INTRODUCTION

Investment Casting Plant

INTRODUCTION

Investment Casting Plant is major section of Precision Engineering Complex, which is a department of Pakistan International airlines. ICP produces a number of cast products of aluminium & steel alloys, & some superalloys. ICP makes these special products to different vendors.

Investment casting, also called lost wax casting, is widely used for producing ferrous and nonferrous metal parts. Unlike other casting processes, investment casting produces net shape parts with excellent surface finish and dimensional accuracy. This manufacturing process is ideal for applications that have relatively low production quantities, because there are some steps which takes much to complete.

ICP also equipped with normal & vacuum heat treatment furnace. Almost all parts are heat treated; normally superalloys are treated in vacuum furnace whereas all other metals are treated in normal furnace. There are some products of investment casting shown in fig. 1, & the flow chart of ICP is given on next page:

Note: - All the pictures pasted in this report are taken from internet.FLOW CHART OF ICP

WAX PATTERN MAKING

Wax pattern making shop has two sub sections according to process step, these sections are described below:1. Injection molding of pattern.

2. Cluster assembly. Injection molding of pattern:

Here is the first step of investment casting. In this process the molten wax is injected into the die through a nozzle, at particular temperature, to take the shape of pattern. Pattern is the actual object we want to eventually be out of metal. The pattern can be made out of anything that holds its shape at least reasonably well. But in investment casting we need a highly fine surface, so we use wax. Some wax patterns are shown in fig. 2.

Investment casting also needs gating system to fill cavity completely, therefore these runner, gates etc are also made by wax but wax used for this purpose is reused. For pattern & gating system there are three Injection Molding Machines two for pattern & one for gating system in ICP.

Cluster assembly:

Cluster assembly is the process of joining a number of patterns in one piece with the help of gating system. It is feasible for casting. This process also involves the finishing of wax pattern surface. Sometimes the pattern surface may damage by the defect of die or by handling, this surface defect must have to remove by filling or cleaning. Here is the picture of cluster assembly in fig. 3.

CERAMIC COATING

Ceramic coating is the process of depositing layers of ceramic materials in the form of fine & finer particles to create a sufficient thick wall of the mould, to hold molten metal for casting. A mold is the negative shape of our pattern. The mold is created from mixture consisting of casting plaster, sand, and water. When they are mixed, they form viscous slurry.

There are some steps given below to produce a sound mould for investment casting; & the ceramic coated cluster is shown in fig 4.1. Mount a holder at the top of cluster.

2. Wash the cluster in 70% alcohol 30% tricholoro ether solution

3. Wash the cluster in 100% alcohol.

4. Allow the cluster to dry for 24 hours.

5. Place in the queue of primary coating

6. Each coat of coating involves three steps, and there are 2 coat of primary coating:

i. Dipping in slurry

ii. Apply sand on wet cluster

iii. Allow to dry for 24 hours

Slurry for steel or superalloy parts is made by alcohol & binder whereas for aluminium casting slurry is made by water & binder. And relatively coarse sand is used for aluminium casting due to low pouring temperature of aluminium alloys.

7. After completing primary coating, backup or secondary coating is applied over the primary coating. It has the same process as mentioned above. But here coarse sand is used for just supporting the primary coating. 10 coats for steel & superalloys & 7 coats for aluminium castings are practiced.

8. Finally they are dipped into the slurry & allow drying in dryer.DEWAXING

Dewaxing is the process of producing a hollow mould ceramic coated wax cluster by melt down of wax. This is done by steam dewaxing machine ICP. The picture of that machine is shown in fig. 5. To dewaxing heat up enough wax to fill your mold in the wax crock pot. It is extremely important to make sure that the temperature of the wax does not exceed 200 F. The flash point of wax (point at which it sets itself on fire) is only slightly above this temperature, and wax fires are very difficult to extinguish.

FIRING

Firing is the process of strengthening the mould for investment casting & cleans the mould from all volatile elements. There is a gas fire furnace in ICP for this purpose. They provide about 10000C for sufficient time to make sure that the mould gets its strength to withstand or hold the molten metal. Here is the firing furnace for investment casting moulds.PRE HEATING

There are two pre heating furnace in casting shop of ICP, one for aluminium and one for steel & superalloys. Pre heating is done just before casting to raise the temperature of mould so that the temperature difference between mould and molten metal which is going to cast into that mould. This lower temperature difference avoids chilled grain, which cause by cool walls of mould. Figure of pre heating furnace is shown in fig. 7.

CASTING

As the temperature of pre heating of mould reaches at about 60% to 80% of require temperature of mould, start melting the metal, so that both melt & mould are ready at pouring time. In ICP they melt grades of known composition, therefore there is normally no need to set the composition but the melt have to clean from all other impurity in the form of slag or any other mean. Impurity usually comes from return scrap, refractory lining or trapped gases. They are usually do three types of melting according to material. This is describes as below:

1. Aluminium casting through pit furnace

2. Steel casting through air induction furnace3. Superalloy casting through vacuum induction meltingAluminium Casting:

In aluminium casting they are making many grades, one of those is 1353 Al-alloy, whose pouring temperature is 720oC. during the melting of aluminium alloy they add some type of degassing tablets to remove impurities & do stirring with a rod or any other thing to allow gases to remove & come on the top surface then it remove in the form of slag then covering salt is add to cover up the surface of the molten bath. Finally they add the grain refiner i.e. titanium in the molten bath & pour into pre heated moulds.

