Cast metals are different from other metals in a few key ways. First, the alloys used for cast parts are different from those used to make forged or machined parts. Cast iron or cast steel, for example, has a much higher component of carbon than rolled steel. This presents specific challenges when you need to weld or adapt cast parts. For example, you cannot readily heat and bend cast iron—it just cracks and falls apart. But you can weld cast parts with the proper equipment and techniques.
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Because they tend to be more brittle, cast parts have a tendency to crack and break. Therefore, most of this chapter is dedicated to fixing broken cast parts. At the end of the chapter, there is a short section on working with other metals, such as copper, lead, bronze, tin, and other alloys. While these metals cannot generally be welded, they are commonly used in brazing, soldering, bodywork, trim, and other automotive applications.
Fixing Broken Cast Parts
When cast parts break, they tend to break cleanly. That is, like a broken ceramic mug, you can fit the pieces back together and come up with a shape that’s very close to what you started with. This is good news for repair work, because you don’t have to deal with a bunch of bent parts. However, you may be dealing with cracks you can’t see, so when you think you’re done, have your cast parts checked for cracks at a good machine shop before you use them.
Exhaust manifolds are among the most common cracked or broken cast parts. These parts are subject to regular heat cycles and vibration in normal service. Some manifolds are made of very thin cast iron, and what iron is there can get worn even thinner over time. Additionally, there are flanges where the manifold bolts to the engine, and these are especially subject to breaks. Most exhaust manifolds are made from basic cast iron, and are comparatively easy to weld.
Engine blocks have an advantage over exhaust manifolds because they’re generally made from thicker castings and better materials are used. However, the application is correspondingly more critical, with precise machine tolerances required through most areas. Welding up engine block cracks caused by freezing water is within the capabilities of most amateurs, but always have your engine block checked for cracks by a professional before you use it. You may have fixed the obvious crack in the outer jacket, but if you missed a flaw in the crankshaft support area or a cylinder wall, you could waste a lot of money when that engine fails.
Many suspension components such as spindles, brake flanges, and steering knuckles are also made from cast iron or steel. In general, vintage cars are more likely to use cast parts. If your project involves boosting the performance of a vintage car, have a good look at all your steering and suspension parts. They were designed for less engine power and much-less-capable tires, and the extra stresses added by hot rodding can spell disaster. The good news is that these parts can be reinforced and in critical cases, replacements are available that have been machined from modern high-strength materials. As always, if you want to use old parts, have them professionally crackchecked before it counts.
Fixing a Broken Cast Iron Part
In this project, we’ll weld up a crack in a 50-year-old cast-iron exhaust manifold from a British sports car. The manifold is made from good quality thick material, so it’s a good candidate for repair. For cast iron, there are two main types of stick welding rod 55 percent and 99 percent nickel. The 99 percent rod leaves a softer material in the weld and is suitable for machining if you need a flat surface when you’re done. The 55 percent rod is a harder alloy and a little easier to weld with.
We chose 99 percent nickel stick welding rod for this repair, and used that rod with the stick welding attachment to a TIG welder. We set the TIG for DC operation and negative polarity, and used 75 amps of power.
Most cast-iron parts benefit from preheating before being welded. Heat the entire part uniformly to minimize the heat shear differential when you weld. You can preheat the part with an oxy-acetylene torch and a rosebud tip. Simply play the flame over the part until it’s all hot. Be sure you do not heat any part of the piece to more than a dull red glow when viewed in a dark room.
If you’re rejoining two parts that have broken apart, position them back in their original orientation. You can grind some bevels into them if they’re thick and penetration is going to be a problem. But especially for auto parts, you want to preserve the key dimensions.
1: When you weld up a cracked part, you put a great deal of heat into the part. You want to be sure you don’t cause the crack to propagate further as you fix it! So begin your repair by drilling two small holes at the ends of the crack. This helps relieve the internal stresses that caused the crack, and allows you to get the repair done without causing further damage. Also, sand or grind the surface “skin” away from the weld area. This skin tends to absorb impurities in the casting process, and can be hard to weld.
2: Check the packaging and documentation delivered with your welding rods and start with the lowest power rating suitable for the thickness of the weld. Start at one end and slowly work your way along the crack. If you have not preheated your piece, you should weld no more than 1/2 inch at a time, and then stop to let the temperature differentials in the piece normalize. Also, stop every inch or so and clean the slag and crust from your weld before continuing. This also gives you a chance to check that the crack is not widening or propagating.
3: Finally, weld up the hole at the far end of the crack and then grind or machine down the weld, if necessary. If you’re welding a part that must hold fluids or gases, plug one end and check for leaks. Small flaws in your weld can create holes in the material and cause leaks when the part is placed under pressure.
