Forging Materials: Aluminum Alloys
Aluminum forging alloys are aluminum with small additions of other elements that enhance the final properties of the component. Aluminum forgings are normally produced in high volume. Because of aluminum’s low density relative to steel, it is not surprising that aluminum alloys are used extensively in industries where component weight is important. Consequently, aluminum forging alloys are used primarily in automotive and aerospace applications.
Figure 3. Typical flow stress for aluminum alloys
Chemistry and Grades
The major element in aluminum forging alloys is, of course, aluminum. Other alloying elements are added to enhance the properties. Copper is added from 0.1 to 5.0%. The copper can form an intermetallic compound with aluminum and produce fine precipitates in the forging if it is properly heat treated. These fine precipitates can produce a metal with higher strength. The forging-alloy grades with copper as the primary addition are 2014, 2025, 2219 and 2618. Magnesium is sometimes added to the 2000 series alloys to enhance the strengthening of the alloy after heat treatment.
Magnesium is also the major alloying element in the forging alloy 5083. It provides strength to the alloy without major loss in ductility. Magnesium (about 1%) in combination with silicon (about 0.75%) and a small amount of copper (0.3%) are used in the 6061 aluminum alloy. The combination of these three alloy additions allows good strength to be achieved after heat treatment.
The most forgeable alloy is 6061 aluminum. It has relatively low flow strength at forging temperatures (Figure 3) and can fill cavities fairly easily. The series of common forgeable aluminum alloys are the ones in the 7xxx alloy series, primarily 7010, 7039, 7049, 7050, 7075 and 7079. These grades are alloyed with zinc (5-8%) along with small additions of copper and magnesium. Small amounts of chromium and manganese are also part of the alloy chemistry. These 7xxx alloys, when properly heat treated, are commercial grades that can achieve the highest strength of all forged aluminum alloys.
The grade of the aluminum alloy is often appended with what is called a temper designation. The designation is normally a letter followed by a number. For example, a very common aluminum alloy is 6061-T6, where the T6 is the temper designation. The most common designations are O for annealed, H for strain hardened and T for solution heat treated. The processing steps to obtain the conditions for these types of tempers are done after forging. The O temper is an alloy with the lowest strength. Most forged aluminum alloys are not strain hardened, which would be cold worked, after forging, so the H temper is not often encountered in forged aluminum alloys. The most common temper for forged alloys would be the T temper, the most common of which are T4 (solution heat treated and naturally aged), T5 (cooled from hot forging and artificially aged) and T6 (solution heat treated and artificially aged).
Figure 1. Very high-magnification aluminum alloy – precipitates are less than 100 nanometers in size
The microstructure of forged aluminum alloys is typical of many nonferrous metals. The structure observed is usually an equiaxed grain structure with some coarse precipitates visible at fairly low magnification.
After the proper heat treatment, some very fine microstructural precipitates often can be observed using a high-magnification electron microscope. Figure 1 shows a transmission electron microscope image of the very small precipitates (less than 100 nm) that can occur in aluminum alloys. These very fine precipitates contribute to the strength of the alloy. As the precipitates grow larger, their number decreases and the strength of the alloy will also decrease. This decreasing strength is called the over-aged condition. Aging is roughly analogous to storing wine. There is a peak time for optimum conditions, and times beyond the peak cause a decrease in quality. Unfortunately, the better wines in our possession rarely last until the optimum time.
As we stated, aluminum forgings are used especially in weight-sensitive applications. The density of aluminum is just over one-third that of steel, while its strength can be a little over one-half, resulting in a higher strength-to-weight ratio. Use of aluminum forgings is increasing in automotive components due to fuel-consumption standards, and aluminum is used extensively in aircraft applications. Aluminum alloys also are common in sporting goods, including bicycles, boats and hiking equipment, as well as in lightweight consumer items such as walkers, wheelchairs and strollers. One significant limitation is that the strength of aluminum alloys starts to decrease in the range of 400°F. Consequently, applications in hot environments are inappropriate.
Figure 2. Typical temperature ranges for aluminum alloys
Forging of Aluminum Alloys
Aluminum is forged at a much lower temperature than any other common metal because of its low melting point. The low density of aluminum contributes to its low capacity to retain heat. Thus, hot tooling is necessary to avoid rapid cooling of the workpiece. The die temperature can often be at or near the billet temperature, a condition often referred to as isothermal forging. Fortunately, most die steels do not temper when used during the isothermal forging of aluminum. Typical forging temperatures range from 775-875°F. Figure 2 gives the temperature ranges for forging, heat treating and using aluminum alloys. The specific forging temperature will depend on the alloy. The flow behavior includes a potential to soften at high strains, resulting in a tendency for flow localization. Very intricate shapes can be forged using isothermal conditions and low press speeds. This type of forging is sometimes called precision forging.
