Aluminium casting

Designing for Eliminating Defects in Aluminium Casting A PROJECT REPORT Submitted by: ANKUR AGGARWAL (08604G) PURSHOTTAM LODHA (08620G) SATYAM PAL (08623G) Under the Guidance of  MR. AMIYA KUMAR SAHOO at

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July 21 to May,2012 Submitted in partial fulfillment of the Degree of Bachelor of Technology

Department of Mechanical Engineering JAYPEE UNIVERSITY OF ENGINEERING & TECHNOLOGY A.B. ROAD, RAGHOGARH, GUNA  – 473 226, M.P, INDIA

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Jaypee University of Engineering and Technology  A-B Road, Raghogarh, Guna - 473 226 (M.P.) Phone: +91-7544-267310-314, 267051 Fax: +91 -7544-267011 Website: www.juet.ac.in

CERTIFICATE

This is to certify that the work entitled Designing for Eliminating Defects in Aluminium Casting is a bonafide work done by Ankur Aggarwal (08604g ), Purshottam Lodha(08620g) and Satyam Pal(08623g) in partial fulfillment of B.Tech and has been carried out under my direct supervision and guidance. This report or a similar report on this topic has not been submitted for any other examination and does not form a part of any other course undergone by the candidate. “

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Signature of Supervisor: Supervisor:

__________________ __________________

Name of Supervisor:

Mr. Amiya Kumar Sahoo

Designation:

Lecturer, Dept. of Mechanical Engineering Engineering

Date:

__________________

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Acknowledgement We express our deepest acknowledgement to our guide Mr. Amiya Kumar Sahoo and we are indebted to him for his guidance and constant supervision as well as for providing necessary information regarding the the project & also for his support throughout throughout the project.His inspiration and perfective criticism is extremely helpful for this project. His support has been immeasurable. We acknowledge the help and superlative presence of the Head of Department of Mechanical Engineering , Dr. J.N. Dube for the opportunities he has given us not only during the project but throughout the course of B.Tech. We also duly acknowledge all the faculty members of our department for guiding us in the making of the project and for solving our problems whenever they surfaced and also for catalyzing our thinking when it seemed to stagnate.Our thanks and appreciations also go to our colleagues in developing the project and people who have willingly helped us out with their abilities. Ankur Aggarwal (08604g) Purshottam Lodha (08620g)

Satyam Pal(08623g) Date:

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Contents

Certificate Acknowledgement

CHAPTER-1 INTRODUCTION 1.1 Metal Casting 1.2 Physical and chemical Properties of Aluminium 1.3 Silica Sand 1.4 Pattern Design

CHAPTER -2 LITERATURE SURVEY CHAPTER -3 DEFECTS AND NON DESTRUCTIVE TESTING CHAPTER -4 PROBLEM FORMULATIONS CHAPTER -5 WORKDONE SO FAR 5.1 Designing of Pattern 5.2 Designing of Gating System

CHAPTER -5 CONTRIBUTIONS BIBLIOGRAPHY

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CHAPTER-1 INTRODUCTION 1.1 METAL CASTING Metal casting is one of the oldest materials shaping methods known. The metal casting industry plays a key role in all the major sectors of our economy. Casting means pouring molten metal into a mould with a cavity of the shape to be made, and allowing it to solidify. When solidified, the desired metal object is taken out from the mould either by breaking the mould or taking the mould apart. The solidified object is called the casting. By this process, intricate parts can be given strength and rigidity frequently not obtainable by any other manufacturing process. The mould, into which the metal is poured, is made of some heat resisting material. Sand is most often used as it resists the high temperature of the molten metal. Permanent moulds of metal can also be used to cast products.

