MP-15 Irons
By Tim Newcomb
September 26, 2014

Steel on the outside and titanium on the inside lets golf club engineers tinker with weight distribution to provide golfers the feel they love from a traditional club and the forgiveness that comes from a club playing larger than its head size.

Combining titanium with steel has turned into an art for Mizuno.

Released in September, Mizuno unveiled a four-year project—the MP-15 set of irons—that inserts a less dense and lighter pure titanium into a 1025E mild carbon steel club head while keeping the thickness of the steel head for solid impact, something traditionally lost in a cavity-back design, Chris Voshall, Mizuno engineer, tells Edge.

“The use of titanium, which is mechanically locked into the head, allows the club to play much larger than its head size in terms of forgiveness while still maintaining all the feel and feedback that is typically lost in a modern multi-material iron,” he says.


While Mizuno has mixed materials before, this “massive improvement” in technology includes the way engineers construct the club head. Every cubic centimeter of titanium used leads to a few extra grams of free weight versus using steel. Engineers used 10 grams of material, including titanium, to replace 38 grams of steel that normally backed the club head, freeing up that weight for precise placement around the perimeter of the club head.

“We went through a number of different prototype iterations before finally landing on not only a size, but also a geometry for the titanium that will give us the most discretionary weight to play with while still maintaining the ultimate in feel,” Voshall says.

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During forging, titanium cools and shrinks less than the steel around it, allowing a locking together of the materials without welding or gluing, “thus helping us to keep the grain structure of the steel that is typically disrupted when the heat of welding is involved,” Voshall says. “The grain structure will make sure that you get the best, softest, most consistent feel.”

The steel’s grain-flow forging, a Mizuno patented process, lets engineers stretch and bend the steel to tightly control the flow of the grains from toe to heel of the club, keeping the grains longer and more consistent.

As part of the new MP-15 launch, the club comes with a standard head weight—optimized for ideal weight displacement—but custom shafts allow players to select lighter or heavier clubs, depending on their fit.

Tour pro Luke Donald was involved in the the process for upgrading the Mizuno Ti Muscle construction, from before the project started some five years ago, right through prototyping.

“We developed multiple rounds of prototypes and traveled to Luke to test each one,” Voshall says. “We made sure he has hit the final round and given it his stamp of approval before the start of mass production.”

Now the Mizuno stamp comes with a precise mix of steel and titanium.

Tim Newcomb covers stadiums, design and gear for Sports Illustrated. Follow him on Twitter at @tdnewcomb.