Aluminum is the most common non-ferrous metal found in automobiles. Engine blocks, cylinder heads, oil pans, bearing carriers, suspension and steering parts, body panels, and structural beams and other components may be made from cast or rolled aluminum.
Welding aluminum is challenging even for experienced welders. The most common method used to weld aluminum is a TIG, but you can also use a spool gun and Argon gas attached to your MIG welder. If you have an oxy-acetylene torch, you can also weld aluminum with that tool.
Working with a MIG welder equipped with Argon and a spool gun makes fast work of aluminum projects. You must use aluminum wire in the gun check with your welding supply shop for the correct material. If you choose a TIG welder for aluminum, the machine must be set for AC operation. Direct current TIG welding on aluminum leads to splatter problems. You also need aluminum TIG welding rods.
Cleanliness is even more important than usual when working aluminum, as the smallest bit of grease, dust, or flecks of ferrous metal causes pitting and slag in the weld. All aluminum parts to be welded should be carefully cleaned to eliminate any oil or grease, and then handled carefully thereafter.
If possible, use dedicated aluminum saw blades and files. This helps keep small flecks of iron or steel from becoming lodged in your aluminum. Even tiny particles of ferrous metals in your weld will contaminate your bead, causing pitting and a porous weld.
Welding an Aluminum Oil Pan
For this project we are adapting a BMW 2.5-liter oil pan for use in a 1.8-liter engine by removing about half an inch out of the middle of the pan. In addition to the cast aluminum upper pan, the stamped steel sump pan also needs modification. And when it’s all done, the two pans have to line up on their bolt holes, and the mating surfaces must be smooth enough to seal. This is a delicate job.
1: Carefully clean all parts in a washer, then bake the aluminum upper pan in an industrial oven to remove oil and grease. When they’re clean, mark the spot to make cuts.
2: Cut the aluminum upper pan with a dedicated aluminum saw blade, and the stamped steel pan with a metal-cutting bandsaw. Then use a dedicated aluminum file to smooth and cut the edges of the aluminum where you’re going to weld. Also, wipe down the edges again to keep things as clean as possible.
3: Clamp the aluminum parts to the steel table. This is important for two reasons. One is that you want the parts to stay perfectly aligned while you tack them together. The second reason is that aluminum has a tendency to warp under welding, and you want to make sure that the mating surface stays as flat as possible.
4: With the welder set at mediumhigh power, tack the two halves into place at several locations along the mating surface. Then fill in the entire length of the piece with weld, bit by bit. It’s important to let each segment cool before continuing to the next step to help prevent warping.
5: The aluminum welding technique is to move the arc and create a hot spot with molten aluminum, then push a bit of welding rod in to fill the gap, then move a couple of millimeters and repeat.
6: With the outside welded, file off any drips that have formed on the inside. Then repeat the welding bead along the inside—this part has to hold oil without leaking. Note the V-shaped notch filed at each end on the gasket mating surface. Build that up with welding rod, then file it flat and smooth.
7: When you’re done, take a straightedge and hold it up against the part. Despite our best efforts, we still got some warpage as the aluminum cooled. There are a couple ways to fix this. If there was enough material, we could take the part to a machine shop and have it machined flat, but since the sump pan only attaches one way and the upper pan is only slightly warped, we’ll just make sure the two halves fit each other.
8: Using a small sander/grinder, wire brush, and files, prepare the edges of the stamped steel sump pan for welding. You could do this part with a MIG welder, but we decided keep on with the TIG. Of course, you must switch to steel welding rod and use a lower level of DC power this time, because this part of the job is working with steel. To get a good fit, bolt the two pieces of the pan securely into their places. The important thing is that the bolt holes line up and the gasket mating surfaces are the same to seal in the oil.
9: As before, tack the two pieces together in several locations to fix the pieces in place without introducing warping. Stamped sheet steel may be very thin, so reduce the welding power. These tack welds also help make sure of good penetration without risk of burning up the thin steel of the pan.
10: Use the same heat-push-move technique to lay a bead along a sheet steel weld. Go over the pan from the outside, with it bolted firmly into place. Then take the pan off and repeat the bead on the inside to fill in any possible leaks.
11: Finally, file the mating surface of the steel sump pan smooth and make sure that everything bolts up cleanly to the aluminum upper pan. There’s a bit of flex in the stamped sheetmetal of the sump, so this should hold oil nicely, and the bolts all line up, too. It’s a job well done!
Working with Copper, Lead, Tin, Bronze and Other Alloys
Unless you’re working with a car from the first few years of the automotive era, there’s not a lot of call for bronze, copper, and other soft metals. But these materials do merit some mention and can be useful to the modern metalworker.
