Heat treatments make it possible to give particular mechanical and technological characteristics to components produced in metal. For additive manufacturing it is necessary to carry out specially designed and verified treatments, able to fully exploit the possibilities offered by the technology and guarantee excellent performance.
Heat treatment is a thermal cycle in which components undergo certain temperatures and certain cooling procedures to obtain specific properties. The Beamit Group uses two types of heat treatment, in vacuum and in air, with special furnaces, ovens and systems. The air system is used for aluminium alloys, while vacuum furnaces are used for a large number of materials such as titanium, nickel superalloys, and steels.
Heat treatment allows the freeing of components from residual stresses that can be generated during printing; it also allows you to give particular mechanical properties to the material chosen according to the applications foreseen for the component.
Subjecting a metal to a thermal cycle alters the microstructure: an increase in toughness and elongation at break is achieved, as well as a corresponding decrease in breaking and yielding loads.
In the field of additive manufacturing, heat treatments are always advisable to avoid residual stresses, but they can be omitted for small components, which do not accumulate many stresses during printing, for applications where particular mechanical properties are not required, or in some cases, for aluminium.
Key issues and challenges to face when carrying out heat treatments on additive productions
Carrying out heat treatments within the company requires the creation of a genuine supply ecosystem that goes far beyond simply acquiring suitable systems: dedicated resources are needed to manage everything related to the heat treatment cycle, both for the metallurgy and design of the curve specific to the customer's design. In addition, there is constant quality control and preventive maintenance.
Our vacuum and air furnaces/ovens are NADCAP accredited, which is required for the aerospace sector. The AMS 2750 divides heat treatment systems into classes based on the accuracy of the maintained temperature. To keep the ovens and furnaces in line with requirements, periodic pyrometry checks are needed, such as the TUS (Temperature Uniformity Survey) test, the SAT test and calibrations of all the instruments.
These checks are vital to guarantee the efficiency of the systems, which is why they are carried out frequently and, in some cases, take a long time; one example is a pyrometry check that can take up to 2 days. Coordination with production and perfect organisation are essential.
NADCAP accreditation also requires employees to be adequately trained: this training is carried out by an external body and only qualified operators can operate the oven or furnace.
Proper training, together with the oven/furnace safety systems which do not allow components to be unloaded when the oven/furnace is hot, contributes to increasing workplace safety.
The types of heat treatment to which additive parts can be subjected
The most commonly used heat treatment is stress relief, which, as its name suggests, serves to relieve parts of the stresses created during the printing process. Useable with all materials, stress relief does not cause significant alterations to the microstructure, even if it involves minimal grain enlargement.
The Beamit group has developed in-house a special stress relief treatment for aluminium alloys.
Solution heat treatment makes the structure and mechanical properties uniform, it tends to remove the very fine microstructure that is generated during additive manufacturing.
Ageing treatment involves phases of precipitation hardenings in the material which increase the hardness or tensile strength and fatigue resistance.
For aluminium, T6 heat treatment is available which involves an initial phase of solution heat treatment at high temperature (between 400 and 500 degrees), followed by quenching in water which cools the pieces very quickly, and finally ageing to confer the required properties.
The heat treatment varies according to the purpose and characteristics of the design. In the aerospace field, heat treatment is used following the AMS specifications according to which each class of material has a designated heat treatment. In racing, where extensive use is made of titanium and nickel alloys, annealing is often used.
Research and development of heat treatments for additive manufacturing
The starting microstructure of the components made using additive manufacturing is different from that obtained using traditional methods; for this reason it is advisable to develop specific heat treatments that allow you to obtain the very best properties, with results similar or even superior to those obtainable with components made with other production methods.
Research and development of a heat treatment usually begins with a literature review aimed at verifying whether heat treatments have already been studied for a specific material, which properties have been obtained and with which temperature ranges. The most promising heat treatments are then selected and tests are carried out to evaluate the mechanical characteristics and select the curve that works best for the intended application.
For materials of which we already have standard specifications, we develop ad hoc heat treatments in order to maximise the mechanical properties by inserting tensile and hardness specimens within a heat treatment cycle.
HIP - Hot Isostatic Pressing
Pres-X, the company of the Beamit group specialising in post-processing for additive manufacturing, is equipped with a special system that allows it to perform both HIP and subsequent post-HIP treatment simultaneously, combining the application of temperature and pressure. HIP treatment allows the material to be densified through the application of pressure, eliminating small pores: in this way a uniform and defect-free microstructure is obtained.
The system belonging to the PRES-X machine fleet makes it possible to carry out HIP, heat treatment and also to perform very rapid cooling, thus conferring particular properties to the components. In addition to maximising the properties of the material, the possibility of carrying out multiple treatments within the same system lets you reduce post-processing times.