Modification of Investment Casting 4

[Salvatio]

I am currently doing a research project and trying to modify the process of investment casting. If every procedure is followed as in a normal investment casting practice, up to the mould cavity stage, instead of pouring molten metal in, will it be possible to use metal powder to firstly fill up the cavity, only then be heated to melt the powder, thus eliminating the need of allowances in pattern design usually needed in foundry practice during the shakeout of the pattern. The powder is left in the cavity to melt, thus occupying the whole cavity. a part of the mould will be open to the atmosphere, such that shrinkage will occur in this portion as in an ingot shrinkage.
Also, the pattern used will only be of a solid wax, which is to be machined by CNC or so, and the mould is made by applying clay or plaster onto the wax pattern. Or a die casting like mould could be made. Additives can be applied to solve the problem of blow holes or preventing gas/air to dissolve in the melt.

My only question is that, can anyone suggest defects and problems that will occur?

 

[COKA]

Salvatio,

When you pour aluminum in a ceramic mould for example, the molten metal passes through a filter to eliminate some impurities. If you fill the mould with powder, how will you ensure that the impurities targeted to be removed with the filter in the regular process will not be there any more. Also, powder is "porous" if I can put it that way, meaning that there is oxygen between the powder granes. And how about the degasing process? When you melt the aluminum ingots to make the molten aluminum, you have to degas the melt before it is ready to be poured. You are mentioning additives, but the degasing process involves putting a "degaser" into the melt on top of the additives.


Coka

 

[NickE]

You would need to highly compact the powder, there is no way that an equivalent voulume of powder will fill a mold properly. There is a maximum packing factor that any size powder will obtain, thus the standard range of density given for most powder metal products. The "pores" cause significant changes in physical properties depending on size/shape/location. Active metal powders would require a flux or complete processing in inert atmosphere. The allowances for shakeout of the pattern would still all apply since you would need to remove the wax properly still. Also the gating and risering of a powder -> liquid -> solid would be annoying.

Coka's points about filtering and de-gassing would also apply.

nick

 

[arunmrao]

Anyone having an exposure to powder metallurgy will be aware the difiiculty in getting a mould filled. This becomes acute with alloy constituents of dissimilar densities. Compacting the powder into the cavity without voids will be a great feat. If you can achieve this succesfully perhaps you would have helped a powder metallurgist realize a dream.

 

[Salvatio]

thanks to all replies. What I'm tryng to do is firstly, make a pattern out of wax. The wax replica will be heated with the mould, to completely melt the wax in order to pour out and removes it as completely. As in used in investment casting, the wax will be removed by heat and pour out, and residual will be burn in the furnace. If necessary, the mould should be made in splits, in which after the removal of the wax, it will be checked and ensured maximum cleaniness inside.
The powder used will be of direct from suppliers for powder metallurgy, which can ensure pureness of the powder, of course, some impurities must be taken into account. Or otherwise, filtering process could be done before the powder is put into the mould.
For a complicated shape, it is impossible to fill the whole mould with powder. More powder will be put in after the initial melting, so that it would fully occupy the volume of the mould.
There will be an opening from the mould to the surface, opened to the atmosphere, as a sprue or riser used in castings. It is this place where powder is added, and shrinkage will take place, and hot pads will be placed around it. And that this "sprue" should be located at the top most of the design.
The need of gating and runners will be eliminated, as for example, if there are 2 same design is to be made in a single mould, to save time, work and space, then these 2 will not be connected togather by means of runners. They will be saperated and each of them will have own "sprues".
Most of the procedures will be similar to the typical investment casting practice.
For higher quality production, the process may use vacumm degasing, if possble, in term of cost, fully vacumm.

 

[NickE]

I still think that you wont get fully dense parts after this step w/o some sort of High pressure compaction, IE HIP.

Also unless the powders are fully processed and stored and "poured" and "melted" in an inert atomosphere you wont get rid of the oxide film on the surface of the powder.

Filtering of a powder metal wont achive anything except be certain that the powder size is passing your filter size.

Since you are still using a wax pattern and dipping in a slurry to create a ceramic mold you will need to draft the pattern properly so that when the wax expands from the heat it wont crack your mold.

Adding solid powder to a liqid metal inside an investment mold sounds tricky and dangerous.

Vaccumn de-gassing would have to be done on the mold while the metal is liquid. Sounds complicated and dangerous again, esp if the metal has a high vapor pressure.

(by the way what metal are you considering?)

All in all I dont think that this is a useful or productive refinement of the investment casting process.

nick

 

[Salvatio]

since that this is still in a testing stage, i will be using Al-5%Ni or Al-30%Cu for their low melting temperature.
Thanks to Nick for reminding me of the expansion of the wax during heating...
Please do tell that if you have thoughts of any problems that will surface during the process, or any methods to improve it.

 

[arunmrao]

Please let us know the method of preparation of the 2 alloys to be studied by you. The difference in densities of the 2 constituents is going to be a problem.

NickE continuing fron where you stopped"All in all I dont think that this is a useful or productive refinement of the investment casting process".

The fault often lies with the research supervisor. In his eagerness to produce research papers such topics are selected and thrust upon naive students.

No doubt unconventional approach or new ideas can be provided by the research academy,but there has to be some purpose to it.Commercial implications of the work I am not concerned, but they should conform to accepted principles of engineering/science.

Students have lot of energy and enthusiasm to do new work,this should be properly channelised by the research supervisor.

There are a lot of areas for modification/improvement of investment cast parts or in powder metallurgy a fascinating field one can work with hard metals, etc. Now trying to synthesize the two areas does not gel.

