Image Credit: H.C. Starck, ATI Wah Chang
Master alloys and alloying additives are alloy-element concentrates, grain refiners, hardeners, deoxidants, and other agents added to a melt or metal-powder blend to produce a particular alloy, modify a melt, or alter processing characteristics. They are designed to improve processes in metallurgy by refining the final properties of different metals. Alloying agents and elements are used as additions to modify or produce alloys in primary metal production melts, foundry melts, and powder metal-blend manufacturing.
Benefits of Master Alloys and Alloying Additives
A master alloy is a pre-alloyed concentrate or mixture of alloying elements. They are used to add all the major alloying elements in one form to the base metal. For example, a 10% addition of 60% aluminum - 40% vanadium composition master alloy to 90% pure titanium will produce Ti-6Al-4V alloy.
Master alloys are essential for adding small amounts of compounds with large melting point differentials. This is because adding a virgin metal to a melt can be inefficient in terms of energy and metal yield. For example, a small addition of pure copper (1083°C melting point) to a large mass of aluminum (660°C melting point) would require heating all the aluminum above 1083°C. A 50-50 master alloy of copper and aluminum eliminates unnecessary heating of the aluminum, allowing the correct percentage of this alloy to be added to the bulk of aluminum at a much lower temperature.
Master alloys are also the most effective method of introducing controlled levels of rare earth metals to more complex alloy systems. They help to control oxygen content within the alloy, stabilizing metals that would normally oxidize in the atmosphere.
Additives and Modifiers
An alloying additive, agent, or modifier any alloy or element combined with a base metal to impart additional specialized characteristics, or to enhance properties. Master alloys are modifiers that contain one or more additives. Additives are pure metal constituents added to a melt to change or enhance certain characteristics, such as manganese or chromium metal additions for steel, which are added to increase hardness and strength.
When selecting a master alloy or alloying additives, industrial buyers need to consider the type of master alloy or alloying agent, its specific function as an additive or modifier, its form, and its purity.
A typical product specification chart for a master alloy or alloy system will list the metals and their percent composition as either a range or maximum.
Table 1 - Product specification chart for an aluminum-based alloy. Table Credit: Reading Alloys.
When working with a specific metal, it is important to know what materials can be alloyed with it to ensure proper alloy selection.
Aluminum is a lightweight, non-ferrous metal with good corrosion resistance, ductility, and strength. Typical alloyants for aluminum include copper, magnesium, manganese, silicon, and zinc. Other elements that can be alloyed with aluminum include nickel, tin, vanadium, molybdenum, chromium, titanium, antimony, bismuth, boron, calcium, and cobalt.
Iron is a soft, magnetic metal. It can be alloyed with carbon in various amounts to make steel. Alloys typically added in addition to carbon include manganese, chromium, molybdenum, vanadium, tungsten, cobalt, silicon, and nickel. Other elements that can be alloyed with iron include titanium, copper, calcium, rare earth metals, zirconium, niobium, selenium, tantalum, sulfur, phosphorous, and nitrogen.
Cobalt is a non-ferrous magnetic metal with high strength and toughness, excellent corrosion and oxidation resistance, and high temperature strength. Alloys typically added to cobalt include chromium, nickel, molybdenum, iron, tungsten, aluminum, manganese, silicon, and titanium. Other elements that can be alloyed with cobalt include vanadium, boron, sulfur, boron, carbon, and phosphorous.
Copper is a soft, conductive, non-ferrous metal with good corrosion resistance, ductility, and strength. Main copper alloyants include zinc (brass alloys), tin (bronze alloys), aluminum, nickel, tungsten, gold, silver, and manganese. Other alloyants of copper include lead, arsenic, antimony, bismuth, chromium, cobalt, iron, magnesium, silicon, phosphorous, titanium, vanadium, and zirconium.
Magnesium is a low density, non-ferrous metal with good ductility, moderate strength, and good corrosion resistance. Primary magnesium alloyants include aluminum, zinc, copper, silver, and zirconium.
