FlowformingMatthew Fonte - Dynamic Flowform Corp.
Well known for its efficiency and economical benefits, flowforming has been widely accepted as the process of choice in the fabrication of difficult-to-manufacture military and aerospace components requiring superior metallurgical and dimensional controls. Flowforming is currently employed for the production of missile outer shells and nose cones, housings for flight and launch motors, casings for rocket motors, large caliber cartridge casings, thin wall mortar tubes, projectiles, warheads, and bomb bodies; as well as tubulars for the nuclear, petrochemical and aerospace industries.
Flowforming is a fairly straight-forward net shape process. First a blank is forged that is short, and round with a hole of cavity to fit over a mandrel. Then it is formed over a mandrel into something that is long and thin walled using a progressive rolling technique involving a set of three rolls in a turning spindle that compress the metal from the OD against the mandrel (at its ID). The three rolls are matched such that each acts both as a steady rest for the others and the metal is compressed and stretched. Both precision wall thicknesses and perfect concentricity are assured by this process. A typical application might be taking a round four foot forged blank of titanium and forming a 20 foot long thin walled tube.
Typical advantages of the process are:
precise thin walls
seamless construction
perfect concentricity
net shape use of material
highly refined microstructure
profitable at > 10:1 length to diameter tubes
best for wall thickness of 1/2" or less
ID must be at least 1" to have sufficient mandrel strength
Dynamic Flowform Corp. has six flowforming machines. Most do production, but one is always reserved for new product development. Mandrels are typically made of 62 HRc D2 tool steel. At Dynamic Flowform the customer owns the mandrel as a tooling charge. If we think of the machine as a big lathe, the mandrel is fastened into the headstock (reverse flowforming), and the three forming rolls (which are staggered both radially and axially) move together in the tool carriage. The rolls each have a precise angle of contact with the metal which forces compression of the metal as the tool carriage moves axially along the mandrel toward the headstock. Initially the rolls are powered to turn synchronized to the mandrel, but once forming is initiated the turning power to the rolls is stopped and they continue to turn by friction contact with the metal being formed.
There are basically two variations of flow forming -- Forward and Reverse. Tubes which are "closed" at one end must use forward flowforming. In this situation the mandrel is fixed to the tail stock, and the blank to be flowformed is attached to the head stock. The 3-roll carriage then moved from the head to the tail of the machine over the mandrel causing progressing deformation of the cylinder walls and extension of the tube. The rolls can be programmed to produce many variations in wall thickness along the progression making complex round tubular shapes. Forward flowforming offers the best dimensional tolerances, but has the disadvantage of requiring a mandrel the full length of the finished tubular ID.
Reverse flowforming is possible only when the finished tubular product is open at both ends. In reverse flowforming the mandrel is attached to the headstock, and the hollow blank initially mounts over the entire length of the mandrel. A "drive ring" is mounted near the headstock which contacts the OD of the blank and assures turning of the blank with the mandrel (mandrels are always highly lubricated prior to each use and the drive ring prevents slippage at the blanks ID). The 3-roll carriage moves from the open end of the mandrel (tail) up toward the headstock, reducing the wall thickness and extending the tube length as it travels. The much of the "finished" tube is left unsupported as it extends toward the tailstock of the machine. The big advantage of this method is that the somewhat expensive mandrel only needs to be as long as the original blank (usually about 1/4 the length of the finished tube) which makes it much less costly than for forward flowforming. As with forward flowforming, the OD can be varied as needed to form tubular shapes with complex profiles of the OD.
For many high reduction flowform jobs, more than one pass (frequently two) of the carriage rolls is required to achieve the full reduction to final dimensions.
High temperature alloys such as Inconel are well suited to flowforming as are most wrought ferrous alloys, titanium alloys, copper alloys and brass. With many lower strength copper alloys and brasses not requiring complex profiles of the OD, "deep drawing" (a simpler net shape process) is easily done and far more economical than flowforming. Flowforming has the advantage on materials which are difficult to deep draw or require achieving complex profiles on the OD in a net shape (no material loss) process.
Flowforming does require a very high quality of material (as the blank) to be successful. Generally, blank mateial is obtained only from the most quality conscious vendors, and is ultrasonically tested prior to use in the flowforming machine.
One of the largest flowformed pieces now in production is a hydrostatic testing pressure vessel which begins as a 54" long preform, and finishes as a 20 foot closed end tube. Most new projects are developed using correct materials but with profiles and all dimensions scaled down to determine correct contact angles, radial and axial stagger, and progression speeds for the 3 forming rolls in the tool carriage. There is some effort to model the process using "D-form" software, but as of yet only properties and parameters are being determined and input to the model and no successful output models have been developed.
Dynamic Flowform Corp. has one location in Delicca, Ma.