metals, slag, and mattes. With this sensor the hydrogen, magnesium, and sodium contents can be determined when aluminium is melted [17, 18]. Resistance Measurement The Liquid Metal Cleanliness Analyzer (LiMCA) is used to control the purity of the melt continuously. The measuring principle is mainly based on the registra- tion of very small resistance modifications in the microohm range in liquid alumi- nium or magnesium caused by non-metallic inclusions. The robust and safe LiM- CA sensor is used in light metal foundries and consists of a heat-resistant tube for sampling and two electrodes, one in a test-tube and the other in the surround- ing melt [5, 19–21]. 4.1.1.2.2 Sensors for Controlling Temperature The temperature of the melt and the mold is of decisive significance for the correct mold filling and the cycle time of the serial casting, which implies the productivity of the company. Temperature sensors with melt contact are based on the principle of conduction, in contrast to the temperature sensors without melt contact. These sen- sors are also separated by protecting tubes or layers of aggressive melts. There is a division between thermoelectric couples and resistance pyrometers. Thermoelectric Couple Measurement Thermoelectric couples (Figure 4.1-3) are based on the thermoelectric effect (See- beck effect). They consist of two wires of different metals with the ends soldered 4.1 Casting and Powder Metallurgy 147 Fig. 4.1-3 Structure of a thermoelectric couple or welded. A voltage arises when the two ends have different temperatures. This thermoelectric voltage depends on the metals used and on the temperature differ- ence between the junction point and the connecting point (summing point) of the measuring instrument. The measurement of the thermoelectric voltage is carried out using high-resistance voltage measuring instruments. If necessary, possible disturbing secondary thermal effects at supplying parts must be eliminated through calibration lines. The measuring range is between –200 and 25008C de- pending on the metals. The following metal pairs are used: platinum/platinum rhodium, nickel/chrome nickel, iron/constantan, and copper/constantan [9, 22, 23]. Thermoelectric couples can also be produced without a protecting tube in very small sizes with a minimum diameter up to 0.5 mm and a free choice of the length. These so-called sheath thermoelectric couples are the most commonly used temperature sensors in light metal foundries because of their flexibility and reasonable price. Resistance Pyrometer Measurement The resistance pyrometer is based on the principle of a change in the electrical re- sistance with variation in the temperature of a conductor or semiconductor. De- pending on the predominant electrical conducting mechanism, a difference is made between pyrometers with a positive (metals) and a negative (high-tempera- ture conductors, negative temperature coefficient resistors, thermistors) resistance- temperature characteristic curve. Resistance pyrometers require analog or digital electrical connections for measurement and for higher demands measuring bridges and compensators are used. Similar to the thermoelectric couple, the ad- vantages of these sensors are the reasonable price, the robustness, the flexibility, and the simple handling. 4.1.1.2.3 Sensors for Controlling the Dosage/Level A correct dosage is decisive for quasi-stationary thermal economy of the mold and therefore significant for the quality of the casting. By reducing the cycle material the economy of the foundry is favored [24]. Contact Electrode Measurement The easiest and most common way to control the dosage is realized with a con- tact electrode. When the melt touches the contact electrode a signal will be sent to the installation control which controls the dosage process [25, 26]. Inductive Sensing In light metal furnaces, inductive level sensors which are protected from the melt by suitable austenitic or ceramic protecting tubes are used to control the level con- tinuously. This principle is based on an induced voltage in a conducting loop in the sensor. This voltage causes an electric current which forms a magnetic field around the sensor. A signal is originated by the variation of the magnetic field by 4 Sensors for Process Monitoring148 the melt [27]. This type of level sensor is expensive, susceptible to wear and costly in maintenance. 4.1.1.3 Sensors without Melt Contact The physical measuring methods and the technical realization of this kind of sen- sors are relatively complicated and complex, although necessary in order to guar- antee continuous production and quality assurance. Since these sensors do not touch the melt, which is often chemically aggressive, and since they are not ex- posed to high thermal stresses, it is unlikely that they will fail. Sensors without melt contact can be divided into different types: sensors for controlling the cur- rent and solidification, for controlling the temperature, for controlling the dosage, pressure, level, and route. 