Processing for properties

19.4 Processing for properties

There is a mantra a few pages back: Composition ⫹ Processing : Microstructure ⫹ Properties. Here is an example to flag up general issues about this interaction.

Aluminum bike frames

The material chosen for the bike frame, for its good stiffness and fatigue resist- ance at low weight (Chapters 5 and 10), is a heat-treatable aluminum alloy. Figure 19.11 illustrates the main steps in the process history. It makes a number of points.

Point 1: the chemistry of the chosen alloy is adjusted at the outset of process- ing, in the liquid state, and cannot be changed after casting. Metallic elements dissolve freely in one another in the liquid state, but it is very difficult to add them in the solid state; diffusion is much too slow.

Point 2: materials processing involves more than one step. Metal casting, powder processes and polymer molding are near-net-shape processes—the raw material is turned into the shape of the component in a single step, leaving only finishing operations (including heat treatment), so the number of steps is few. By contrast, wrought products (those that undergo some deformation processing) involve a longer chain of processes. The initial casting of wrought alloys is most economic on a large scale, so standard compositions are cast into large ingots as in Figure 19.11(a), which are transported to different factories for processing into different products. For the bike frame, the ingot is sliced into billets and extruded into tube. The figure shows, in (b), a temperature history for the mate- rial as it is shaped directly from a solid circular billet, around 200 mm in diam- eter, to a hollow tube, 30 mm in diameter and 3 mm thick. The extrusion is done hot to reduce the strength and increase the ductility. This enables the material to undergo the large plastic strains, allowing fast throughput at relatively low extrusion force. The quality of surface finish is determined at this stage.

Point 3: design focuses on the properties of the finished product, but some of these same properties (strength, ductility, etc.) are critical during processing. The design and the processing characteristics can be in conflict—strong alloys are more difficult to process and are thus more expensive. So in choosing materials for a component it is important to examine their suitability for processing as well as for performance in service. Returning to the extrusion step of the bicycle frame, we note that extrusion produces the required shape and it also enhances

Ingot casting

Cast ingot

10 m emperature, T

Time, t

Extrusion

Extruded tube,

rapidly cooled

Extrude

Air/water

emperature, T

Time, t

Stock tube, cut to

Age hardening

length & aged m (c)

T emperature,

Time, t Welded

bike frame

Welding

(d)

Weld metal Heat-affected

zone

emperature, T

Time, t

Figure 19.11

A schematic process history for the manufacture of a bike frame in heat-treatable aluminum alloy. (a) Casting of large ingot, cut into billets for extrusion. (b) Hot extrusion into tube, incorporating the solution heat treatment and quench. (c) Sections cut to length and age hardened. (d) Assembly into the frame by arc welding.

464 Chapter 19 Follow the recipe: processing and properties

the microstructure. The hot deformation refines the as-cast grain structure and helps homogenize the alloy. Forced cooling of the tube as it emerges from the extrusion press quenches it, trapping solute in solution, thus obviating the need for a subsequent solution heat treatment prior to ageing. The tube is then cut to length and aged as in Figure 19.11(c), giving it its final strength. In a non- heat-treatable aluminum alloy, strength comes from solid solution and work hardening, not precipitation, so the extrusion is then done cold to maximize the work hardening, with no further heat treatment. Deformation and thermal his- tories are critical for controlling shape and final properties simultaneously, but depend in the detail on the particular alloy variant being processed.

Finally, the frame is assembled by arc welding. In the heat-affected zone the precipitates carefully produced by age hardening disappear back into solution. As the whole age-hardening cycle is difficult to repeat on the completed frame, the heat-affected zone round the weld ends up with different, inferior properties to the rest of the frame (Figure 19.9). Hence the last point in this short journey through a typical metal-processing operation is a reminder: watch out for joining steps (especially welding) that undo the good done by earlier process steps.

With this background, we now examine how diverse combinations of chem- istry and processing are used to manipulate properties, in all material classes. The emphasis is on the interaction between processing and the ‘ideal’ properties stored in a property database, which can be achieved if all goes to plan. But the bicycle frame case study shows that we need to keep our eyes open in this game, and to be aware of why we often need to compromise between design objectives and manufacturing realities. The importance of the ‘Seek Documentation’ stage in material and process selection should not be underestimated!