Metal Casting History (India) 3000 BC Earliest castings include the 11 cm high bronze dancing girl found at Mohen-jo-daro. 2000 BC Iron pillars, arrows, hooks, nails, bowls and daggers or earlier have been found in Delhi, Roopar, Nashik and other places

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1.2 PHYSICAL AND CHEMICAL PROPERTIES OF ALUMINIUM Aluminum has the symbol Al. From the Latin word "alumen" meaning "alum". The ancient Greeks and Romans used alum in medicine as an astringent, and in dyeing processes. Atomic Number = 13, Atomic Mass = 26.98, 13 protons, 13 electrons, 14 neutrons. First isolated by Hans Christian Oersted of Denmark in1825. Most common use: 1. Cans and foils 2. Kitchen utensils 3. Residential house siding 4.Industrial applications where a strong, light, easily material is needed 5.Wire for electricity, although its electrical conductivity is only about 60% that of copper per Area of cross section, it is used in electrical transmission lines because of its lightness and price 6.Alloys are of vital importance in the construction of modern aircraft and rockets. Physical Properties:

Aluminum is a shiny, silvery white colored metal that is light in weight and strong. The density of aluminum is 2.7 g/mL, which means the metal will sink in water, but is still relatively light. Chemical Properties:

The surface of aluminum metal is covered with a thin layer of oxide that helps protect the metal from attack by air. So, normally, aluminum metal does not react with air. If the oxide layer is damaged, the aluminum metal is exposed to attack. Aluminum will burn in oxygen with a brilliant white flame to form the trioxide aluminum(III) oxide, Al 2O3. Aluminum metal is obtained from the pure hot molten Al 2O3 by an electrolytic method. The electrolysis of the hot oxide in a carbon lined steel cell acting as the cathode with carbon anodes is the most common method. Melting Point: 660.32 °C and Boiling Point: 2519 °C Young Modulus: 70 GPa (in alloys) Ductility & Malleability: High Hardness: 420 MPa Density: Low 2700 kg / cubic metre Electrical Resistivity: Low 2.65 x 10-8 Ohm metre Reflectivity: High 71% unpolished and when polished: 97%

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1.3 SILICA SAND

For many years there has been a requirement for high quality silica sand in the Foundry industry. Foundries use the clean, well graded sand to produce moulds for the production of metal castings which are the basis of engineering and manufacturing industries across the UK. High purity silica sand is washed and graded to remove any impurities and then dried before it is delivered to the foundry. Because this type of sand has a higher melting point than iron, copper, aluminium and other alloys it can be mixed with a binder to produce a mould that can hold the molten metal when producing a casting. The sand is classified into different sizes which affects the amount of binder required and the final surface finish of the casting. The sand mould is later broken away from the casting and can be recycled within the foundry or used elsewhere in the production of asphalt or concrete products. 1.4 PATTERN DESIGN A pattern is a replica of the object to be made by casting process with some modification. Material for pattern: wood, wax, metal etc

The main modifications are:1. The addition of pattern allowances, 2. The provision of core print 3. Elimination of fine details 1.4.1 TYPES OF PATTERN 1. Single piece pattern 2. Split pattern 3. Gated pattern 4. Cope and drag pattern 5. Match plate pattern 6. Loose piece pattern 7. Follow pattern 8. Seep pattern 9. Skeleton pattern 1.4.2 PATTERN ALLOWANCES 1. Shrinkage allowance 2. Draft allowance 3. Machining allowance 4. Shake allowance 5. Distortion allowance

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CHAPTER -2 LITERATURE SURVEY 2.1 ANALYSIS OF FOUNDRY DEFECTS AND PREVENTIVE ACTIVITIES FOR QUALITY IMPROVEMENT OF CASTING

In this paper, Casting as complicated production processes carries risk of failures occurrence during all the process of accomplishment of the finished products. It’s very important to have an opposite process inspection, to perform preventive activities, and to make use of research techniques for better loss prevention. According to the paper we discussed the example of foundry direction, and uses of quality control Tools and find out that this was the point for good conclusion in relating to quantitatively activity of  analyzed firm past at usage of selected method of researches it gives base to introductions of  correctness in casting process. Efficiency of usage of improvement methods in of quality of casting process perfectly is perfectly visible on represented examples. Most important failure of the process are given below 1. Displacement 2. Micron 3. Slaggy 4. Heterogeneous 5. Shrinkage 6. Hot cracks These defects are due to workers negligence, material factor and failure in technology of  casting. Elimination of these non conformances can bring reduction of quantity of failures above 50%.

