Chapter 3. Literature Review. 3.1 Automation in Casting Industry in General

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1 Chapter 3 Literature Review 3.1 Automation in Casting Industry in General Detailed literature survey allied with the topic has been carried out and a brief representation is indicated herewith. This literature review was carried out with an aim to investigate automation in Casting Industry and related cost [4]. Explored areas where simple automation is and can be used using same principles as of currently used expensive automation systems [8]. Literature review showed that currently automation is employed in sand casting, pour systems and various areas of casting industry depending on types of basic principle of foundry system [10] i.e. sand castings, Die castings, Investment or Lost was casting process etc. from the literature review it is clear that only certain areas are focused with high expenditure for automation to sustain quality of castings. Current casting Industries are focusing more and more recently for automation systems to comprise 2 or more processes at a time e.g. in green sand, making molds with loose sand and compress/squeeze them on one line. Many foundry are now also looking at automatic fettling system to reduce physical labor involved. Die casting foundries are now using automatic manipulators for pouring at large scales for its eciency and lower cost. Unfortunately there is not muc literature available on lower thermal masses pouring using bottom pour system. Most of the dispensable mold casting processes are using mold making and systems by automation. Investment casting industries are looking at using robot systems for shell building. This is one of the areas that's it also very labor oriented and has lots of variability if manually operated. g. 3.1 shows an automatic sand mold making systems which is commonly adopted for a green sand process. Pouring process is also automated in sand casting areas where tilt pour and bottom pour are used using motorized crane system. g. 3.2 shows an an example of tilt pour using same practice as pouring weight for larger castings is more labor intensive and prone to variation in ow rate, tie cycle. Fig. 3.3 shows an example of automatic die closing and opening for every pour cycle in die casting process using universal robotic arm. 33

2 Figure 3.1: Automatic sand mold preparation system [70] Figure 3.2: Semi-automated pour system in green sand process [71] Figure 3.3: Robotic mold system in die cast process [72] 34

3 3.2 Investment Casting Process and Related Automation Investment casting process is used for making as cast precision castings with intricate designs and with excellent as cast nish. Literature review showed that in international market for investment castings, mainly aerospace, jewelry, defense customers were involved. Big companies are able to invest in shelling and pouring process mainly [5]. While doing literature about automation and optimization of process related to it, it was seen that semi capacity investment casting plants are still using most of the traditional process procedure to produce castings but are facing lots of problems related to defects. These defects were mainly related to variability coming from human factors involved. Figure 3.4: Kuka industrial robot[4] Figure 3.5: Example of automated pouring [13] 35

4 Even though there is some work done in areas only for investment castings process area [5] for shell improvement and pouring techniques like vacuum pour, gravity pour etc., most of the foundries are unable to adopt it easily due to costs related to it. g and g show general examples of automated system for shelling (Kuka Industrial Robot) and Automated pour in investment castings process respectively. 3.3 Pour System Automation/Bottom Pour for Pour for Low Carbon Alloy Steels As mentioned earlier, lots of methods using hi-tech technology like Robotic arms are used in bigger foundries for consistency in multilayered critical shell dips [4]. Semi-automated systems are being worked on at cheaper costs for medium capacity Investment casting Foundries. There are certain pour systems are developed like gravity pour for better yield and better quality [1]. Also some eorts are done to get rid of air trap issues by changing pour head using automation [16]. So far there is no reference found that any foundry is using bottom pour system in Investment casting foundry. Bottom pour is used in sand castings or shell castings using cast iron, Aluminum i.e. metals with long cooling curve. Investment castings usually use super alloys and high grade steel with higher cooling curves. Heat transfer plays important role in Investment casting process and is critical from the point of shell preheat temperature and pour temperature of metal [19]. Therefore some special insulation materials were used to reduce and optimize heat transfer rate for better quality castings. There is not much work done in bottom pour of small thermal masses (Usually <400 kg) of ally steel in Investment Industry yet. 3.4 Ceramic Coatings/Composites for Minimal Heat Transfer Since thermal heat transfer in casting industry plays an extremely important role for molten metal and surroundings, continuous eorts are made on this subject. Metal when taken out from melting furnace transferred in pouring ladle to pour in molds, the transfer time is very important. Also the metal temperature at furnace and when actually poured in molds should have a minimum dierence i.e. T. Now there are mainly 2 Medias where metal can lose its heat during this transfer 1. Atmosphere 2. Ladle inner surface area In pour study we are using SS 304 alloy which has less uidity at high temperature due to its composition and when transferred to ladle for poue to molds, a selection of composite 36

