**3. Die and Expansion Designs**

The experimental setup of the extrusion process is shown in Figure 3a. The setup is installed in a hydraulic press capable of 260 kN (Greenerd Press and Machine Co., Nashua, NH, USA). The press is omitted from figures. As indicated previously, extrusion was performed at 52% and 68% radial strain. The only difference between the two extrusions is the size of the extrusion ledge (i.e. the dimension L, as shown later) to increase the strain from 52% to 68%. The off-the-shelf die set is customized to perform both the extrusion and expansion processes. The extrusion die and the expanding post are easily swapped to either perform the extrusion or expansion process. A cut-away view of the extrusion setup is provided in Figure 3b,c. Critical dimensions are shown in Figure 3c and are tabulated in Table 2.

**Figure 3.** (**a**) Experimental setup of extrusion process which provides 52% radial strain. Omitted is the hydraulic press. Also depicted is annealed copper and aluminum test metals ready for extrusion. For size perspective, the test metals are 89 mm in height. This setup is identical to the process which enacts 68% radial strain except for dimension L. (**b**) Section view of extrusion setup. (**c**) Die cavity and extrusion ledge. The tubular bimetal is omitted.

**Table 2.** Critical dimensional values of extrusion die.


For either the extrusion or the expansion process, both configurations utilize the punch as the mechanism to enact deformation. This was tactically chosen to keep the setup contained to one die set installed in one hydraulic press. Both the die and the expansion post use a floating alignment method. To ensure the punch is always axially aligned neither the extrusion die, nor the expansion post are fixed in place; rather, they both self-align to the punch during setup.

The die is of sufficient length to fully encapsulate the length of the bimetal tubes. This is to ensure it is forced into the extrusion ledge. The extrusion edge geometry, which has a 30 ◦ transfer from the initial diameter to the extrusion diameter, has rounded and smooth radii. Just below the extrusion ledge is a diametrical relief for ease of tube removal after the tube is extruded into and past the extrusion ledge. The inner mandrel floats collinear to the die and remains collinear when the bimetal tubes are installed inside the die. The floating mandrel is positioned such that only the least amount of the mandrel is below the extrusion ledge to aid in the removal of the bonded bimetal tube. After extrusion is performed, due to the setup in a hydraulic press, the bimetal tube is removed by removing the die and floating mandrel. For this reason, the floating mandrel is not attached to the vertical support below it. A relief is cut into the bottom portion of the die set, the width slightly larger than an extruded tube, to assist in bimetallic tube removal. Lastly, the punch is designed to insert into the die and have the floating mandrel insert in it. Clearances between these components are less than 2.54 × 10 <sup>−</sup><sup>2</sup> mm.

The expansion post for the expansion process, shown in Figure 4, has a smooth transitional ledge, at 10 ◦ , used to expand the bimetal tube such that it can fit over a tube of its original extruded size. A few variations of the expanding post can be used in which the expansion diameter is increased in predetermined increments to aid in processing. Incorporating these variations allows a step-up approach to achieving the final desired inner diameter of an expanded bimetal, if needed. Alternatively, the diameter may be increased directly in one expansion step. For the testing performed, all tubes were directly

expanded in one step. Sitting at the base of the expanding post is an oversized washer which can be used to aid in the removal of the bimetal.

used to aid in the removal of the bimetal.

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**Figure 4.** (**a**) Experimental setup of expansion process. Omitted is the hydraulic press. (**b**) Section view of expansion setup. Expansion setup uses the same die set as the extrusion process.

The die, punch, mandrel, and expansion post are the main functional components that are performing the extrusion or expansion. The material for these components is AISI A2 tool steel. This material is commonly used in extrusion dies and other high stress material forming processes. The hardness range for these components is 58 to 62 HRC which is a typical range for extrusion and forming dies. The A2 material has desirable characteristics which are tabulated in Table 3. The wear resistance and toughness are improved with the coating described below.


**Table 3.** Cold work tool steel relative ratings (A = greatest to E = least) [63].

The die, punch, mandrel, and expanding post are coated in a thermal diffusion process, which is a typical coating process for blanks, dies, and components used in similar high stress forming operations. The coating provides additional lubricity and reduces reactionary stresses during extrusion. Additionally, the tool toughness and hardness are promoted, and in general, the life of the components are extended. The coating data is tabulated in Table 4.


**Table 4.** Coating data for die, mandrel, punch, and expansion post [64].