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2.2 FOUNDRY QUALITY CONTROL ASPECTS AND PROSPECTS TO REDUCE SCRAP REWORK AND REJECTION IN METAL CASTING. Objective of quality control in metal casting industry The aims and objectives of foundry quality control are to improve the casting industries income by making the product more acceptable to the customers by providing long life, greater usefulness, versatility, aesthetic concepts.

Metal casting industries are actively involved to reduce the scrap rejection and rework during the manufacturing process of the components. To achieve this, the production concerns must follow the quality control procedures correctly and perfectly without any negligence. Timely implementation of the modified techniques based on the quality control research is a must to avoid defects in the products. Some of the solutions and quality control aspects are explained in a simplified manner to eliminate the unawareness of the foundry industrial personnel who work in the casting manufacturing quality control departments. This review paper provides very valuable information to the young manufacturing and mechanical engineers who have interest to start their career in the manufacturing concerns of medium and large scale captive foundries. This paper discusses all about the general quality control aspects in a detailed manner. Emphasis of quality and quality control in foundry The inspection of castings has the dual purpose of ensuring that the product confirms to design requirements and of providing information needed for quality control in foundry. Casting practice and quality control in foundries Castings can be difficult to get right. Important rules are proposed as necessary and sufficient for the manufacture of reliable and quality castings .It is proposed that they are used in addition to existing necessary technical specifications such as alloy type as follows • Start with a good quality melt. • Avoid turbulent entrainment of the surface film on the liquid. • Avoid laminar entrainment of the surface film on the liquid. • Avoid core blows by adequate venting. • Avoid shrinkage.

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CHAPTER -3 DEFECTS AND NON DESTRUCTIVE TESTING

GREEN SAND MOULD The Sand Casting process - aka Green Sand Mould process - is commonly used in the casting of aluminum, brass/bronze, and iron products. The term "green sand" is known principally because of the moisture content within the sand. The sand undergoes a "mulling" process in which various clay and chemical additives that act as binders are blended with the sand, which results in a compound which is suitable for the sand moulding process. This prepared sand mixture is then compressed around the pattern at specific pressures and temperatures, to ensure it will maintain its shape throughout the remainder of the ca sting process. The blended sand and binders are compacted around the pattern, taking on the shape of the desired casting. Sometimes the design of the casting entails internal passageways being formed into the mould. This is done by using sand cores which are made of a similar sand mixture. The cores are strategically placed to form the necessary passageways in the casting. The two halves of the mould are subsequently closed and metal is poured into the cavity and left to solidify. After solidification has taken place, the sand is vibrated until it is released from the casting. The finishing process can then be completed by grinding, machining, plating and painting.

3.1 GATING SYSTEM A well designed gating system is a prerequisite for achieving perfect casting. The key objective of a gating system is to ensure smooth and complete flow of molten metal from ladle to the casting cavity 3.2 OBJECTIVE OF THE GATING SYSTEM the four main points, which enables a proper gating system, are: Clean molten metal. Smooth filling of the casting cavity. Uniform filling of the casting cavity. Complete filling of the casting cavity The mould cavity must be filled with a clean metal so that it prevents the entry of slag and inclusions into the mould cavity, which in turn minimizes the surface instability. If the mould has smooth filling then it helps to reduce the bulk turbulence. If it has a uniform filling it means that the casting fill is in a controlled manner. Complete filling of the cavity makes the metal thin with minimum resistance at the end sections.

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3.3 MAIN ELEMENTS

Pouring basin: This is otherwise called as bush or cup. It is circular or rectangular in shape. It collects the molten metal, which is poured, from the ladle. Sprue: It is circular in cross section. It leads the molten metal from the pouring basin to the sprue well. Sprue well: It changes the direction of flow of the molten metal to right angle and passes it to the runner. Runner: The runner takes the molten metal from sprue to the casting. Ingates: This is the final stage where the molten metal moves from the runner to the mould cavity. Slag trap: It filters the slag when the molten metal moves from the runner and ingates. It is also placed in the runner.

