Formation of fasteners having threads formed of coil structures

The method of forming a composite nut that includes positioning within a case a separately formed helical coil structure which is formed of metal in work-hardened condition and has a radially inner portion shaped to form an internal thread and a radially outer portion with an outer surface proximate the case;

• directing a high energy beam generally axially along the inner surface of said case and along said outer surface of the coil structure in a relation forming a fusion bond between said case and said radially outer portion of said coil structure;
• locally heating said radially outer portion of the coil structure by said high energy beam, during formation of said fusion bond, to a temperature locally adversely affecting the work-hardened condition of said radially outer portion;
• maintaining said internal thread formed by said inner portion of the coil structure in threaded engagement with an external thread on a highly heat conductive heat sink mandrel during said heating;
• and conducting heat away from said inner portion of the coil structure through the engaged threads of said heat sink mandrel during said heating and at a rate maintaining said inner portion of the coil structure at a temperature substantially lower than said radially outer portion, and low enough to prevent destruction of its thread shaped configuration, and to prevent as great an adverse effect if any on the work-hardened condition of said inner portion as on said outer portion, and thereby maintain said inner portion and the thread formed thereby substantially harder than said outer portion.