Lead Alloy Pours – Composition, Uses and Methods

This article is about the various alloys of lead, lead casting methods and lead alloy applications. The article also goes into some detail with respect to some important lead alloys and their uses.

The heating of metal to its liquid form and then pouring into a mold – better know as casting - is a common method of creating the basic or complex shapes used in numerous applications. This is definitely true for lead casting, which is used in many areas including lead bullets, lead fishing sinkers and large scale castings for ship ballast keels. Other uses include counter balance weights for elevator weights and lead casks for nuclear storage and transportation. Nuclear casks are discussed in more detail later in the article.

Because lead is very soft and ductile, it is often melted with other metals to create lead alloys with specific required properties, commonly with Antimony, Tin, Arsenic, Bismuth, Copper, and Zinc.

Lead antimony alloys provide additional hardness to the lead and are often used in storage battery grids, sheet lead and pipe castings. Antimony contents of lead-antimony alloys can range from 0.5 to 25%, but they are usually 2 to 5%.

Lead arsenic alloys are also used to increase hardness; arsenical lead is used for cable sheathing and is important in the production of round dropped lead shot.

Tin adds ductility and strength to lead. Lead alloyed with tin, bismuth, or other elements, either alone or in combination, forms alloys with particularly low melting points. Some of these alloys are referred to as fusible alloys; in particular lead tin alloys are used in lead solder and historically as pewter.

Several lead based alloys are referred to as Babbitt metal, lead Babbitt or bearing metal, and are used in the bearing surface on rotary bearings. Babbit is discussed later in the article.
Lead Casting Methods
There are several methods of casting lead alloys, depending on the type of piece to be made and the resources available to the person making the casting.
Sand Casting is one of the most popular and simplest types of casting that has been used for centuries. Castings can be made in a range of sizes from pieces that fit in the palm of your hand to entire train chassis. It can take several days to prepare a sand casting mold. The sand is bonded together using clays, chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little maintenance.
Investment Casting - (also known as lost-wax casting) is a process that has been practiced for thousands of years, originally beeswax was used but today modern specialized high technology waxes are used to create high-quality components.
Investment casting derives its name from the fact that the pattern is invested, or surrounded, with a refractory material. The wax pattern is carved into an exact replica of the piece to be produced and then is invested or surrounded by a refractory material. When the molten metal is poured into the mold the wax melts and the metal takes up its shape.
Permanent mold casting – is a metal casting process that employs reusable molds ("permanent molds"), usually made from metal. lead alloys are often cast in this manner
The die casting process forces molten metal mold cavities (which are machined into dies). The die casting method is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency.
Centrifugal casting - uses a temporary sand mold held in a spinning chamber.
Nuclear Casks
An important application for lead pours is for nuclear radiation casks, where the casting allows for a piece with no seams – except for the lid.
Nuclear casks are heavily lead shielded containers used to store and or ship radioactive materials. Such as spent nuclear fuel or other high-level radioactive waste. Casks are often made from lead, concrete, and steel. Casks must meet regulatory requirements and are not intended for long-term disposal in a repository.
Spent nuclear fuel shipping casks are used to transport spent nuclear fuel used in nuclear power plants and research reactors to disposal sites. Each shipping container is designed to maintain its integrity under normal transportation conditions and during hypothetical accident conditions. Casks are designed and tested to withstand crashes, fire, water immersion and puncture. To be certified, a cask design must withstand a sequence of four tests that measure its performance in specified crash and fire accident conditions.
Lead storage and lead containers are a critical aspect of nuclear containment and radiation shielding. Dry cask storage is a method of storing high-level radioactive waste, such as spent nuclear fuel that has already been cooled in the spent fuel pool for at least one year. The fuel is surrounded by inert gas inside a large container. These casks are typically steel enclosed lead cylinders that are either welded or bolted closed, the cylinder provides leak-tight containment of the spent fuel, and also provide radiation shielding to workers and members of the public.
Plastic shelled lead storage containers for radioactive material, known in the industry as 'Pigs', the internal container is a solid cast lead container fabricated for nuclear material containment. Safely store your uranium ore, radioactive Isotopes, and other radioactive samples.
Lead Babbit

The eponymous babbitt is named after Isaac Babbitt

Babbitt formulated a metal alloy in 1839 that was successfully used as the bearing surface in metal bearings. Also know as Babbitt metal, white metal or bearing metal, the exact formulation invented by Babbitt is not know, however several compositions have been created, common ones involve alloys of lead and are known as lead babbitt:

• 80% lead, 15% antimony, 5% tin

• 76% copper, 24% lead

• 75% lead, 10% tin

• 67% copper, 28% tin, 5% lead

Babbitt metal is often used as the surface layer in a the moving surface of multi-metal structure bearings. Babbitt metal is soft and easily damaged so it seems at first to be an unlikely material for bearings, however the detail structure of the metal consists of hard crystals embedded in softer material, when this material wears it creates lubricant paths within the bearing between the hard crystals that provide the actual bearing surface.

Traditional Babbitt bearings are cast in place around a shaft loosely embedded in a cast iron pillow block, through a hole that eventually becomes a lubricating hole. After casting the metal is trimmed back, lubricating oil ways are cut into the surface and the bearing is “run in” at low loads and revolution..

The expression a "run bearing" also derives from this style of bearing, since failure of lubrication will lead to heat build up due to friction in the bearing, eventually leading to the bearing metal liquefying and running out of the pillow block.

Before the advent of low cost electric motors, power was distributed through factories from a central engine via overhead shafts running in hundreds of Babbitt bearings.

Lead based babbitts are more economical and used for low speed, uniform speed and moderate pressure as in line shafting, electric motors and general machinery. These babbitts are also used in applications subjected to shock loads.

Conclusion

Lead alloys find many uses still today. In this article we have discussed many of the applications for and the composition of lead alloys. We have also discussed ways of producing lead casting molds and some specific uses for lead alloy pours.