Steps in Making Sand Castings There are six basic steps in making sand castings: 1. 2. 3. 4. 5.

Patternmaking Core making Molding Melting and pouring Cleaning

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Pattern making The pattern is a physical model of the casting used to make the mold. The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting. If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities. Core making Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings. Thus the void space between the core and mold-cavity surface is what eventually becomes the casting. Molding Molding consists of all operations necessary to prepare a mold for receiving molten metal. Molding usually involves placing a molding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity and finishing and closing the mold. Melting and Pouring The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled. Cleaning Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects and general quality is performed .

3.4 Elements of gating system The main components include pouring basin, sprue, well, runner and ingates. Gating systems can be classified based on the position of the parting plane and also on the basis of ingates position as follows, i) Horizontal systems

Suitable for flat castings filled under gravity and commonly used in sand casting of ferrous metals and gravity die casting of non-ferrous metals.

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ii) Vertical systemsSuitable for tall castings and used in high pressure sand and shell mould and die casting processes.

iii) Top gating systemswherein the molten metal enters at the top of the casting promoting directional solidification from bottom to top of the casting.

iv) Bottom gating systems wherein the molten metal enters from the bottom and fills the mould slowly. The number and position of ingates is very important in designing the gating systems. The diagram given below describes the different considerations in the channel layout in horizontal gating system with side ingates. Heuristics for ingates location i) Side feeders When side feeders are used their efficiency can be improved by filling the first stream of hot molten metal through ingates, thereby reducing the fettling effort and subsequent marks on the casting. ii) Thick sections

This allows molten metal to flow to other sections with minimal cooling reducing breakage during fettling of ingates. iii) Clear path In case of sand casting it is essential to allow the metal to flow with minimal obstructions or direction changes to curb turbulence. In such systems ingates should never be placed opposite to a core. iv) Low free fall here the ingates are located where the free fall of molten metal in the mould cavity is low thereby minimizing oxidation and erosion at the point of impact.

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Gating System The assembly of channels which facilitates the molten metal to enter into the mold cavity is called the gating system (Figure17). Alternatively, the gating system refers to all passage ways through which molten metal passes to enter into the mold cavity. The nomenclature of gating system depends upon the function of different channels which they perform.   

Down gates or sprue Cross gates or runners Ingates or gates

The metal flows down from the pouring basin or pouring cup into the down gate or sprue and passes through the cross gate or channels and ingates or gates before entering into the mold cavity

Goals of Gating System The goals for the gating system are    



To minimize turbulence to avoid trapping gasses into the mold To get enough metal into the mold cavity before the metal starts to solidify To avoid shrinkage Establish the best possible temperature gradient in the solidifying casting so that the shrinkage if occurs must be in the gating system not in the required cast part. Incorporates a system for trapping the non-metallic inclusions

Hydraulic Principles used in the Gating System Reynold's Number Nature of flow in the gating system can be established by calculating Reynold's number

RN

=

Reynold's number

V

=

Mean Velocity of flow

D

=

diameter of tubular flow

m

=

Kinematics Viscosity

= Dynamic viscosity / Density

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r

=

Fluid density

When the Reynold's number is less than 2000 stream line flow results and when the number is more than 2000 turbulent flow prevails. As far as possible the turbulent flow must be avoided in the sand mold as because of the turbulence sand particles gets dislodged from the mold or the gating system and may enter into the mould cavity leading to the production of defective casting. Excess turbulence causes   

Inclusion of dross or slag Air aspiration into the mold Erosion of the mold walls

