Melt treatments of copper alloys.

AFS Copper Alloy Technology Transfer Committee (3-H)

In casting copper alloys, melt treatment plays an important role in the quality of the final casting. Specifically, gas absorption and oxidation during melting and pouring must be carefully controlled to avoid porosity defects, misruns, poor surface, inferior mechanical properties, etc. Consequently, most copper alloy foundries practice some form of melt covering to control absorption and oxidation during melting, and deoxidation/degassing techniques in the furnace or transfer ladle just prior to casting.

Many of these techniques were summarized in a 1989 AFS Transactions paper. Much of the melt treatment technology for copper alloys, however, evolved during the era of crucible melting and "wheelbarrow" foundries, and the questions arose whether today's modern, induction-furnace-based foundries still used these same techniques. Furthermore, the need to control impurities and defects has increased dramatically in today's modern competitive marketplace.

With these thoughts in mind, the Molten Metal Process and Technology Transfer Committees of the AFS Copper Alloy Div., via two surveys, canvassed U.S. foundries to discern which practices were in use today. More than 400 foundries were surveyed, and response rate was 10-15%. Following are the specific issues addressed: alloy, melt cover used, how much, how often replaced, furnace size, furnace type, charging practice, skimming practice, results and gas control.

The questions asked in the deoxidation survey were specific to which deoxidants and degassifters were employed (phosphorus copper, calcium boride, lithium, etc.), amounts, where and how added, results, analysis, tracing erratic results to specific causes, etc.

Geographically, most responses came from Midwestern copper-base foundries, as might be expected, although foundries in the South, East and Far West were also represented. The most surprising result of both surveys was simply that there was no real surprise. Today's melt treatment practices are still similar to those of yesteryear - typical melt covers are employed to reduce melt oxidation and limit gas absorption, and deoxidation/degassing is still a must. If anything changed, it's that the traditional practice of deliberate melt oxidation followed by deoxidation formerly used in crucible melting to avoid excessive hydrogen pickup from fuel-fired furnaces is no longer in vogue, as most foundries now operate with induction melters.

Melt Cover

Most respondents producing the red brasses, coppers, and manganese bronzes either used graphite or charcoal as a melt cover, or added nothing at all. When carbonaceous covers are used, care must be exercised over the particle size grading and impurity levels (ash, moisture, other) to minimize quality problems and melt losses.

On aluminum bronzes, phosphorus bronzes and some red brasses, crushed glass or proprietary chemical flux covers were used. In all cases, only enough material to cover the bath was added. While some foundries replaced this each heat, others simply replenished the cover as needed during the day to maintain an adequate cover. Melt losses ranged from 1-6%, undoubtedly related to the percentage of returns being re-melted in the charge, furnace temperatures maintained and size of casting being poured.

Copper alloy foundries should: use the melt cover that is appropriate for the specific alloy; control the amount used; monitor and control the impurity content of carbonaceous melt covers; and optimize cover use, skimming practice and melt loss in the skim commensurate with the melt quality necessary to produce specific castings.

Deoxidation

Most foundries routinely use 1-2 oz phosphorus copper shot per 100 lb melt to deoxidize red brasses and tin bronzes. While occasional chemical analysis of residual phosphorous is conducted, most foundries rely on observations of resultant gas porosity, swells, fluidity and fractures to determine if deoxidation has been adequate. Lithium is a common deoxidant/degasser for the high copper alloys and again, little analysis is performed. Rather, it is the observed activity in the ladle, piping in the riser, increased elongation and improved fluidity, combined with a green, glassy metal appearance, that is most used to signal effectiveness.

Calcium boride is also routinely used to produce high conductivity copper alloys. Actual measurements of electrical conductivity, observed shrink, elongation values and "feel" (vibration in a carbon rod in the melt) is used as a floor technique to qualitatively determine oxygen content. The normal addition rate is 2-3 oz of calcium boride per 100 lb of melt.

Other deoxidants/degassifiers include special copper-aluminum alloys, manganese and magnesium for nickel-bearing coppers, zincs and proprietary fluxes, including intermetallics for aluminum bronzes. These are usually plunged into the furnace or transfer ladle just before pouring. Allowances must be made for the full reactivity of the deoxidant followed by adequate skimming of the slag or reaction products from the surface of the bath before pouring.

In some cases, argon or nitrogen is used in a lance application for degassing. In this case, the purge gas collects hydrogen, and to a certain extent, precipitated oxide phases from the melt. Rotary impeller degassing, which is common in aluminum foundries, is not as of yet utilized in copper alloy foundries to any great extent.

Deoxidation practices should be conscientiously applied for desired results, and those results should be measured and analyzed to optimize the effectiveness of such treatment. Practices in the AFS Casting Copper-Base Alloys handbook should be followed as applicable.