Graded Microstructures in Cold Formed Metallic Components

The room temperature forming of metallic materials enables the design of components that possess tailored, graded mechanical properties. During the room temperature forming process known as cold heading (sometimes referred to as cold forming and cold forging), forces are imparted to a cylindrical metal slug resulting in plastic flow of the material into a geometry that matches that of the surrounding tooling. Of the immediate interest to the metallurgist are the various parameters that influence material performance in the final application. Pertinent aspects of cold heading are briefly summarized, and specific examples of graded microstructures and cold work in both simple and more complex geometries are presented.

Key Production Parameters Affecting Net Part Properties

Generally speaking, the cold heading method of part forming is intended as a low-cost, high throughput technique. The relative robustness of equipment, low proportion of material waste, and accommodation of poorly machinable materials make this an economically attractive production process. 

Strain Magnitude

Material deformation follows a carefully calculated or empirically derived series of steps [1], typically ranging from one to six seperate operations to form a finished part. Strengthing of materials by cold work results from the generation, movement, and eventual interference of dislocations [2] during material flow. Pure metallic materials will work harden at relatively lower rates than their respective alloys. This is due to solid solution or precipitation strengthing in alloys, which act to make the path of travel for dislocations tortuous [3]. The effect of this room temperature deformation is the focus of the latter half of the present report. 

Temperature

Cold heading, as the name implies, relies on a series of plastic deformation steps that take place with equipment and material starting at room temperature. In this context, “cold” refers to temperatures that are well below the recrystallization temperature of the metal. The degree of temperature rise resulting from adiabatic heating during deformation and frictional heating from machine motion is relatively low. Forming operations conducted at these cool temperatures generally prevent the structural transformations that occur during elevated temperature wrought processing, such as those that may be found in forging or extrusion. Specifically, cold heading precludes dynamic recrystallization. Absent this, the microstructure of most structural materials will experience refinement at room temperature. Effects on material properties are generally: increases in hardness and strength, and concomitant decreases in elongation and fracture toughness. 

Strain Rate

Cold heading and forming machines and tooling are often optimized for high volume operation. Therefore, the time spent deforming each part is on the order of a fraction of a second per form operation or “hit”. The short duration of each hit means that strain rates in the heading operation are high in magnitude, and vary widely depending on the location within the part. Strain rate sensitivity therefore plays a significant role in the formability of various alloys and tempers. Soft, annealed materials with low strain rate sensitivity (that is, with a weak dependence of flow stress on strain rate) will best accommodate high magnitudes of strain during forming. When working with exotic materials, strain rate sensitivity may be unknown. Some work has shown that the split Hopkinson bar technique [4] may provide valuable insight into material behavior in a cold forming operation [5].

Key Graded Material Properties Resulting from Cold Heading

Case I: Simple Pins Made From 302SS & Grade 4 Ti

Uniform, axisymmetric geometries such as that of a pin or nail are readily accomplished with cold heading techniques. The metallurgist may find examples of graded properties even in this uncomplicated geometry, as shown in the below examples for grade 302 austenitic stainless steel (Figure 1) and commercially pure grade 4 titanium (Figure 2). Vickers microhardness measurements were also performed on the larger diameter “head” region and the smaller diameter “shank” region, shown in Table 1. As one may expect, the head region experiences a relatively higher magnitude of strain during forming.

Cold-Heading_302-stainless-steel

Cold-Heading_Grade-4-Titanium

alloy-pin-cold-forming-hardness

The aforementioned components were produced using a fixed tool geometry and machine setup, corresponding to uniform strain magnitudes and rates, respectively. It is interesting to note, then, that the titanium exhibits nearly twice the proportional increase -- 37%, versus 21% in 302 stainless steel -- in hardness from the shank region to the head region. This is due in part to differences in the magnitude of work hardening between the austenitic, face centered cubic (FCC) stainless steel, and the hexagonal close packed (HCP) titanium material. The HCP crystallographic structure has fewer active slip systems, which results in a higher flow stress in materials having this atomic arrangement. 

Case II: Complex Ablation Catheter Tip Made from Pt-10 wt.%Ir

One key medical component that is not commonly associated with the cold heading technique is the ablation catheter tip. These round nosed components are utilized in the targeted destruction of defective myocardial (heart) tissues that cause arrhythmias [6]. Platinum alloys, especially those with iridium content such as 90 wt.% Pt-10 wt.% Ir, are recognized for their bioinertness and particular suitability for tissue contacting electrodes [7]. An overview image from scanning electron microscopy of the cold formed catheter tip blank is shown in Figure 3.

Cold-Formed-Ablation-Catheter-Tip

Platinum-iridium alloys are dominated by a continuous face centered cubic solid solution with limited low temperature immiscibility [8], and therefore exhibit primarily solid solution strengthening. Even in this relatively simple binary alloy, significant work hardening is observed during the first of two cold forming operations required to form an ablation catheter tip. This is illustrated in Figure 4. In this presentation, conventional metallographic preparation and electrochemical etching using a solution of H2 O-HCl-NaCl [9] was used to provide grain relief, and 50gf Vickers hardness indentations were used to measure microhardness. 

Residual texture from the wire drawing process that precedes cold heading can be observed in the left-to-right orientation of Figure 4. This is a result of annealing the material at the final diameter without recrystallizing in order to limit grain growth, or “coarsening”. The material flow during the cold heading process is mostly focused in forming the blind hole at the left and the round “nose” at the right. These features, unsurprisingly, show stronger responses to metallographic etching consistent with higher grain boundry and dislocation density, and substantial increases in microhardness. The bulk material hardness is approximately HV50g 180, while areas of severe deformation climb to HV50g 250, a 39% increase.

Cold-Form-Ablation-Catheter_Microstructure-and-Hardness

Summary

When expertly employed, the process of cold heading may enable a designer to achieve superior component performance by tailoring the microstructure and mechanical properties of the resultant component. Key factors to consider are the magnitude of deformation, rates of imposed strain, and strain hardening behavior of the material being deformed. 

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