Bernoulli's Equation

h = height of liquid P = Static Pressure n = metal velocity g = Acceleration due to gravity r = Fluid density Turbulence can be avoided by incorporating small changes in the design of gating system. The sharp changes in the flow should be avoided to smooth changes. The gating system must be designed in such a way that the system always runs full with the liquid metal. The most important things to remember in designing runners and gates are to avoid sharp corners. Any changes in direction or cross sectional area should make use of rounded corners. To avoid the aspiration the tapered sprues are designed in the gating systems. A sprue tapered to a smaller size at its bottom will create a choke which will help keep the sprue full of molten metal. Types of Gating Systems (Figure18a, 18b) The gating systems are of two types:  

Pressurized gating system Un-pressurized gating system

Pressurized Gating System 

The total cross sectional area decreases towards the mold cavity 15

   

Back pressure is maintained by the restrictions in the metal flow Flow of liquid (volume) is almost equal from all gates Back pressure helps in reducing the aspiration as the sprue always runs full Because of the restrictions the metal flows at high velocity leading to more turbulence and chances of mold erosion

Un-Pressurized Gating System     

The total cross sectional area increases towards the mold cavity Restriction only at the bottom of sprue Flow of liquid (volume) is different from all gates aspiration in the gating system as the system never runs full Less turbulence

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Riser Riser is a source of extra metal which flows from riser to mold cavity to compensate for shrinkage which takes place in the casting when it starts solidifying. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally. Risers are known by different names as metal reservoir, feeders, or headers. Shrinkage in a mold, from the time of pouring to final casting, occurs in three stages. 1. during the liquid state 2. during the transformation from liquid to solid 3. during the solid state First type of shrinkage is being compensated by the feeders or the gating system. For the second type of shrinkage risers are required. Risers are normally placed at that portion of the casting which is last to freeze. A riser must stay in liquid state at least as long as the casting and must be able to feed the casting during this time. Functions of Risers   

Provide extra metal to compensate for the volumetric shrinkage Allow mold gases to escape Provide extra metal pressure on the solidifying mold to reproduce mold details more exact

Design Requirements of Risers 1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area)2 of the riser must be greater than that of the casting. However, when this condition does not meet the metal in the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers. 2. Riser placement: the spacing of risers in the casting must be considered by effectively calculating the feeding distance of the risers. 3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers, although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the riser can be shaped as hemisphere.

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Molding Material and Properties A large variety of molding materials is used in foundries for manufacturing molds and cores. They include molding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of molding materials is based on their processing properties. The properties that are generally required in molding materials are: Refractoriness It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest. Permeability During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity. Green Strength The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape. Dry Strength When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material. Hot Strength As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength. Collapsibility The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings.Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity. 18

Casting Defects

The following are the major defects, which are likely to occur in sand castings     

Gas defects Shrinkage cavities Moulding material defects Pouring metal defects Mould shift

Gas Defects

A condition existing in a casting caused by the trapping of gas in the molten metal or by mould gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal. Causes

The lower gas-passing tendency of the mould, which may be due to lower venting, lower permeability of the mould or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mould mixture, and excessive moisture present in the mould.   

Metal contains gas Mould is too hot Poor mould burnout

Shrinkage Cavities

These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mould. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities. Moulding Material Defects

The defects in this category are cuts and washes, metal penetration, fusion, and swell. Cut and washes

These appear as rough spots and areas of excess metal, and are caused by erosion of moulding sand by the flowing metal. This is caused by the moulding sand not having enough strength and the

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molten metal flowing at high velocity. The former can be taken care of by the proper choice of  moulding sand and the latter can be overcome by the proper design of the gating system. Metal penetration

When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mould wash was applied on the surface of the mould. The coarser the sand grains more the metal penetration. Fusion

This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high. Swell

Under the influence of metallostatic forces, the mould wall may move back causing a swell in the dimension of the casting. A proper ramming of the mould will correct this defect. Inclusions

Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mould can prevent this defect. Pouring Metal Defects

The likely defects in this category are  

Mis-run Cold shuts.

A mis-run is caused when the metal is unable to fill the mould cavity completely and thus leaves unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mould cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design. A cold shut is caused when two streams while meeting in the mould cavity, do not fuse together properly thus forming a discontinuity in the casting. When the molten metal is poured into the mould cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the poured metal and thick enough walls in the casting design.

