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I am hoping somebody may be able to guide me on this.
I have tried making various bronze alloys with some success but more often not.
Take an example, Tin bronze alloy which has Copper, Tin, Lead and Zinc in its formulation.
I melt the copper, then add the tin followed by the lead and all is good. Add the Zinc and all hell breaks loose as it boils off and burns with it's greenish flare and white/yellow smoke.
Now, I am used to casting Brass alloys and the flaring is minimal when I choose to pour but this reaction is disconcerting and hazardous too.
Surely an awful amount of Zinc is lost in the process too?
Would it be better to add the zinc percentage using brass alloy since this is already a proportion of copper and zinc?
Aluminium bronze was a complete failure as it was so gooey and silicon bronze not much better in my experience.
Either way, all the experiments I have tried have left a horrible black skin on the sprue/pour basin but my Lost PLA castings remarkably did work out.
Any guidance would be gratefully received
Rich
 

GreenTwin

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I have been acquiring the materials to make lead-free bearing bronze castings, and have researched it quite a bit.

Silicon Bronze:

I considered making silicon bronze, which is what the art folks use for cast sculptures, and it is mostly copper.
A common brand name for silicon bronze is Everdur.

Nominal Composition:

  • 95% Cu
  • 4% Si
  • 1% Mn
While silicon bronze is suppose to be very easy to melt and pour, without the zinc burnoff problems, what I need for model engine work is the wear characteristics of leaded bronze.
I have phased out almost all leaded metals from the shop, with the exception of some steel hex stock which I use to make screws/bolts.

Boat Shaft Bronze:

I have a lot of old bronze boat shaft, and so I made some bearings from that material, and the stiction was so bad that no matter how loose the bearing was, the shaft would not rotate in it.
Boat shaft bronze seems to be the worst material in the world to make bearings from.

I found this info on the net:
Most of the old "bronze" shafts were made of what is commonly referred to as Tobin Bronze.
Tobin Bronze is 60%copper, 39.25% Zinc and 0.75% Tin.



Leaded Bearing Bronze: (I have gotten away from any leaded products for foundry use, but this is for general information)

Leaded bearing bronze (C93700 or SAE64) (Copper=80%, Tin=10%, Lead=10%).


Lead-Free Bearing Bronze C89835, Bismust Tin Bronze

(Copper=87%, Zinc=3%, Tin=6.7%, Bismuth=2.2%, Antimony=0.35%, Iron=0.2%, Lead=0.09%, Nickel=1%, Phosphorus=0.1%, Silicon=0.005%, Sulfur=0.08%)

So the above recipe for lead-free bearing bronze is what I intend to cast into bearing shapes.
I have been gathering the various metals, and I think I have everything except the lead, which I will probably omit.

For phosphorus, I am using the material commonly used to solder copper pipe for refrigerant lines, and it is an alloy, so I have to calculate the correct amount in order to get my 0.1% phosphorus.
You don't want to dump raw phosphorus into molten meltal I don't think.

As with many volatile additives, such as the magnesium that you add to gray iron in order to cast ductile iron, the magnesium has to be alloyed with some other metal, else it will just cause a huge fire or explosion when added to molten metal.

How to Melt:

I started asking around about how to add the various metals, and what to melt first.

Obviously anything you add to molten metal must be very dry (perhaps even oven baked), because any slight residual moisture on the surface of anything can cause a molten metal explosion in a crucible.

Here is one video I found by one of my casting buddies:
I have not tried these methods, and so can't vouch for anything in this video.




Melt Cover:

Some folks use broken glass on top of their brass/bronze melts, which seals the molten metal from the atmosphere during the melt, to prevent zinc burnoff.
I am reluctant to add glass to a melt, since I am not doing sculptures, but rather making engine bearings, so I don't want any foreign material in the bearing.
They say you can skim off the glass at the end of a melt, but can you get it all out? Not sure.


So to try and address some of your questions:

1. Can you use or make a product such as silicon bronze, and basically avoid the zinc ?

2. I think if you must have zinc, you could add a calculated amount of brass towards the end of the melt.
I would not add raw zinc to a molten pool of metal, but it may be possible under certain conditions that can contain the reaction safely.

3. Zinc does burn off during a brass melt, and the fumes should be avoided.
I did melt and pour some brass, and a lot of the zinc did seem to burn off, with lots of smoke (zinc fumes to be avoided), but the zinc burnoff did not seem to otherwise negatively affect the casting.

4. I have seem some aluminum bronze, and have a sample, but it is basically unmachinable because it is extremely hard.

5. I would not worry about a skin on the spure or basin, if the casting turned out well.
I am not sure exactly what your black deposit is, and that sounds a bit unusual, and I would not expect that.

So perhaps try to make an Everdur-type silicon bronze alloy, to avoid zinc burnoff problems.

Or perhaps try to make a lead-free bearing bronze, which has very little zinc in it, and use some brass as a source for the zinc.


That is my 2 cents worth, coming from someone who has not yet made lead-free bearing bronze, but who is about to try it.

