A powerful tool
Heat treatment is an essential post-processing step in additive manufacturing to tailor the material in the produced part to specific needs and requirements. The path to success is finding the right combination of material and heat treatment.
Additive manufacturing (AM) lets us produce complex metal designs that are not possible using traditional methods. But without post processing, the manufactured parts would be nothing but fancy design, too brittle or porous to be used in real applications.
While post processing involves everything from part removal to surface treatment, one of the most important post-processing steps is heat treatment.
“Steel, iron and many other elements are polymorphic materials, which means they exhibit different crystallographic phases in different temperature and pressure domains,” says Markus Schneider, manager modeling, simulation and fatigue at GKN Sinter Metals in Germany. “Consequently, their material properties – chemical, thermal, electric, magnetic or mechanical – are different since they result from the corresponding microstructure. Heat treating is an essential way to tailor those material properties.”
A beautiful, sustainable idea
In fact, he says, heat treatment is such a powerful tool that it would almost be possible to use just one alloy and modify it to all special needs and requirements.
“That’s a beautiful idea and a more sustainable one, too, since the recycling process would be so much easier,” Schneider says. “However, heat treatments alone cannot cover the whole material property spectrum. For sure, there is still the need for special macro and micro alloying elements and methods. The right combination of materials and heat treatments is essential.”
Another strong argument for heat treatment is the engineering principle of local quality. “Economic and environmental considerations drive us to more intelligent part design,” he says. “In many cases the bulk material properties do not have the same importance as the surface material properties, since the surface guards the structure against chemical, thermal or mechanical attacks. This insight supports a variety of post treatments such as coating, sputtering, plating and heat treatment.”
One common reason for applying post-process heat treatment in AM is residual stress relief. Residual stress is caused by the heating and cooling of the metal as the part builds layer by layer. These internal stresses must be relieved before the part is removed from the build plate; otherwise, the part may warp or even crack. “This method is a game changer if flat and thin structures should be printed,” Schneider says.
Other heat treatment processes are aimed at giving the specific part its final properties. The range of heat treatment options is similar to those for competing technologies such as machining, casting, stamping or forging, though a bit smaller since the number of available AM materials is low compared with other processes.
“But in general, I see no reason not to apply the whole heat treatment spectrum,” Schneider says. “A few methods will not reach the same importance as in the other industries. For example, induction hardening is great for regular shaped and symmetrical parts like rings, pipes and gears. The limiting factor is the shape of the inductor, which should be tailored to the shape of the part and the kinematics. Complex-shaped AM parts will be a challenge. There is also a strong economic disadvantage – induction hardening is great for big lots since the automatization effort is significant.”
In what way is the AM method linked to the choice of heat treatment?
“The majority of heat treatments are linked to the chemical composition and not to the printing technology,” Schneider says. “However, for one-step AM methods such as powder bed fusion and direct energy deposition, you can cluster it in terms of the introduced heat and the heat distribution. Soft annealing and residual stress relief treatments may be required if there is a high and non-uniform heat (temperature gradients, fast heating or cooling) distribution. In two-step AM methods such as binder jetting, sintering is part of the manufacturing process to consolidate the green part after the printing.”
Choosing the process
Since the choice of heat treatment processes is typically linked to the chemical composition of the material, a lot of knowledge can be adopted from the heat treatment procedures for welded structures, Schneider says.
Some parts, however, require a special type of heat treatment depending on the expected loadings, requirements and applications. For example, steel gears will be case-hardened and aluminum chassis parts will be aged.
“The most beautiful way is to test it,” Schneider says. “Prepare fatigue test coupons – or any other kind of test coupon – and make an S-N line with and without heat treatment. Test the corrosion resistance (stainless steels), the thermal conductivity (copper), the strength (aluminum), the creep rate (nickel base materials and titanium) and so on, with and without heat treatment. Compare the mean values and the corresponding standard deviations.”
More than hardness to consider
Schneider says a common mistake is the belief that hardening is the most important factor for AM post-process heat treatment.
“Unfortunately, it is still very common to correlate everything with the hardness,” he says. “Heat treatment should be applied to make the product more reliable, predictable and valuable. To obtain a reliable product you need to know the material’s sensitivities regarding notches, static and cyclic stresses, its residual stresses, work hardening, internal defects and roughness, its heat treatment response and density dependence, the material and manufacturing scatter, as well as the effect of its chemical composition.”