Molybdenum is a refractory metal with avery high melting point and a relatively high density. As a base metal, molybdenum is typically alloyed with titanium, zirconium, and carbon. It also can be alloyed with tungsten.
Nickel is a non-ferrous metal with high strength and toughness, excellent corrosion resistance, and superior elevated temperature properties. Typical nickel alloyants include chromium, copper, aluminum, manganese, molybdenum, silicon, iron, zinc, silicon, cobalt, and magnesium. Other possible alloyants for nickel include tungsten, titanium, carbon, niobium, phosphorus, tantalum, lanthanum, boron, and calcium.
Titanium is a non-ferrous metal with excellent corrosion resistance, good fatigue properties, and a high strength-to-weight ratio. Elements in titanium-based alloys typically include aluminum, tin, molybdenum, and vanadium and can also include chromium, manganese, selenium, iron, silicon, columbium, tantalum, zirconium, and nitrogen.
Zinc is a moderately low melting, non-ferrous metal widely used in the production of die cast components. Primary alloyants of zinc include aluminum, tin, magnesium, copper, and iron. Other possible alloying elements are lead, cadmium, silicon, cerium, antimony, cobalt, titanium, and vanadium.
For more detailed information on the properties of different types of metals and alloys, please visit the Metals and Alloys section on GlobalSpec.
Alloys added to a base metal melt or powder can be classified as either master alloys or alloying additives. These products can further be distinguished and selected by the specific function they perform.
Modifiers include any materials added to enhance or alter certain properties in a melt or alloy. Properties include corrosion resistance, fluidity, machinability, ductility, stability, strength, hardness, and weldability.
Microalloying modifiers are additions added in small amounts that have a significant impact on properties or processing. For example, rare earth element (RE) additions in small amounts can dramatically alter surface energy and melt penetration in a weld pool,refinement of the grain structure, and inclusion morphology.
Deoxidizing or killing agents such as aluminum, silicon, or calcium are added into a steel or iron alloy melt to absorb the melt, resulting in the absence of gas release during the solidification process. In steel production, killing stops any additional reaction between the carbon and oxygen, preserving the carbon content desired. Rimmed, unkilled, capped, or semi-killed steels will have greater variation in composition across the cast ingot.
Desulfurizing agents reduce or control the morphology of sulfur in a steel or alloy. Sulfur tends to wet the grain boundaries, resultingin hot shortness or brittle condition steel or iron alloys during hot working processes. Manganese additions to steel or iron alloys form manganese sulfides, which are nodular in nature resulting in strengthening of the grain boundaries.
Grain refiners, nucleants, or nucleating agents promote rapid crystallization and a finer crystal size by providing many points for crystals to start growing during solidification.
Inoculants, nodulizers,or spheroidizers are added to cast irons to form spherical shaped graphite nodules, resulting in ductile cast iron. Without nebulization, graphite flakes form resulting ina more brittle cast iron.
Alloy Form and Quantity
Figure 1 - Depiction of various final master alloy forms, including waffle plate and powder.
Image Credit: Affilips N.V.
Master alloys and alloying additives generally are bought from the manufacturer as waffle plate, cast ingots, cut ingot pieces of varying sizes, or powders based on the requirements of the customer. Industrial buyers should consider the most convenient product form for their system or process.
The quantity of the order will also need to be considered. Some larger manufacturers may have a minimum production requirement, while some smaller companies may be limited by their output rates.
Purity and Quality
Purity is a measure of how well a metal or alloy has been refined. Any excess element in an alloy can be considered an impurity. Products will generally contain some small levels of impurities such as oxygen or sulfur. While these levels are sometimes insignificant, they should be monitored when producing or selecting an alloy, since trace amounts of some elements can have significant effects on the alloy's quality and material properties. For example, carbon steel becomes "red-short" with excess sulfur, making it brittle.
The overall quality of a master alloy is largely dependent on the quality of the raw materials used in its production. Purity and quality are oftentimes assured through a quality management system, which can be certified to standards such as ISO9001: 2000. Some companies may offer conformance forms for each product batch detailing the chemical analysis results according to the specifications requested by the client.