.
Edit:
As a general approach, I plan to melt the metals that make up the bulk of the alloy first, and add the trace amount alloys after the other metals are molten, probably by plunging them (being ready for a reaction with lots of safety gear).
I will probably wrap all the trace metals in aluminum foil, and plunge then to the bottom of the melt.

If you try to add the trace materials at the beginning of a melt, I would guess they would burn off during the melt.

I add a small amount of ferrosilicon to my iron melts, and I add it to the molten metal immediately before I pour, since the ferrosilicon effect only lasts a few minutes after you add it to the metal.


Edit02:
I notice in the video that he uses pennies as a source of zinc.
Modern pennies (US coin) are copper-plated zinc.
Old pennies are solid copper.

.
 
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abby

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I have made many kilos of gun metal ( red brass in the US) to the 85-5-5-5 recipe. This is an ideal alloy for steam engine castings.
I melt the copper first under a flux cover then add the tin followed by the lead , I add the zinc in the form of brass bar (60/40).
I pour at around 1100°C after removing as much flux as possible , a little dry sand sprinkled over helps to coagulate the flux into a thick mass which will stick to the skimming tool , de-oxidize with a phosphorus copper plunge tube.
Aluminium Bronze machines well but requires very sharp tooling , I have cast replacement Jaguar gear selector forks in aluminium bronze and reamed the bores but only a brand new reamer had any effect.
A suitable flux can be made from borax , potash , salt and charcoal in equal quantities.
An interesting alloy to make is the Japanese leaded bronze which was used for casting sculptures. This is a straight alloy of copper and lead and produces beautiful patinas on the sculptures. It contains 25% lead !.
Try this mix and pour into an ingot mould.You will see that as the ingot solidifies silver globules of lead ooze from the alloy. to encorporate all of the lead the ingot/s have to be melted and poured 3 times.............ain't that typically Japanese.
Dan.
 

GreenTwin

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I heard years ago that borax could/should be used as a flux.
Another fellow tried it, and it destroyed his crucibles in just a few melts.

Borax is said to be rough on crucibles.
I have never used it.

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Thanks guys.
There is a lot of useful info there!
On the Borax issue I can confirm how badly it ruined one of my crucibles first time I tried it.
It leaves a gooey glassy residue which attacked the crucible material.
I have used it to coat a laboratory ceramic crucible as advised by the supplier to form a glassy skin but will avoid on foundry crucibles.
 

abby

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Obviously you do what you think best for your situation but I have plumbago crucibles that have done more than 100 melts using this flux that show no signs of degradation.
If you have a gooey mess than you are not hot enough , the flux will be very liquid at the correct heat and require a little dry sand to coagulate for removal.
Dan.
 
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Thanks Dan,
I will take onboard your expertise as my experimentations are pretty novice level.
Also, I have had mixed results due to cheap crucibles and now only buy Salamander.
Rich
 

abby

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A well known supplier of foundry chemicals and sundries is UK based (probably multi-national now) Foseco Ltd.
Their factory/warehouse is not far from me and when I started serious foundry work I purchased all of my requirements from them.
For brass melting and grain refining they recommended their CUPRIT 49 flux.
This white powder is placed in the bottom of the crucible and the metal added , then more flux is sprinkled over. It gives really good cast brass,
however it eats crucibles .
I was getting around 20 melts before the crucible was so thin that I had to throw it away. I believe fluorides in the flux were responsible.
For bronze they recommended their CUPREX flux and this was totally different , being a black briquette.
This flux was kind to my crucibles , Salamander are not cheap but safety comes at a cost ! and I got many more melts without detrimental effect.
These findings were from over 50 years ago and Foseco's products may well be different now , but my own concocted flux is as near to the cuprex as I could come to without laboratory analysis.
Bronzes are odd creatures and over the years I have experimented with many recipes , mainly for the resulting patinas on sculpture which was my main business interest.
Quite small differences in alloying additions can make huge differences to the physical properties which is important in real world engineering but quite frankly for most modelling use I really don't think it makes a lot of difference , except perhaps for bearings , and for the small amount we need it is probably much cheaper to buy a stick of PB than try to make it.

home made gun-metal casting

DSCF5168.jpg


unexpected flaw

DSCF5176.jpg


a pair of good castings

DSCF5167.jpg


Dan.​
 
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Very interesting Dan,
Was that an investment casting?
I agree it is often easier to buy a piece but how else do we have fun? LOL!
Maybe whipping myself with a Birchwood sapling would sometimes be more pleasurable than the frustrations in casting at times (grin).
Rich
 

abby

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I agree , casting is no.1 in my book ! opening every mould , or in my case flask , to see the results is an exciting experience.
Yes they are investment castings.
dan.
 

GreenTwin

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Looks sort of like a hot tear in that photo above.

There is so much to casting beside making the pattern and mold, and pouring in molten metal.

It has been my favorite hobby so far, but was not easy to learn, and the learning really never stops.

.
 
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