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The mis-run and cold shut defects are caused either by a lower fluidity of the mould or when the section thickness of the casting is very small. Fluidity can be improved by changing the composition of the metal and by increasing the pouring temperature of the metal. Mould Shift

The mould shift defect occurs when cope and drag or moulding boxes have not been properly aligned.

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Coke-Fired Furnace

      

Primarily used for non-ferrous metals Furnace is of a cylindrical shape Also known as pit furnace Preparation involves: first to make a deep bed of coke in the furnace Burn the coke till it attains the state of maximum combustion Insert the crucible in the coke bed Remove the crucible when the melt reaches to desired temperature

3.5 NON-DESTRUCTIVE TESTING METHODS 3. INTRODUCTION

Non-destructive Testing is one part of the function of Quality Control and is complementary to other long established methods .By definition non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suitability for service .The technique can be applied on a sampling basis for individual investigation or maybe used for 100% checking of material in a production quality control system. Non-destructive Testing is not just a method for rejecting substandard material; it is also an assurance that the supposedly good is good. The technique uses a variety of principles; there is no single method around which a black box may be built to satisfy all requirements in all circumstances.

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What follows is a brief description of the methods most commonly used in industry, together with details of typical applications, functions and advantages. The methods covered are : 1 Radiography 2 Magnetic particle crack detection 3 Dye Penetrant Testing 4 Ultrasonic flow detection

1.Radiography

This technique is suitable for the detection of internal defects in ferrous and nonferrous metals and other materials. X-rays, generated electrically, and Gamma rays emitted from radioactive isotopes are penetrating radiation which is differentially absorbed by the material through which it passes; the greater the thickness, the greater the absorbtion. Furthermore, the denser the material the greater will be absorbtion. X and Gamma rays also have the property, like light, of  partially converting silver halide crystals in a photographic film to metallic silver, in proportion to the intensity of the radiation reaching the film, and therefore forming a latent image. This can be developed and fixed in a similar way to normal photographic film. Material with internal voids is tested by placing the subject between the source of radiation and the film. The voids show as darkened areas, where more radiation has reached the film, on a clear background. The principles are the same for both X and Gamma radiography. In X-radiography the penetrating power is determined by the number of volts applied to the X-Ray tube - in steel approximately 1000 volts per inch thickness is necessary. In Gamma radiography the isotope governs the penetrating power and is unalterable in each isotope. Thus Iridium 192 is used for 1/2" to 1" steel and Caesium 134 is used for 3/4" to 21/2" steel. In X-radiography the intensity, and therefore the exposure time, is governed by the amperage of the cathode in the tube. Exposure time is usually expressed in terms of mille ampere minutes. With Gamma rays the intensity of the radiation is set at the time of supply of the isotope. The intensity of radiation from isotopes is measured in Becquerel’s and reduces over a period of time. The time taken to decay to half the amount of curies is the half life and is characteristic of each isotope. For example, the half life of Iridium 192 is 74 days and Caesium 134 is 2.1 years. The exposure factor is a product of the number of curies and time, usually expressed in curie hours. The time of exposure must be increased as the isotope decays - when the exposure period becomes uneconomical the isotope must be renewed. As the isotope is continuously emitting radiation it must be housed in a container of uranium or similar dense shielding material, whilst not exposed to protect the environment and personnel. To produce an X or Gamma radiograph, the film package (comprising film and intensifying screens - the latter being required to reduce the exposure time – enclosed in a light tight cassette) is placed close to the surface of the subject. The source of  radiation is positioned on the other side of the subject some distance away, so that the radiation passes through the subject and on to the film. After the exposure period the film is removed, processed, dried, and then viewed by transmitted light on a special viewer. Various radiographic and photographic accessories are necessary, including such items as radiation monitors, film markers, image quality indicators, darkroom equipment, etc. Where the last is concerned there are many degrees of sophistication, including fully automatic processing units. These accessories are the same for both X and Gamma radiography systems. Also required are such consumable items as radiographic film and processing chemicals..

