Bronze casting question

[Bruce Hooke]

I have a bronze casting that has some small pits in it that are too deep to grind out (they are pinhead size in diameter but fairly deep). They appear to be where the runners (where the molten bronze flowed from one casting to the next one) connected to the castings. What I am wondering is:

A. Is there a reasonable and reliable way to fix these pits?

and

B. Is the foundry not doing something right or is this just something that happens with bronze casting?

Thanks.
- Bruce

[stef]

Hi Bruce:

Based on your description of the defect it is difficult to comment on what it could be with out further information.

How big is the casting?

What is it?

Where is the defect?

A photo or sketch would be nice as well.

There could be any number of reasons for the defect you described thus the additional questions. This will also allow for you to develop a repair/removal strategy or to disscuss the issue with your foundry.

Stef

[Bruce Hooke]

Stef,

Thanks. The casting is about 4" long x 1 3/8" diameter and has walls that are about 1/8" to 5/32" thick except at the tip. It weighs about 10 ounces. It's a canoe setting pole shoe. Here's a picture of one (this picture does not show the defect):

http://www.bghooke.com/images/shoe-1.jpg

I should have a picture of the actual defect when I get the next roll of film back in a week or so. The defect is near the pointed end but still on the side rather than on the rounded point. I uncovered it when I was grinding off the remains of the runner. It's actually a small cluster of pits that I could almost get away with just grinding off, but there is one deeper hole that is about 0.05" in diameter and 0.05" deep.

Here's a cross section of the casting:

http://www.bghooke.com/brucehooke/Images/shoepolesection.gif

[Jon Etheredge]

You can get bronze brazing rod and fill the pits. A welding supply can probably help you to choose an appropriate filler rod.

[stef]

Hi Bruce:

It's a nice looking piece of work. How long is the finished unit?

If you are comfortable with brazing Jon's suggestion would be the way to go. If you still have the casting runner you already have the correct alloy available all you need is some flux.

Another method would be to drill out the area and put a bronze bolt in prior to grinding flush.

Yet another way would be bronze powder addition to wooden boat builder's wonder googe as a fill to the hole. They make art castings this way.

The first method gives you a structurally sound repair which is how "professionals" would repair similar defects in larger castings. The other methods are more cosmetic but would give a serviceable repair.

With respect to the casting and defect. Based on your sketch, photo, and description, there could be several reasons for the defect. 1. Small gating leading to inadequate feeding of the casting. 2. Inadequate riser of casting metal feeding the casting. 3. To low a superheat on the casting metal. 4. Gas in metal. Depending on the orientation of the casting and the casting method some of these will not apply and other reasons will become evident. Without seeing the defect or knowing the casting method and practice, I would guess that #1 is the most probable mechanism with #3 a close second.

Stef

[Bob Smalser]

If it's not structural, the simplest way to do it is to fill the holes with soft solder @ 450 degrees.

If it is structural, then silver solder @ 1100 degrees will provide a 40kpsi fix. Be careful heating to those temps, however, as severe overheating can melt the bronze.

Welding supply houses and home stores have the MAPP gas and solder/flux required.

I recently turned down the shafts of a pair of bronze oarlocks on a lathe, and had to drill a hole for the tailstock mounting to do it....I simply filled the hole with soft solder and its practically invisible.

[Bruce Hooke]

Thank you! It sounds like brazing is the best way to go (plus talking to the foundry to see if we can avoid this in the future). I want to do this right because I am selling these shoes. I dug out WoodenBoat 115, which has an article about making your own brass and bronze hardware using brazing techniques and brazing does not sound that hard. I've never done it but I have soldered lots of stuff and I have also done some arc welding so I should be able to figure it out.

I am also happy to hear that the likely causes sound like they should not be that hard to avoid on the next pour so hopefully the next time around I will not have this problem to deal with at all...

The finished unit is 4 1/2" long.

[Paul Scheuer]

I've been told that internal voids are inherent in the casting process. When I had my jewelry collection stripped, I found lots of scarry pits. Silver solder worked well to fill the voids, and didn't show at all when the pieces were buffed. Some of the cast metal seemed to transport to the silver. A filled 0.050" hole should be invisible.

[Todd Bradshaw]

There is also a market for canoe rudder gudgeons and leeboard bracket parts if you get to the point where you want to expand your line of offerings.

[NormMessinger]

Musings from the mostly ignorant (me): The big steel casting foundary I toured here in Omaha a while back has a whole section devoted to fixing flaws in castings. They were welding fill metal in to truck differentials when I visited. Only a small percentage of the castings required repair. On the smaller pieces, they were producing hardware for power poles, they just recast those few that did not meet specs.

Braze welding is simple enough that even I can do it. A class at the local community college made it so. Flow on enough fill metal (the grinder is your friend) and a useable part can be made. The difficulty is matching color.