4.1.1.3.1 Sensors for Controlling Current and Solidification Precise knowledge of the melt current, the solidification and the thermal econo- my of the mold is an important factor in the design of casting dies. With this knowledge it is possible to attain perfect heating and cooling circuits, cycle times, and temperature distribution for directional solidification. For the continuous cast- ing process the control of the position of the solidification contour is of great im- portance since the continuous cast velocity and the charging depend on this posi- tion. If the meniscus is not respected, liquid metal may flow over or run out [28– 30]. X-ray Imaging X-rays from a radioactive source, typically a rod-type emitter (eg, Co-60) in a lead protector, radiograph the mold. Since solid metals absorb X-rays better than melts owing to their higher density, the position of the solidification contour can be de- tected by a scintillation meter. The sprue, ie, the melt current during die casting, can be supervised and the position of the solidification contour can be directed in a continuous casting mold. Figure 4.1-4 shows a schematic diagram of this super- vising method for continuous casting [30–35]. The complex protection of the workplace against radioactive radiation reduces the number of applications of this supervising method. X-ray processes and com- puted tomography (CT) are additionally used for nondestructive component test- ing and for the quality testing of safety components. Defects in casting, eg, inclu- sions, sink-holes, pores, cracks, etc., can be detected [37, 38]. 4.1 Casting and Powder Metallurgy 149 Thermal Imaging Cameras for thermal imaging visualize infrared radiation, ie, thermal radiation from object surfaces. Since the atmosphere is not transparent to thermal radiation over the whole radiation spectrum, these cameras are divided into near-, medium- and far-infrared cameras according to the sensitivity of their sensors [29, 30]. The flow of metal melts is examined for model molds consisting of a solid mold with die sinking and even face and which is closed by a moveable, transparent mold half of solid foam (aerogel) (Figure 4.1-5). Owing to its transparency to visible light and thermal radiation in the near-infrared range, the flow and solidification of steel, lead, aluminium, and magnesium melts, etc., can be observed [29, 39]. Since the assembly is complex and the use of the aerogel slab is difficult, ther- mal imaging for the examination of the flow of melts is only used in research or for the design of molds. 4.1.1.3.2 Sensors for Controlling Temperature If the metallurgical melt flow is correct, up to 100% of rejects in die casting can occur owing to the wrong temperature of the mold. Non-contact temperature sen- sors permit a correct mold design and effective continuous control of the melt temperature at positions difficult to access or at temperatures that destroy contact sensors [40]. 4 Sensors for Process Monitoring150 Fig. 4.1-4 Principle of X-ray imaging 4.1 Casting and Powder Metallurgy 151 Fig. 4.1-5 Filling of a model mold Thermal Imaging For thermal imaging of the mold temperature, as shown in Figure 4.1-6, mainly far-infrared cameras are used due to the emission spectrum [29, 41]. With these examinations a relationship between the die casting temperature, the flow tem- perature of the cooling system, and the cast cavity could be found [40]. Further, thermal imaging is used for the verification of simulation results and mold de- signs [41, 42]. Another application of this type of camera is the supervision of the cast tem- perature for continuous casting. Additionally, conventional cameras are used for the observation of the billet surface, the billet orientation, etc. [43]. Pyrometry Pyrometry is based on the same physical rules of thermal radiation and thermal imaging. In contrast to thermal imaging cameras, pyrometers detect the tempera- ture only at intervals, but they are more economical, easier to use, and they have an excellent accuracy of up to ±1%. In general, foundries use total radiation py- rometers for low temperatures and ratio or two-color pyrometers for higher tem- peratures as emitted by iron and steel melts. Total radiation pyrometers can easy be tested as their signals are directly subject to the Stefan-Boltzmann law. For the two-color pyrometer two partial radiations in different wave ranges are considered. Ratio pyrometers measure the temperature of the object by the ratio of the radia- tion density of two different spectral regions. The advantage is that the transmis- sion distance does not influence the measuring results [31, 44, 45]. In the steel industry, pyrometers have been used since the 1950s for the super- vision of melt temperatures [31, 46]. Additionally, they are used for continuous casting for the control of the billet temperature, ie, for the control of the cooling system [47]. 4 Sensors for Process Monitoring152 Fig. 4.1-6 Thermogram of a model mold