Description of Alloys Used in Aluminum Softball Bats (What Are the High-Tech Al Bats Made Of?) by Ronald D. Noebe, Ph.D. and Mark McDowell, Ph.D.

Everyone is familiar with the most common Aluminum (Al) alloys used in today's high-tech softball and baseball bats. They are "CU31" and "C405". But what does that mean? When we went to look up the composition for these alloys we found that they did not exist in any alloy handbook. This, of course, raised our curiosity. So we checked with some of our industrial contacts and discovered that CU31 and C405 are internal ALCOA designations used for marketing purposes, and are essentially "forced" on the bat manufacturers. The primary reason is to make this code recognition a mystical part of the bat industry. In this respect, ALCOA and the bat companies have been quite successful. After all, everyone is always talking about the latest "C405" bat. Even though to the average person and even most metallurgists, the term has absolutely no meaning.

So what are the high-end softball bats made out of? The answer is quite simple. The sports industry is looking for the highest possible strength, in a lightweight metal, with reasonable raw material and processing costs. After all, high performance and low cost are what drives all industries from automotive to aerospace. The material that best meets these goals and which all high-end softball bats (with the exception of composite bats) are made out of today are 7xxx series aluminum alloys. This four-digit designation (Xxxx) is used by all industries in the identification of Al alloys, (instead of the myriad trademark and other designations such as C405, Cu31, and DF24A), so that engineers know exactly what they are working on and designing with. The first digit indicates the alloy group, and the remaining digits identify the alloy composition in more detail.

More specifically, many of the softball bats are made from the alloys 7055, 7050, and in past years may have been fabricated out of 7046, 7075 and 7005 in descending order of strength and cost-to-produce. The chemical compositions of these and several other aluminum alloys, as defined in Woldman's Engineering Alloys Handbook, are listed in [ Table 1 ].

In general, 7xxx series alloys contain 4 to 8 wt.% Zinc (Zn) and 1 to 3 wt.% Magnesium (Mg), creating a family of wrought, heat-treatable aluminum alloys. "Wrought" means that the metal undergoes some type of deformation processing, such as extrusion, swaging, and ironing as in the case of softball bats. "Heat treatable" means that the strength of the alloys are governed or manipulated by different heat treatments, which result in various precipitate structures. The 7xxx alloys are strengthened primarily by MgZn₂ precipitates. Copper (Cu) additions of 1 to 2 wt.% are added to these alloys when even greater strengthening is justified, such as for high performance aircraft applications or premiere softball bats. In fact, the highest room-temperature strengths of any aluminum alloys are developed from Al-Zn-Mg-Cu alloys. This makes them ideal alloys for bats where you want to maximize room temperature strength so that you can use the thinnest wall sections possible. The 7xxx alloys also contain very small additions of other elements, such as zirconium (Zr) and in some cases, chromium (Cr). Zr is used for additional strengthening (due to Al₃Zr precipitation), grain refinement, and recrystallization inhibition. Cr is used primarily to give the alloy better stress-corrosion resistance and is used in applications where Al alloys will be in contact with salt-water environments such as marine applications. Cr does not have a significant benefit in terms of strength but may help increase the recrystallization temperature of the alloy. However, it is not nearly as potent as Zr in this respect. 

ALCOA's Cu31: Prior to 1996 all the top-of-the-line softball bats offered by the bat companies were made out of an alloy designated Cu31. This extremely popular Al-Zn-Mg-Cu alloy is more commonly known as 7050. The trademarked designation Cu31 means that not only is the alloy 7050 but that it was supplied by ALCOA. However, being the curious engineers that we are, we actually measured the chemical composition of two bats stamped Cu31 (one from Louisville Slugger and the other from Grover). The results of these analyses were identical and are shown in [ Table 2 ]. As you can see from comparison of Tables 1 and 2, the bats stamped CU31 fit the chemistry profile for alloy 7050. The only discrepancy we could find was the trace level of zirconium (Zr) in these bats, which was slightly higher than called for in the specifications. However, these levels are so small, that it is difficult to determine whether the difference is real or some error in our measurement technique.