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Advantages of Radiography

• Information is presented pictorially. • A permanent record is provided which may be viewed at a time and place distant from the test. • Useful for thin sections. • Sensitivity declared on each film. • Suitable for any material Disadvantages of Radiography

Generally an inability to cope with thick sections.

• Possible health hazard. • Need to direct the beam accurately for two-dimensional defects. • Film processing and viewing facilities are necessary, as is an exposure compound 2.MAGNETIC PARTICLE INSPECTION

Magnetic particle inspection is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferroelectric materials such as iron, nickel, cobalt, and some of  their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC). The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous iron particles are then applied to the part. The particles may be dry or in a wet suspension. If an area of  flux leakage is present the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication .

Advantages of Magnetic Particle Crack Detection

• Simplicity of operation and application. • Quantitative. • Can be automated, apart from viewing. (Though modern developments in automatic defect recognition can be used in parts of simple geometry e.g billets and bars. In this case a special camera captures the defect indication image and processes it for further display and action) Disadvantages of Magnetic ParticleCrack Detection

• Restricted to ferromagnetic materials. • Restricted to surface or near surface flaws. • Not fail safe in that lack of indication could mean no defects or process not carried out properly 24

3.DYE PENETRANT TESTING

This method is frequently used for the detection of surface breaking flaws in non ferromagnetic Materials. The subject to be examined is first of all chemically cleaned, usually by vapour Phase, to remove all traces of foreign material, grease, dirt, etc. from the surface from within the cracks. Next the Penetrant (which is a very fine thin oil usually dyed bright red or ultra-violet fluorescent) is applied and allowed to remain in contact with the surface for approximately fifteen minutes. Capillary action draws the Penetrant into the crack during this period. The surplus Penetrant on the surface is then removed completely and thin coating of powdered chalk  is applied. After a further period (development time) the chalk draws the dye out of the crack, rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background. The process is purely a mechanical/chemical one and the various substances used may be applied in a large variety of ways, from aerosol spray cans at the most simple end to dipping in large tanks on an automatic basis at the other end. The latter system requires sophisticated tanks, spraying and drying equipment but the principle remains the same Advantages of Dye Penetrant Testing

• Simplicity of operation. • Best method for surface breaking cracks in non-ferrous metals. • Suitable for automatic testing, with r eservation concerning viewing. Disadvantages of Dye Penetrant Testing

• Restricted to surface breaking defects only. • Decreased sensitivity. • Uses a considerable amount of consumables ULTRASONIC FLOW DETECTION

In ultrasonic testing (UT), very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipe work corrosion. Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood and composites, albeit with less resolution. It is a form of non-destructive testing used in many industries including aerospace , automotive and other transportation sectors.

Advantages

1. High penetrating power, which allows the detection of flaws deep in the part. 2. High sensitivity, permitting the detection of extremely small flaws. 3. Only one surface need be accessible. 25

4. Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces. 5. Some capability of estimating the size, orientation, shape and nature of defects. 6. Nonhazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity. 7. Capable of portable or highly automated operation.

Disadvantages

1. Manual operation requires careful attention by experienced technicians 2. Extensive technical knowledge is required for the development of inspection procedures. 3. Parts that is rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect. 4. Surface must be prepared by cleaning and removing loose scale, paint, etc., although paint that is properly bonded to a surface need not be removed. 5. Coolants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT). 6. Inspected items must be water resistant, when using water based coolants that do not contain rust inhibitors.

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CHAPTER -4 PROBLEM FORMULATIONS Designing for eliminating the defects in aluminium casting for the g iven object.

1. 2. 3. 4. 5. 6.

Designing pattern and gating system for Aluminium casting through sand moulding process Preparation of cavity using sand moulding process Performing casting for the same Identifying the casting defects (if possible measuring them) Redesign the same casting to eliminate defects After rigorous designing and cross checking suitable and adequate design will be proposed for given Al casting.