I guess if I were going to repair the flaw in the casting in question I would grind out the flaw to where I had good metal all around, braze weld, grind and polish. Bob's methods would be fine form my personal use if I couldn't braze but I wouldn't want to purchase such a piece unless the price was right.

[Bruce Hooke]

Todd,

That sounds interesting. Do you know where I could track down details on what the parts in question should look like?

Norm,

Well, I'm glad to hear that at least the big guys have to deal with similar issues. I agree that while a solder fix would be fine for my own use it's not something I'd be happy buying or selling, except maybe as a clearly marked second, but I'd rather not go down that road.

I think the next step is to talk to the foundry and find out if this is likely to be a regular issue. If it's not then I may just keep the couple of castings that have this defect around for when someone asks for a second or for my own use. If it is going to be a regular problem then it will be more important for me to get good at brazing.

- Bruce

[Bob Smalser]

Many silver solder/brazes have a slight copper hue when buffed....given the other color irregularities common in bronze, I don't believe the casual observer would notice the repair.

http://www.brownells.com/aspx/NS/Store/ProductDetail.aspx?p=6647&title=SILVER+SOLDER+PASTES+FOR+THE+PRACTICAL+GUNWO RKER

http://www.brownells.com/Images/Products/322100650.jpg

The gap-filling flavor of this 1100-degree silver braze ground in flux is the cat's meow for this sort of thing using MAPP gas....so simple to use, any (fill in somebody you have in mind) can do it.

[Dan Miller]

Bruce,

I have a page on my website that shows Old Town sailing canoe hardware: http://www.dragonflycanoe.com/otacc/index.html

Unfortunately, the rudder gudgeons are not shown well, but we could rectify that... I have standing orders for gudgeons, goosenecks, tail nuts (lever bolts), hook bolts and thumb screws. These are the parts that are always missing! So far I haven't had much trouble getting vintage leeboard brackets, so if you do them, they'll need to be in the $100/pair range, not the $200/pair that other repros are currently going for.

If you do try to do these, let me know - I may be able to provide originals for patterns and place a good order.

Cheers,
Dan Miller
Dragonfly Canoe Works

[Todd Bradshaw]

Of the half dozen or so canoe gudgeons in my book these are the ones that most folks would like to have.
http://www.imagestation.com/picture/sraid133/p8d260d8b870437ddbd7408ca5d786310/f78ed9ae.jpg
http://www.imagestation.com/picture/sraid133/pf6457715e2747874dd4c6882a6d90e43/f78ed98f.jpg

Bronze angle brackets for the ends of leeboard crossbars would certainly be nice instead of the standard aluminum ones and I'd still like to try my cam-operated leeboard clamp idea.
http://www.imagestation.com/picture/sraid133/pc788a2c588481468ca65df91634daf6d/f78ed97b.jpg
http://www.imagestation.com/picture/sraid133/p962672e76d1b7d8d1e5e662507360bdb/f78ed95e.jpg

[Bruce Hooke]

Thanks Todd and Dan. These do sound like some ideas worth investigating. It seems like the first step is for me to get Todd's book, something I've been meaning to do for a couple of years now. So, I just ordered it through my local bookstore. Then I'll have to think through these parts, how hard they would be make patterns for and finish after casting, etc.

So, I'll let you know what I figure out and decide when I've had a chance to work through all of this...

- Bruce

[Bruce Hooke]

OK, I've got an update to add...

First off, here's a picture of one of the more extensive defects:
http://www.bghooke.com/brucehooke/Images/Shoe_Defect.jpg

Now, I went to my local welders supply store and got some bronze filler rod and some flux to go with it. The guy there who knew what he was talking about seemed to think that I was headed in the right direction but suspected that a propane torch would not be hot enough. This proved to be true. Even with the shoe sitting on top of a gas burner on my stove I could not get the rod to melt. As a final test I even point the torch flame right at the rod rather than heating the shoe and letting that try to melt the rod. Everything turned a nice glowing orange color, the flux glazed over very nicely but the rod just sat there. So, I think I need a hotter setup, which ideally probably means an oxyacetylene system. However, before I go down that route I am going to wait until the foundry reopens in a week (after their summer break) and see what they have to say. They have been very helpful throughout this process and I've certainly learned a lot about casting, patternmaking and lots of related topics!

Thanks again for the great advice I've received from you all!

[NormMessinger]

Bob mentioned MAP gas above. It might be hot enough but I wonder having never used the stuff.

[Bob Smalser]

MAPP gas is made for that job.....

....acetylene can be too dirty without some practice at it.

[Bruce Hooke]

Thank you for the clarification on MAPP gas torches. I was not familiar with MAPP gas. I purchased a MAPP gas torch today with a swirl tip...