5 material for inner coating was very important to get minium heat transfer rate of molten SS 304 at about C. It starts solidifying at approximately C. Literature review was done extensively to select the most appropriate available composite mix for this work. A composite material is a material system consisting of a mixture of two (or) more micro constituents which are mutually insoluble diering in form and composition and forming distinct phases. Thus using it is possible to have such combination of properties like high strength and stiness, corrosion resistance, and ability to withstand extreme high temperature conditions. The important advantages of composites over the common bulk materials are as follows: 1. Higher specic strength. 2. Lower specic gravity. 3. Higher specic stiness. 4. Lower electrical conductivity. 5. Better corrosion and oxidation resistance. 6. Can be fabricated easily. 7. They are tough having good impact and thermal shock resistance. [43] Some types of ber reinforced composites are described below: 1. Glass Fiber : Reinforced polymer composites employ glass bers for improving the characteristics of especially polymeric matrices containing nylons, polyesters etc. These composites possess lower densities higher tensile strengths and impact resistance and resistance to corrosion and chemicals. parts, storage tanks transportation industries, plastic pipes etc., Applications: Automobile 2. Carbon Fiber : Reinforced Composites are employed in situations requiring excellent resistance to corrosion, lighter density retention of properties even at high temperatures. Applications: Structural components of aircraft, sport materials. 3. Alumina Based Composites : Reinforced metal composites have improved specic strength, stiness, abrasion resistance and dimensional stability. Applications (i) Matrix - Al2O3Aluminum alloy reinforced with Al O or carbon bre used in components of automobile engines (ii) Matrix - Ni and Co based alloy reinforcement with Al2O3(or) tungsten used in components of turbine engines. Refractory materials most commonly used for lining furnace walls are synthesized mainly from silica-alumina geo-materials. The control of the quality of refractory demands a wide knowledge in various scientic elds. In fact, the production chain has a number of steps (choice of raw material, mixture preparation, drying, pressing, ring etc.) requiring 37

6 Table 3.1: Thermal properties of Al2O3 Specic Heat Coecient of Thermal Expansion Thermal Conductivity 880 J/kg-K 8.1 x 10 6 / 0 C 18 1W/m-K Table 3.2: Thermal properties of SiO2 Specic Heat Coecient of Thermal Expansion Thermal Conductivity 1000 J/kg-K 5.6 x 10 7 / 0 C W/m-K perfect mastery and which the slightest error or neglect of any parameter results in failures in the nal refractory product quality. Elements Al2O3and SiO2 [42] seem to be very important for refractory lining of foundry ladles to minimize heat transfer between the ladle lining and molten metal. Till now, many research studies have been conducted, until now, on mullite (3Al2O3-2SiO2) as a mineral phase with many properties: low thermal expansion and conductivity, excellent creep resistance, high temperature strength, and good chemical stability [44]. These studies have focused on their synthesis methods, their phase equilibrium, their micro-structures and their thermo-mechanical properties [44, 45]. Tests have shown that a slight and regular variation of the thermal shrinkage/expansion between C and C, which helps to provide a good thermal shock resistance and low heat thermal heat conductivity with the composite material containing Al 2O3 and SiO2. Thermal properties of Al2O 3 and SiO2 are shown in tables 3.1 and 3.2 respectively. Based on this a right combination of Alumina silicate can be used for optimized heat transfer for ladle lining using alloys with high rate of cooling like SS Heat Transfer of Molten Metal and Preheated Shells In pour process of investment casting shell process, as mentioned in chapter 3, preheating of shells is very critical to reduce heat transfer. According to AFS (American Foundry men's Society), reliable and realistic thermal properties data for investment casting shell molds can increase the accuracy of solidication simulations and predictions of shrinkage. Investment casting shells exhibit several phase transformations during ring and pouring that can aect their transient thermal properties. These properties depend on time, temperature and process history. Mingzhi Xu, Simon Lekakh and Von Richards, Missouri Univ. of Science and Technology, Rolla, Mo., studied the thermal properties (thermal conductivity and specic heat capacity) of seven industrially produced ceramic molds. They used an inverse method, where pure nickel was poured into ceramic molds equipped with thermocouples. Simulation software then was used to simulate virtual cooling curves that resembled the 38

7 experimental curves by adjusting the temperature dependent thermal properties of the ceramic mold [51]. The thermal properties data obtained from this method were compared with measurement results from laser ash in the hope the dataset will serve to improve the accuracy of investment casting simulation. The paper, Thermal Property Database for Investment Casting Shells, provides their analysis of this study [50]. Now heat loss during the metal pour is a main concern in this process. This happens due to surrounding temperature, heat dissipation rate, movement of the mechanism etc. The overall Heat Transfer Coecient (HTC) between the ambient and shell mold, ha can be given as a function of the convection heat transfer coecient, hma, and shell emissivity [13] as h a = h ma + h r = h ma + σε(t 2 + T 2A )(T + T A ) where h r is an eective radiation HTC, T is the temperature of the surface [K], and T A is ambient temperature [K]. There are dierent correlations for the HTC depending on the characteristic length of the surface and surface orientation, i.e., vertical and horizontal (Most of correlations on natural convection are given in terms of the Nusselt number(n ul ), and Rayeigh number(r al ) N ul = h mal K...Eqn 1 R al = gβ(t s T )L 3 v α...eqn 2 where g is gravitational acceleration, β is the thermal expansion coecient, v α is the kinematic viscosity of the uid, L is the characteristic length scale and α is the thermal diusivity, T s is the surface temperature and T is the ambient temperature. For air at ambient temperature, the thermal conductivity k = 2.6 E 2 W/mK and gβ = 9.07E+7 [1/m 3 K]. The characteristic length scale, L for a surface can be computed as the ratio between the surface area (A) and its perimeter(p) i.e. L = A. The HTC is determined from the ρ Nusselt number as h ma = K L N ul...eqn 3 These equations are process based and are analyzed using simulations method only. So it is based on each and every foundry process for mal=king shells, environment and handling and pour mechanical systems. Above literature review is very useful for deciding preparation needed to achieve our objective for the problem denition of this work. Next chapter discusses plan of work to start the experimentation on practical grounds. v α 39