In the world of golf, designers and manufacturers are constantly searching for new materials and developing new equipment. They're trying to satisfy the appetite of the golfing public for any tools that will improve their game, and the professionals for anything that may give them an edge.
Most innovators focus on the golf club, particularly the head. Generally, they want a face insert for woods and irons that is of such strength that it will permit the design of heads that are lighter, larger and offer greater energy transfer. All manufacturers are looking for the ultimate new material that would achieve the most coveted of all player objectives - greater distance and accuracy off the tee or fairway.
This discussion is limited to golf club face plates, although the authors acknowledge the usefulness of various excellent metals and composites for other sections of the head and shaft as well. No matter what the head design, the club head face is key to flight of the golf ball because that is the surface that makes the all-important contact with the ball.
Many high tech materials have been investigated and used in the heads of recent lines of golf clubs. Two stand out as mainstays that are used widely in club design -Dynamet Ti-6Al-4V alloy and CarTech Custom 630 (17Cr-4Ni) stainless steel.
Two next-generation alloys have been introduced to the golf industry. The alloys, originally developed as candidates for aerospace applications, are CarTech AerMet® alloy and Carpenter CarTech Custom 465® stainless. Both of these aerospace alloys have been found to offer a desirable combination of properties for club head design. They have made a significant impact in golfing since they have been adopted recently by both club designers and manufacturers.
CarTech AerMet® alloy is a premium-melted, ultra-high strength steel that was developed initially as a candidate for use in the landing gear of U. S. Navy carrier-based F-18 E/F jet fighters. For a wide variety of commercial as well as military applications, it has offered a unique combination of high strength and hardness, fracture toughness, exceptional ductility, and resistance to both fatigue and stress corrosion cracking.
CarTech Custom 465® stainless is a premium melted, martensitic, age-hardenable stainless steel that, when aged to the H-1000 condition, provides a superior combination of high strength, fracture toughness and resistance to stress corrosion cracking compared with other high-strength PH stainless alloys. In its H-900 condition, with 1795 MPa (260 ksi) peak ultimate tensile strength, it still retains high notch tensile strength and fracture toughness.
For those who design club heads, as well as those who swing them, we have documented the mechanical properties of these four alloys (Table 1) as a means of comparing their likely performance. The data provides key measurements of strength, hardness, density, elongation, elasticity and strength-to-weight ratio. Typical chemical compositions for the alloys are given in Table 2.
Chart 1 shows the hardness and tensile strength of the four alloys in our matrix. Assuming that the distance a golf ball is driven relates to hardness and tensile strength of the face insert, more of the energy of a swing should be transferred to the ball with the two aerospace alloys at 54 HRC, 2083 MPa (302 KSI) tensile strength and 51 HRC, 1795 MPa (260 KSI) tensile strength, than with the CarTech Custom 630 (17Cr-4Ni) and titanium alloys at lower hardness and tensile strengths.
The specific strength (strength-to-density ratio) and stiffness (modulus of elasticity) of the four alloys are compared in Chart 2. All three of the high strength steels have densities that are nearly identical: CarTech AerMet® alloy, 7889 kg/m3 (0.285 lb./in3); CarTech Custom 465® stainless, 7833 kg/m3 (0.283 lb./in3), and CarTech Custom 630 (17Cr-4Ni), 7806 kg/m3 (0.282 lb./in3) (Table 1) Dynamet Ti-6Al-4V alloy is much lower, with a density of 4429 kg/m3 (0.160 lb./in3)
It may come as a surprise that titanium, known for its low density, no longer has an edge over other alloys in specific strength. The specific strength of CarTech AerMet® alloy can match that of Dynamet Ti-6Al-4V alloy, or can exceed it, depending on heat treatment.
Titanium, when aged at 482 oC (900 oF), can reach a specific strength of 26.2 Km. Without aging, Dynamet Ti-6Al-4V alloy achieves a specific strength of only 20.7 Km.
When aged at 468 oC (875 oF), CarTech AerMet® alloy attains a specific strength of 26.9 Km. This is slightly higher than that of titanium, and at a much higher hardness (54 HRC vs 40 HRC). CarTech Custom 465® stainless has nearly as high a specific strength (23.3 Km) as titanium, but again at a much higher hardness (51 HRC vs 40 HRC).
The specific strength of CarTech Custom 630 (17Cr-4Ni) is substantially lower, at 17.8 Km, than that of the other three alloys, with only slightly more hardness (44 HRC) than titanium.
The three steels in the group of four metals studied have approximately 70% higher stiffness or modulus of elasticity than Dynamet Ti-6Al-4V alloy (Chart 2). These alloys will be stiffer and support a greater load. Therefore, inserts of those high modulus, high strength alloys will deform less elastically at point of impact than titanium, and thus transfer more energy to the ball. Likewise, all three steels have substantially higher elongation properties than Dynamet Ti-6Al-4V alloy (Table 1). The higher the alloy's elongation percentage, the more ductile and tougher the face plate that strikes the ball.
Good Head Design
On the basis of measured properties, it follows that good club head design requires a face insert made with alloys having the best combination of high specific strength and high modulus of elasticity. With these attributes, the designer may be able to enlarge the "sweet spot" without adding unacceptable weight. With a larger sweet spot and more energy transfer, the golfer is capable of driving the ball farther and straighter without swinging harder.
Tests have determined that the impact between the club head and golf ball can generate a stress of up to 3,000 psi. Thus it is imperative that the face plate be made of a material that is strong enough to resist cracking or permanent deformation. When the material has a high specific strength and high modulus of elasticity, it is also possible to design a thinner face that resists damage from striking a ball or other hard object.
New developments in equipment and materials are cascading into the realm of golf at a pace no one could have imagined a few years ago. Yet, in today's perspective, that's not too surprising in view of the exploding population of golfers - most recently estimated at more than 50 million worldwide - who are demanding improvements. Anything to help them lower their handicap, and enjoy the game more.
Designers and manufacturers are responding, working and testing intensively to find and use the most suitable materials. In this great sport, particularly, the innovators have been willing to try any new material that offers the right combination of high strength, stiffness, weight, hardness, toughness and durability.
For now and the near future, the alloys discussed here are among the preferred choices. They and improved variations are likely to serve the needs of the golfing public for at least the near term. It will be up to materials developers to keep up with the trends and the insatiable desire for something that will provide better performance and satisfaction.
By James M.Dahl, Paul M. Novotny and James W. Martin
Carpenter Technology Corporation