Copper sheet is useful for cutting custom gaskets and making certain decorative pieces. Pure copper is quite soft and forgiving, so you can form it cold without too much trouble. You can also use thicker copper and brass sheets to make engine dress-up items such as brackets to hold your ignition coil, TDC pointers for your crankshaft, and other non-stressed items.
Lead has a long history in automotive work it was used as the basic body filler before the invention of plastic based body fillers. Because lead melts at a little over 600 degrees F, you can melt it with a propane torch and let it fill a dent or gap, then easily file or sand it smooth. It’s very cool and old school to do bodywork with lead, but obviously take all due precautions as lead is poisonous to your brain and will accumulate in your body over time.
With older cars European cars especially you are likely to find some bushings and other components made of bronze. Bronze is an alloy of copper and tin and while it’s harder than copper, most of the time the bronze pieces you find will be hopelessly worn out. Replace them with new bushings made of nylon, Delrin, or aluminum. The original automaker used bronze because it was inexpensive and we have better materials now.
Brazing and Soldering
Occasionally you need to join unlike metals together. Usually this involves fasteners—you bolt the aluminum part to the steel part, or rivet the aluminum sheetmetal to the steel bodywork. But sometimes the interaction requires a watertight seal or just a connection that is more solid.
Most people have used a soldering iron on electrical components. The theory is simple you melt a soft metal filler material (usually an alloy of lead, tin, and other metals) and use it to glue a wire to a fixed point. The molten soldering metal flows and sticks to both parts and when it freezes, it has enough grip to hold the piece in place as long as no one pulls too hard on it.
Brazing and soldering are similar processes in that they create an attachment without melting either of the two (or more) parts that are being held in place by the braze. Brazing rods are typically made of bronze, brass, or other soft metal alloys.
If you look at metal under a microscope, it’s not as smooth as it appears to the naked eye. It has a rough surface with space between metal crystals and fibers. Molten brazing metals will flow and soak into those spaces and when the brazing material freezes, it holds to the surrounding metal.
You can take advantage of that property and use brazing to join aluminum to steel, for example, with a weld-like joint. The joint can be as strong as a weld, but make sure that the braze is cleaned, assembled, and performed well to get that strength.
Race car builders sometimes braze suspension parts onto a race car so that in the event of an accident, the braze is the part that breaks. If the suspension tears away in a crash without damaging the frame, it’s easy to repair! This requires a detailed knowledge of the stresses on the parts in normal operation and a high degree of confidence in the quality of the braze.
On many older cars, you can also see evidence that brazing has been used to fix cracks in sheetmetal and to fill small holes. This was popular among amateurs for many years because brazing was easier and less expensive than a welding fix.
More often, brazing has been used to assemble radiators, oil coolers, and other tanks that require a fluid-tight seal. But the process has lost some favor with the advent of widespread and affordable TIG welding for aluminum.
To braze a joint, both parts must be absolutely clean. Wire brushing the parts at the mating surfaces helps, too. The two parts must be mated as closely as possible. Brazing is not as good for filling gaps as welding, because the strength of the braze depends on getting as much grip into the surrounding metal as possible.
A corollary of getting the grip in the parts to be brazed is that you need to have an ample mating surface. If you try to braze a butt joint between two thin pieces of metal, you won’t get a strong joint because there’s very little metal to take the braze just a square millimeter or two. So, always plan to overlap your parts when brazing. The braze soaks into the joint between the two parts and that’s what makes it strong. You can also braze fittings (such as aluminum and plumbing fittings) onto brass or aluminum tanks. The mating surface isn’t large, but then again, it’s an unstressed joint.
1: Get two pieces of aluminum and clean them thoroughly. Give them a little wire brush treatment to rough up their surfaces. Make sure that they fit well together—two pieces of flat 1/8-inch plate work well for brazing.
2: Set the pieces of aluminum up on your welding table and put some aluminum brazing flux powder on them. Overlap them by at least an inch. Make sure they’re closely mated with no gaps!
3: Start a braze by heating the surrounding metal up to the melting point of the brazing rod, but not any higher! You’ll know it’s right when the flux melts and starts to flow. The aluminum itself is far from melting at this point.
4: Dip your aluminum brazing rod in a jar of flux and feed a little into the hot area. It should melt and create a puddle. Let the puddle flow into the joint and fill the area. Then move on as if you were gas-welding the joint. The molten rod will actually be sucked into the braze joint.
5: Turn your project over and braze the lap joint on the other side in the same way. Then let the parts cool normally and test your braze.
Written by Russell Nyberg & Jeffery Zurschmeide and Posted with Permission of CarTechBooks