A golf club head comprises a face portion improved in the durability by increasing the strength of the toe-side upper region of the face portion. The face portion is formed from a unidirectionally rolled plate of a titanium alloy having alpha phase, and at least in the toe-side upper region, the titanium alloy has alpha phase crystals of a hexagonal closely packed structure whose hexagonal symmetry axis (a) is oriented in the direction of a line (k) drawn between the sweet spot (SS) and the toe end point (TP).
1. A method for manufacturing a golf club head having a club face having a sweet spot (SS) and a toe end point (TP), comprising:
designing a contour shape of the club face so that the club face has a pointed toe end on the upper side of a horizontal line (HL) drawn passing through the sweet spot (SS) such that said toe end point (TP) which is the farthest point on the edge of the club face from the sweet spot (SS) on the toe-side thereof is positioned at the pointed toe end, and a straight line (K) drawn from the sweet spot (SS) to said toe end point (TP) is inclined at an angle (delta) of from 15 to 35 degrees with respect to the horizontal line,
cutting out a primary face plate for forming the club face from a unidirectionally rolled plate of an alpha+beta titanium alloy having a rolled direction (RD) such that said straight line (K) becomes in parallel with the rolled direction (RD), wherein said titanium alloy has alpha phase crystals of a hexagonal closely packed structure whose hexagonal symmetry axis is oriented in the rolled direction (RD),
forming a thicker central part and a thin annular part of the face plate by depressing the rear surface of the primary face plate with a die pressing machine, and
making the golf club head by using the pressed cut-out face plate and a main shell body to which the face plate is fixed.
2. The method according to claim 1 , in which the thicker central part has a substantially constant thickness of not less than 2.80 mm but not more than 3.50 mm, and the thin part has a substantially constant thickness of not more than 2.70 mm but not less than 2.10 mm.
3. The method according to claim 1 or 2 , which further comprises:
forming said main shell body by casting a metal material as a single-piece structure provided with a front opening, and
welding the face plate to the main shell body so that the front opening is covered with the face plate.
4. The method according to claim 1 , wherein
the primary face plate is provided with a bulge and/or a roll of the club face by plastic forming as a separate process after the process of forming the thicker central part and the thin annular part.
5. A method for manufacturing a golf club head,
the golf club head comprising a face portion defining a club face for striking a ball, the club face having a sweet spot (SS) and a toe end point (TP), wherein the toe end point (TP) is positioned on the upper side of a horizontal line passing through the sweet spot (SS) and on the toe-side of a vertical line passing through the sweet spot (SS), and a straight line (K) drawn from the sweet spot (SS) to said toe end point (TP) is inclined at an angle (delta) of from 15 to 35 degrees with respect to the horizontal line,
the method comprising the steps of:
cutting out a primary face plate for forming the club face from a unidirectionally rolled plate of a titanium alloy having a rolled direction (RD) such that said straight line (K) becomes in parallel with the rolled direction (RD),
providing a variable thickness for the face portion by plastic forming of the primary face plate using a pressing machine, or alternatively by cutting of the primary face plate using a NC milling machine,
providing curvature of a bulge and/or a roll for the club face by plastic forming of the primary face plate provided with the variable thickness, and
making the golf club head by using the curved face plate and a main shell body to which the face plate is fixed.
6. The method according to claim 5 , which further comprises:
forming said main shell body by casting a metal material as a single-piece structure provided with a front opening, and
welding the face plate to the main shell body so that the front opening is covered with the face plate.
7. The method according to claim 5 , wherein
the unidirectionally rolled plate has a modulus anisotropy ratio Epd/Erd between the tensile elastic modulus Erd in the rolled direction (RD) and the tensile elastic modulus Epd in the perpendicular direction (PD) to the rolled direction is not less than 1.10, but not more than 1.60.
8. The method according to claim 5 or 7 , wherein
the unidirectionally rolled plate has strength anisotropy ratio Spd/Srd between the tensile strength Srd in the rolled direction (RD) and the tensile strength Spd in the perpendicular direction (PD) to the rolled direction is not less than 1.20, but not more than 1.60.
9. The method according to claim 5 , wherein in the step of providing the variable thickness, the face portion is provided with a thicker central part having a thickness of not less than 2.80 mm but not more than 3.50 mm, and a thin part having a thickness of not more than 2.70 mm but not less than 2.10 mm.