For most forging applications, the billet material is subject to some deformation processing (cogging, rolling or extrusion) to break down the cast grain structure. With aluminum, continuous-cast billets are more common. Cast billets can be homogenized when a more consistent microstructure is necessary. Some porosity exists in cast billets, which must be “healed” by forging. Of course, extruded and forged billets are also used. Billets are generally saw cut. Abrasive saws are not used due to the high ductility and low melting temperature of aluminum.
Most forging billets are heated in gas or electric furnaces. The melting point of aluminum is significantly lower than other common metals, so convection is the dominant mode of heat transfer due to the relatively low forging temperature required. Radiation is dominant in metals with forging temperatures over 1500°F. Since aluminum is not ferromagnetic, induction heating is impractical.
Aluminum alloys are generally forged on hydraulic presses because of their high strain-rate sensitivity. The use of hammers and mechanical presses are the exception rather than the rule. Ram speeds are generally on the slow side, with 1 inch/second or less being typical. Hot dies, including isothermal, are common. In most cases, the tooling temperature is within 250°F of the workpiece temperature. For net-shape/precision forgings, isothermal tooling and low strain rates are the norm.
Hammer forging of aluminum alloys needs to be done with care. Because of the high deformation speeds with hammers, one can experience hot shortness in the workpiece. Lubrication has evolved during the last 25 years, with lead-bearing lubes phased out in the 1980s. Currently, oil-based lubricants are being replaced with water-based and synthetic lubricants. Lubrication is a critical aspect when forging aluminum because of the metal’s strong tendency to gall (i.e. pieces of the aluminum workpiece adhering to the die).
Due to aluminum’s low flow stress and high ductility, very complex shapes can be forged from aluminum alloys relative to other metals. Potential defects include laps, galling, flow localization and unfills/suck-ins. Flow localization can appear to be a lap or crack. Many high-production aluminum parts are finish-forged without defects. Low-volume parts (generally aerospace) are forged in the finished tooling, removed to grind out defects and coined to size using the same tooling. This would be impractical with other alloys due to the formation of scale or grain growth.
After forging, high-volume parts (automotive) are immediately heat treated on automated lines. One-off parts, such as some used in the aerospace industry, are air-cooled. Post-forging cold work is performed by coining to control size and residual stress in critical applications.
Generally, aluminum alloys can be strengthened by one of two mechanisms – work hardening or precipitation hardening. Work hardening occurs when the aluminum alloy is deformed at room temperature, which is done to increase the strength. Most forged components will not have any significant deformation imparted to them after the shape has been achieved by forging. As a result, work hardening is not used to strengthen aluminum alloys after forging.
The second mechanism of precipitation strengthening can be used on almost all of the commercially available forging alloys. To achieve high strength by this method, a multi-step heat-treating process is required. The first step is the solution treatment, in which the alloy is heated to a high temperature but below the melting point. During this step, many of the alloying elements (e.g., copper) in the metal dissolve into the aluminum, creating a solid solution. The second step is to quench the metal from this high temperature – usually with water as the quenchant. Quench cracking does not occur as readily in aluminum alloys as it does in steels, so the severity of the quench is not often an issue. The solid solution created in step one is retained at room temperature by the quench step. The third step is the aging treatment. A natural aging can occur in a number of alloys. In natural aging, the aluminum alloy will increase in strength as time goes by. The alloy will achieve a maximum strength, and if aging continues the alloy will enter the over-aged state and start to lose strength.
Some naturally aging alloys are stored below room temperature before they are put into service. An artificial aging in which the quenched component is heated to an intermediate temperature and the increase in strength occurs is more common. Artificially aged alloys can over-age if they remain at this intermediate temperature for too long.
For both naturally and artificially aged alloys the strength increase is due to the formation of small precipitates that come out of the solid solution. These precipitates form because the atoms in the metal can diffuse and will react with other atoms to form compounds that precipitate. As an interesting historical note, precipitation hardening in aluminum alloys was discovered by Alfred Wilm of Germany in 1902. He wanted to see if quenching aluminum alloys would increase the strength similarly to steel. He did his first experiments on Friday and was frustrated to measure a decrease in strength after quenching. When he resumed his testing on Monday he was astonished to see that the metal had greatly increased in strength due to the natural aging that occurred.
This first article on nonferrous forging materials explored aluminum forging alloys, which are used in components where low weight is important. The forging of aluminum is best conducted on a hydraulic press under isothermal conditions. The final properties of the alloy are achieved via a proper precipitation-hardening heat treatment. We will explore the forging of titanium alloys in our next article.
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both the Department of Defense and industry. These teammates are determined to ensure the nation’s forging industry is positioned for challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). This work was originally prepared for the FIA Theory & Applications of Forging and Die Design course by Scientific Forming Technologies Corporation.
Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at 303-273-3793 or firstname.lastname@example.org. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or email@example.com.