Pechiney's Cu31: The alloy known as Diamond Flex 24A or DF24A, which is used in Steele's XLT3 bat is also alloy 7050 [ Table 2 ]. In this case, the extruded tubes were supplied by Pechiney, which is headquartered in Courbevoie, France. Pechiney is the largest aluminum producer in Europe and the fourth largest producer in the world. But for the time being, Pechiney has only a very small market share in the U.S., supplying extrusions exclusively to one of the smaller bat manufacturing facilities, Paragone Sports of California. The major difference between Pechiney's version of alloy 7050 and ALCOA's is that Pechiney's had a slightly higher Mg content, which could have the potential for slightly higher strength if heat treated correctly.

ALCOA's C405: C405 is the trademarked designation of aluminum alloy 7055 when supplied by ALCOA to the bat industry. Worth was the first to introduce C405 in softball bats in 1996 and since then, essentially all major producers of softball bats use C405 in most, if not all, of their top-of-the-line bats. In this case, we measured the chemistry of three different bats from different manufacturers (Louisville Slugger, Worth, and Steele's), [ Table 2 ]. Not only did all three bats fit the chemical profile of alloy 7055 [ Table 1 ], but within measurement error were exactly the same composition. This is an indication of ALCOA's ability to supply a reproducible and consistent material to all companies.

C405 Plus is the same alloy as C405. The "Plus" indicates that the bat manufacturers (or ALCOA) had developed a better heat treatment for the alloy, which leads to higher strength. C405 Plus was introduced in TPS bats in 1997. Improvements in the heat treatment of this alloy continue today.

C405 Ultra is the '98 version of C405, which has undergone even further optimization of the heat treatment process resulting in an even stronger version of the alloy. C405 Ultra will be introduced in the 1998 line of Easton bats. But the composition of C405 has remained the same throughout the years. It is the manner in which the alloy has been heat treated that has changed. But the outcome is what counts -- stronger softball bats. 

C555 : Based on our initial samples, the trademarked alloy C555 has essentially the same composition as alloy 7055 (trademarked C405) except for the addition of 500ppm of Scandium.

AM-70R is the alloy used by American Modern Metals to produce the bat shells used in the 1998 line of Steele's bats, One Cryo's Equalizer, and most Power-Flite bats is straight Al alloy 7149, without even a significant Zr-modification.

Sc500 is Easton's designation for the alloy used for the aluminum shell in the Easton Redline. This is probably the most significant change in alloy design for a softball bat since the introduction of C405 back in 1996. This alloy was developed jointly by Kaiser Aluminum and Ashurst Technology for exclusive use by Easton. The composition for this alloy (which we analyzed twice) does not fall exactly in the range for any of the common 7xxx alloys but is most similar to 7055 [ Table 2 ]. What is so special about this alloy is the presence of a dilute Sc addition. Scandium is a very rare, expensive, and lightweight metal. When added to Al in dilute quantities it is responsible for increased strength from Al3Sc precipitation, enhanced weldability, and significant resistance to recrystallization. Resistance to recrystallization means that Al alloys containing Sc will have a finer and more consistent grain size retaining a higher degree of "work" in the microstructure (higher dislocation content) after wrought processing and heat treatment than comparable alloys without Sc. This translates into significantly higher strengths and better overall properties than for a comparable alloy without Sc. The effects of Sc are enhanced in the presence of Zr due to the precipitation of fine Al₃(Zr_xSc_1-x) precipitates. Because this alloy contains both Sc and Zr it was obviously developed to take advantage of this synergistic strengthening effect.

So which is the best bat on the market? The answer to that question is getting more difficult. The best bat is the one that gives you the best price for performance ratio. If money is not an object, then the bat with the thinnest walls will probably be your choice. There was a popular phrase coined in 1997 that stated "He/She who has the thinnest walls, wins it all" and by and large this is true. The thinner the walls, the more velocity off the bat, the harder the ball is hit, the farther the ball will travel. There is an added benefit from extra walls as in the better designed multiple wall bats. These bats will generate even more velocity off the bat and combine this with players getting bigger and stronger, just about every player can reach the 300' fence. The bottom line is to purchase a bat that will give you performance plus durability for the climate that you normally play in.