Now, here's the latest update -- I tried the MAPP torch and despite keeping it on the casting for a long time I could never get the rod to really melt. Small bits of the rod would break off and get stuck to the surface with the flux but I was expecting the rod to melt and flow like solder...isn't that what should be happening?

One other thing I'm confused about is isn't the rod pretty much the same material as the casting? If so, won't the casting be melting when the rod melts -- if I can ever get the rod to melt?

Thanks!

[stef]

Bruce:

The point in "bronze brazing" in steel systems is to have a higher melting point and stronger alloy than "solder", while not melting (welding) the steel. Well, with bronze:bronze systems you are actually welding the materials. You probably do not have enough energy output to melt the casting which is thermally massive relative to the rod.

If you can get the bronze rod to melt in your mapp torch you are half way there. I would practice getting the rod to melt by placing it in different positions in the flame front ie tip, middle, edge. This will give you an idea how long it takes to hit the melting point of the alloy and where your hottest flame is. When you are comfortable with this then get ready to try it with the casting in the same spot of the flame front for a minute or more before you put the rod into the picture for melting. You may also have to get a separate flux to apply to the casting.

:cool: Another tip would be to use cutting glasses to better see what is happening in the flame front area. :cool:

Based on your descriptions so far with the defect in the massive tip of the castng you would have little danger of melting it. I would be very concerned over a defect repair in the thinner regions of the casting. :eek:

Good luck,

Stef

[Bruce Hooke]

Thanks Stef. I am using shade 3 glasses, which do help a bit with seeing what's going on, as well as providing some protection for my eyes.

I'll give your suggestions a try. When it actually comes to making the repair, how much should I be trying to melt the rod using heat from the casting and how much should I just be putting the torch flame on the rod to melt it? Thanks!

[Ross M]

Bruce

A sound repair absolutely requires that the workpiece be hot enough to melt the filler.

The heat source is NEVER used directly on the filler. In fact, I would not even heat the pits directly, but rather play the torch in circle around them. This minimizes the level of oddball oxides that your flux must deal with.

"Is the foundry not doing something right or is this just something that happens with bronze casting?"

Absolutely - but that is not to say that elimination of porosity is easy or cheap.

There are about a dozen key causes, but probably the most common is gating and riser errors. These passageways to and from the void that forms your part MUST allow for smooth steady flow into the bottom and easy movement of displaced gases upward. (rereading the thread, Stef said it best).

Also, the severe change in cross-sectional thickness is not helping your foundryman at all.

Somewhere in the 600's section of your library there are bound to be good books.

Best wishes,

Ross

[Bruce Hooke]

Thanks Ross. What you are saying about how the heat should be applied fits with what I thought, so that's good to know. I am a bit puzzled about how the casting can get hot enough to melt a rod of same material without the casting also melting, but I'll trust that it works.

I am also not convinced now that the MAPP torch puts out enough heat to do the job, but I'll mess around with it a bit more tomorrow...I like Stef's idea of using the rod to get a better feel for the torch and where it is hotest.

[Bill Perkins]

Now that you know the location and depth of the pitting the casting process produces , I wonder if it would be most cost effective to increase the dimension of the casting at that point accordingly and then grind down ? Might the problem be lessened or disappear with extra thickness at that transition point ? My sense is that a more gradual transition in cross sectional area on the interior of the plug would also help .

Since you have the Mapp torch , I hope you'll try the solder Bob mentioned that's apparently designed to work in that temperature range .I'm thinking of going this route , and would be interested to see your results . It's certainly strong enough .

[ 08-09-2004, 11:28 PM: Message edited by: Bill Perkins ]

[Ross M]

"I am a bit puzzled about how the casting can get hot enough to melt a rod of same material without the casting also melting, but I'll trust that it works."

I am not certain either - I take it for granted that you welding supplies man sold you an alloy having a slightly lower melting point than the parent metal. This may or may not be the case.

If you know your parent metal ID fairly closely, I could find a good filler for you. If you know the ID of the filler as well, we might find your man was correct.

The absolute last word on brazing technique and supplies, IMHO, is the Handy & Harmon company. You might Google them.

Back to the casting vendor: Porosity is not inevitable. Pratt & Whitney knows people who can cast turbine blades, fir-tree root and all, of the meanest alloys on earth, as a single crystal. Really.

Ross

PS: Those blades, roughly the same weight as your object, probably cost $5k each... horses for courses

[Bob Smalser]

I wish you'd switch to 1100-degree silver braze, Bruce.

If MAPP won't melt that rod, then you are approaching the melting point of your casting....it takes some experience in heat control to "weld" bronze, or even to "braze" it with lower temperature rod.

Silver braze is a good compromize....and looking at your cosmetic application, soft solder would serve just as well and be a whole lote easier at 450 degrees.