Please do not take me to be critical ,but I have provided my view point.(Perhaps this is one reason that I am not a popular examiner,rarely does one recommend my name)

 

[saldanha]

What are the benefites of this aproach?

 

[unclesyd]

Didn't one of the H.I.P'ing people take a similar approach severals years back?

 

[Salvatio]

This method of modification is still under study. Thanks to arunmrao for his great view points. However, this is an individual project, hence with only limited resources.
Parts made with investment castings often do not require any further machining, because of the close tolerances that can be achieved. By modification of it, runners and gating systems can be eliminated or bring to a minimum, thus a higher yield of casting. The requirement and time of making the latter is thus also reduced. Problems with pouring melt can too be eliminated, since that the powder will be melted within the mould. The main aim is still, to reduce cost, productivity time and improve quality.
There will be some extend of permeability in the mould design, and that air is forced out by applying pressure.

unclesyd: is the approach successful? would you know where could i find more info about it?

Thanks.

 

[Salvatio]

I think I've confused most people here... What i'm going to d with the powder is not to consolidate nor to densify it, but to melt it, inside the mould. The reason i use powder is such that it's the easiest way to fill a cavity other than using liquid metal, instaed using a bulk of metal. Compaction of the powder will not be needed since it is to be melted.

 

[MechEng13]

Salvatio,

I believe you need to reread NickE's post. The problem that you will run into with your idea is that the Volume of powder in the mold will not fill the mold once it is in it's liquid state. When the people in the Powder metal industry make parts they heavily compact the powder into a green state. Then it is oven cured at high temps during which I believe from what I recall there is additional shrinkage.

In the powder state there will be pockets. Also the density of the molten powder will be lower than that of the powder adding to the lowered volume to fill the mold.

Hope you see a solution...I'd be curious to see it work



Alan M. Etzkorn
Product Engineer
Nixon Tool Co.

http://www.nixontool.com

 

[Salvatio]

Thank you MechEng13, i know that there will be a change in volume after the melt of the powder. As I've stated, additional powder will be added once the volume decreases. The question is, what will happen once solid powder is added into liq metal, as in ice being added into water. Oxides of the powder wont be much of a problem since that powder will be de-gas/deoxidised, before it goes for packaging.
Indeed that the volume will fall-rise-fall, in the process of adding in powder, but if the added powder is melted by the melted powder, and of course, the furnace is still operating, maybe in several attemps to fully fill the volume, what kind of problems will arise?

Problems faced in powder metallurgy is that it's unable to achieve 100% of soild, without having porosity.

Problems faced in casting is, the pouring-solidification process.

again, thanks to all for helping me out.

 

[COKA]

De-gassed powder before it goes for packaging?? I won't make any coments to that because I have no experience in powder metallurgy. But correct me if I am wrong... that package will have to be vacuum packed in some inert atmosphere so that no oxygen gets inside....? (This was me thinking out loud...) Then it will have to be somehow fed to the mould where it will not be in contact with oxygen, because as soon as it starts melting and gets in contact with oxygen, you'll have to think about conventional de-gassing. And even then, I see lots of porosity occuring here.

Coka

 

[NickE]

Coka- my point exactly. Unless all of the handling and processing of the powder is done in vaccumn and/or inert atomosphere the powder will acquire an oxide film. This film will, unless fluxed, require a very high temp to melt. Also the gass that is entrapped in the pores btw the powder particles will still need to be removed, you can't expect it all to be removed by floating to the surface of the liquid.

nick

 

[saldanha]

Another thing that you have to remember is that the volume change of the metal takes place during solidification. so even if you had more powder after volume change, it might not be of any effect because some area have solidify already or are becoming dificult to reach by metal in any other form than the liquid form. the process may become even worst after the addition of more powder due to temperature difference causing the liquid metal to solidify rapidly thereby making the process of filling cavities more dificult. Unless if you are using a matterial which shows some volume expension during soludification like cast iron.
maybe you can use some form of selective heating so that you can control the melting porcess so that you fill the aection that may pose serion contraction problems. just a suggestion

 

[rd400guy]

It's an interesting idea, but here are the problems I see:

First, what method of heating are you using to melt the powder? Depending on the size of the part, you're going to have to dump in a significant amount of heat to melt the powder. Furnaces specifically designed to melt metal have very thick refractory linings, and/or in some cases water cooling. Your mold would have to withstand the same amount of heat as a furnace, as the powder melts, which would mean a VERY thick mold. Removing the mold after the part has solidified would be a huge problem. In conventional casting, as soon as the metal is poured in, it starts to cool and forms a shell, so the mold doesn't have to be as thick, but to actually melt the metal inside the mold would require a lot more "mold" than you realize.

Second, I agree with everyone else that you'll have porosity problems.

Third, I fully agree with NickE that the oxide (and possibly nitride) coating on each individual powder particle will provide enough of a problem that it will require COMPLETE vacuum/inert processing to make it a non-issue. With the monetary investment required to do that, I think it would be much more cost effective to pour molten metal, and cut off the "excess" sprue, runners, risers, etc., which can always be used later as raw material for remelt.

Fourth, with respect to adding powder to make up for density/shrinkage issues, now you'll have to add even more heat to melt the "cold" powder you're adding. In terms of ice and water, if you had hot water and needed more hot water, would you really want to add ice? The risers in conventional investment casting are the reserviors that hold the extra "hot water" you'll need as solidification progresses and the casting shrinks.

I've rambled on long enough, but if you do try it, please let us all know how it works.