CALCULATIONS FOR PATTERN DESIGNING BEFORE ALLOWANCE (PATTERN)

27

SHRINKAGE ALLOWANCES FOR VARIOUS METALS

Material Grey cast iron

Pattern dimensions , mm

Shrinkage Allowances (mm/m)

Up to 600

10.5

600 to 1200

8.5

Malleable iron

-

11.8

Chromium steel

-

20

Aluminum

-

13

Aluminium bronze

-

20.0 to 23.0

Calculations for shrinkage allowance 1. For dimension 9 cm, Allowances = 9*13/1000 = 0.117 After allowances = 9+0.117=9.117cm 2. For dimension 4cm, Allowances = 4*13/1000 = 0.052 After allowances = 4+0.052=4.052cm 3. For dimension 3 cm, Allowances = 3*13/1000 = 0.039 After allowances = 3 +0.039=3.039cm

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MACHINING ALLOWANCES ON PATTERN

Dimension ,mm

Allowances ,mm Bore

Allowances ,mm Surface

Non ferrous Up to 200

2.0

1.5

201 to 300

2.5

1.5

301 to 900

3.0

2.5

1. for dimension 9.117cm After allowances=9.117+0.0015=9.3cm 2. for dimension 4.052cm After allowances=4.052 +0.0015=4.2cm 3. for dimension 3.039cm After allowances=3.039 +0.0015=3.2cm

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DRAFT ALLOWANCES

Pattern material

wood

Height of surface, mm

Draft angle of  surfaces, degree

Draft angle of  surfaces, degree

External surface

Internal surface

51 to 100

1.00

1.50

101 to 200

.75

1.00

201 to 300

.50

1

After Allowances (Pattern)

Mathematical Calculation Density of aluminium ( ρ) : 2.7 g/ cm³ Melting Temperature : 660.37 °C Volume of casting =290cm³ Mass of casting = volume X density of metal used = 290*2.7Kg = 783gm •

•

•

•

•

Mass of metal poured = actual casting mass X 100 casting  yield 

•

Casting yield for aluminium =25  – 45 (40 taken) 30

•

•

Mass of metal poured (W) =(783×100)÷40 =1.96kg Pouring rate =

mass of  metal  poured   pouring time

ASSUME pouring rate 0.5s for aluminium

Calculating the Choke Area Choke Area = •

   t  •

•

•

•

•

•

•

W   C 

2 gH 

Effective head of casting(H): h t ==H4.5 cm W= casting mass ρ= density of molten metal t = pouring time=6.5 sec C=efficiency factor=0.95 H=effective metal head g= acceleration due to gravity

From this formula choke area is 1.18cmɜ From the choke area we can get choke diameter (d) from the formula 2 Area = d  /4 So the diameter (d) is = 1.2 cm Here the gating system we use unpressurised system for low turbulence and other benefit in good casting For unpressurised gating system the ratio of sprue : runner: ingates is 1:3:3 2 Area of the runner = 3.6cm Length and height √   =1.8 (top width is 1.2 and height is 1.2cm) Length of the runner is =6.4 (bottom width is 0.8cm and length of runner is 9.2 cm Design Assumption 2 Here we are using two ingates so area of each ingates =1.8cm Dimensions of gates we know width and height ratio is 4 2 2 So 4 h =1.8cm From this we get h=0.67 and w=2.68 cm Height and width of ingates 31

Height =.67 and width =2.68 Sprue dimensions Top dia of sprue is =1.414×1.23 =1.69cm Height of the sprue =4.5 cm And bottom dia of sprue is 1.2 cm Dimensions of risers 3 Riser=3×0.065×290=56.55cm Design Assumption Here we use two Risers 3 So, volume of one riser is 29 cm . For cylindrical riser =π/4 d3h Then dia of riser = 3.29 and it will also the high of riser because aspect ratio of riser is 1. 2 Area of  cylinder = πdh+ 2π (d)  /4 2 = 3.14*(3.3) +π(3.3)/2 2 =51.31cm

32

DESIGN OF CASTING PATTERN, SPRUE AND RISER

Runner

Sprue

Casting pattern

33

34