[Bruce Hooke]

Originally posted by Bill Perkins:
Now that you know the location and depth of the pitting the casting process produces , I wonder if it would be most cost effective to increase the dimension of the casting at that point and then grind down ? Might the problem disappear with extra thickness at that transition point ?Some of the pits are over 1/8" deep so increasing the size of the casting would be tough. Also, it's looking like there may be hidden voids that would be exposed by deep grinding...



Since you have the Mapp torch , I hope you'll try the solder Bob mentioned that's apparently designed to work in that temperature range .I'm thinking of going this route , and would be interested to see your results . It's certainly strong enough .I think I'm going to keep going down the route I'm currently on for a while but if I do try the solder Bob mentioned I will let you know how it works. The color of the metal in the castings is pretty uniform so I have a feeling any variation would stand out, but I do have some other ideas for which that solder might be just the ticket.

[Bruce Hooke]

Well, I've been digging a bit more on the web. It seems like there are two standard silicon bronze casting alloys, both of which have the same melting and pouring temperatures. The best information I found was on the Atlas Metal website (http://www.atlasmetal.com/). On that basis, it seems unlikely that the bronze rod I got would be "off" in temperature (note: I did not know and the welding supply guy did not ask about specific alloys), but I suspect this also means that the rod melts at pretty near the temperature of the casting.

I think part of why this may work at all is because of the spread in temps between when silcon bronze starts to melt and when it is fully liquid. The melting points listed on the Atlas Metal website are:

Melting Point (Liquidus) 1790 F
Melting Point (Solidus) 1590 F

If nothing else I'm figuring out that what I am trying to do is not easy so I really need to work with the foundry to cut down on these problems!

Bob, the reason I am holding off on going to a lower temperature solution is because I am selling these castings so I really feel like I need to get them right, rather than just almost right. Of course I may be a bit bullheaded too... :D

In any case, I think I need to stop pushing at this problem until I can talk to the foundry...

There appears to be a bunch of information on the Handy & Harman Website: http://www.handyharman.com/ but it's getting too late tonight for me to absorb more information.

[Ross M]

"Also, it's looking like there may be hidden voids that would be exposed by deep grinding..."

Be aware - the best possible filling operation is going to give you a product that corrodes (develops patina) slightly differently at the repair. If this is detectable, you may pay a price with your reputation. 5 years after the fact.

This, and your statement I quoted above, really makes me want to say "Talk to your casting vendor. Ask what it would cost to eliminate this condition."

Make it clear, on the improved mold purchase order itself, that the purchase order is contigent on elimination of porosity in the finished part; as evidenced by XX parts produced at a cost not to exceed XX each. Or somesuch.

If he will not fix a cost, find yourself a new vendor.

This is just an opinion,of course, and your mileage may vary.

Ross

[Bruce Hooke]

Thanks Ross. Yet more reasons to get this right from the start. I suspect that when I show the castings to the foundry owner he will be eager to fix the problem. These castings were made when he was away on a trip and so he never saw them, which may be part of the problem...

[stef]

Hi Bruce:

With respect to your welding abilities I do not think you could melt the casting. It will be radiating heat faster than you would be putting it in. Unless you are using a large tourch.

The trick/art is to get the area of the repair to the liquidus temperature quickly and add your filler and get out. The area to be repaired has to be hot enough to fuse with the filler metal and also has to be clean so as not to impede the fusing process.

I have reviewed your earlier posts and noted that you had already used a preheat system while trying to get the propane to work. This is a good thing to be doing. Have you tried it with the MAPP?

If you have access to an oxyfuel system it is quite easy to have the energy input and temperatures to do this. The caloric value of the flame system is the same only the rate of energy output and flame temperature are much higher( I won't bore you with the details ) but there are some warnings. I have noticed several mentions of not using OXY/MAPP with high >60% copper which is similar to restrictions placed on oxy/acetylene systems. This is also why knowing the alloy used can be important.

After pondering you photo of the defect it should be relatively easy to address in subsequent castings. As you are discovering the cost in time and money to repair a casting defect can easily exceed the value of the unit. It is always best to do it right the first time.

If you are interested here is a link to a cu-sn phase diagram (http://www.metallurgy.nist.gov/phase/solder/cusn.html) which if you know the compostion of your bronze you can find the solidus and liquidus temperatures among other metallurgical tidbits.

Another interesting item, metallurgically speaking is the actual crystal structure of the metal in the casting. This is a link to some photo micrographs (http://www.copper.org/resources/properties/microstructure/phos_bronze.html) of bronze in various forms. Notice the scale of the structures in the photographs and the range of alloys present.

Earlier in the thread I mentioned bronze powder (http://www.tapplastics.com/shop/product.php?pid=76). This is the type of product I wast mentioning. It is used to make art :( castings with resin systems.

I hope I have not muddied the waters any further.

Stef