They allow castings to cool. Wait at least 6 hours before breaking the castings open. This is more than enough time, but it is much better to wait too long than not long enough.Steel Casting:

In steel casting they melt the require grades of steel in 60 Kg air induction furnace. Here they also add 40% shop return of same grade to maintain the composition & as the melt is ready they pour it into pre heated mould. The air furnace is shown below in fig 8.

Superalloy Casting:

During my internship I didnt get the opportunity to see the process of vacuum induction furnace but fortunately I understand the process by the help of helpful employees.

Vacuum induction furnace uses a vacuum to improve the quality of the casting and minimize porosity. Typically it consists of an upper and a lower chamber. The upper chamber or melting chamber housing the crucible, and the lower casting chamber housing the investment mould. Both chambers are connected via a small hole containing a stopper. A vacuum is pulled in the lower chamber, while pressure is applied in the upper, and then the stopper is removed. This creates the greatest pressure differential to fill the molds

Here in the vacuum furnace the pressure lowers up to 10-3 bars by using a combination of mechanical pump, roots pump, diffusion pump, holding pump & hydraulic pump. The crucible furnace in this vacuum chamber occupy 10 Kg molten metal & pour in the same environment, to get very accurate material in single crystal or polycrystalline form. In the class of superalloys they do casting of Rene 125 & Incoloy 718C, which are the nickel base alloys for temperature application.FETTLING

Fettling is a time-consuming and difficult process, but it is also extremely crucial if you want your casting to look good. Here are the basic steps:

Break up the ceramic mould.

Clean the surface of part with hot water jet with 200-400 bar pressure

Remaining sand is remove by sand blasting if require

Remove the sprues and gates from cluster

Slightly machine the separation point of part HEAT TREATMENT

eat treatment is a operation of heating & cooling to get the desire properties of material. In ICP the four heat treatment furnace, three furnaces are normal environment furnaces whereas one is vacuum heat treatment furnace. The operations are being done on these furnaces are described below:

Normal Heat Treatment Furnaces:

In these furnaces usually aluminium alloys are treated, i.e. A356, A357 etc. the operations are being done on these grades is:

Solution Treatment

In this treatment the charge is heated up to 550oC, where it allows soaking for 12 to 18 hours then water quench. This form the super saturated solution in metal.

Precipitation Hardening

This is the second treatment on the same the grade to get precipitate of the dissolved particles to get strength.

Vacuum Heat Treatment Furnaces:

In this furnace steels & superalloys are usually treated i.e. carbon resisting steel, 17-4PH, ASM 5376, Incoloy 718 C & Rene 125 & the operation of treatment are

Homogenizing

Solution Treatment

Precipitation Hardening

STRAIGHTENING

After heat treatment the parts are distorted by rapid change in temperature, so we have to straight them on their original position. For this purpose ICP uses different fixers to get the right parts & they have also measures the parts with requirementsINSPECTION & TESTING

As per the requirement of customer ICP test & inspect the materials. The have the facilities of following testing:

Fluorescent particle inspection

Fluorescent particle inspection is a non destructive testing process which is use to detect surface crack on non porous materials. In ICP there is a complete setup FPI, with certified peoples. Here is a picture of FPI on fig. 11.

Emission Spectrometer

Spectrometer is a rapid chemical analyzer which can tell us the whole composition of metal within 40 seconds. A common spectrometer is shown in fig 12.

Radiography

Radiography is also a non destructive testing process which is use to detect any internal discontinuity i.e. crack, flaw, slag etc as shown in fig. 13.

Tensile test

Tensile test tells us the strength of material, which is why every cluster in ICP consists of two specimens of tensile round bar. A common tensile test machine is shown in fig. 14. Hardness test

There three hardness tester in ICP Lab, i.e. Brinell, Rockwell & micro Vicker hardness tester, hardness testers tell us about the materials that how is can resist the penetration on it.

Plasma coating testing

To check the strength of plasma coating material, they coated on same button then mount it on the fixers with the help of epoxy & test it as in tensile test.

Metallography

Metallography is the study of microstructures of materials. There are 2 microscope in ICP, which is use in daily routine for microstructure of all material & also for R&D purpose. RECOMMENDATION

To get the fine structure & enhance mechanical properties add 5% amount of inoculants content i.e. cobalt aluminate (CoAl2O4) in the surface layer of the ceramic mould.

To lower the porosity content in investment casting, properly pre heat the ceramic mould, means pre heat at that temperature where all the gaseous content have removed.BIBLIOGRAPHY Foundry Technology by P.L. Jain

Handbook of Investment Casting by James E. Sopcak

Internet

Materials and Processes in Manufacturing by Degarmo & E. Paul.Fig 1 Some Products of Investment Casting

WAX PATTERN MAKING

START

CERAMIC COATING

DEWAXING

PRE-HEATING

CASTING

FETTLING

HEAT TREATMENT

STRAIGHTENING

INSPECTION & TESTING

FIRING

Fig 2 Some Wax Pattern

FINISH

Fig 3 Cluster Assembly

Fig 4 Ceramic Coated Cluster

Fig 5 Steam Dewaxing Machine

Fig 6 Firing furnace for Moulds

Fig 8 Pit furnace for Aluminium

Fig 9 Air induction furnace for Steel

Fig 10 Vacuum induction furnace for Superalloys

Fig 7 Pre heating furnace for moulds

Fig 11 Fluorescent particle inspection

Fig 13 Radiography inspection

Fig 14 Tensile Testing Machine

Fig 12 Emission Spectrometer

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