Solution Treatment and Over-age (T7)

Development of the solution treatment (ST)

A20X was initially developed from the age hardenable casting alloy A201. Hence A20X is an Al-Cu alloy with other alloy additions that make it a unique material. Like many similar alloys, A20x develops its high strength from the precipitation of certain second phases. In order to optimise the precipitation reaction the alloy must first be solution treated to ensure sufficient solute atoms (alloying elements such as the copper, magnesium and silver) are in solid solution. The aim is to create a saturated solid solution that is in a thermodynamically unstable state. This is achieved by heating the metal alloy to a high temperature, and then after a period of time rapidly cooling by quenching.

In the original cast metal this heating cycle was developed to occur over a 48 hour period. This length of time was found to be necessary in order to allow the larger second phase particles to slowly diffuse into the solid solution. Furthermore, this was also a stepped process involving 3 different holding temperatures slightly below, then above 500°C, and finally above 530°C. The resulting microstructure after quenching is then an even equiaxed grain structure with high solute concentrations in each grain of the alloy.

Additive manufactured alloys have a much finer sub-granular cellular structure when compared to their conventionally produced equivalent metals. Also, in most alloys there is epitaxial growth from the base substrate that any parts are built on. This then leads to the formation of larger columnar grains, with micron scale sub-granular structures. However, in A20X there is no large grain formation, and very little epitaxial solidification. Therefore, it followed that since rapid solidification naturally results in higher solute concentrations, the developed the heat treatment cycle for the additive manufactured A20X would be a shorter stepped cycle.

This first developed heat treatment cycle for AM part made with A20X alloy was reduced to just 24 hours. All parts processed this way, and then correctly aged, have been consistently found to be the same, and even higher, strength as the cast alloy. This cycle was the standard cycle applied to all AM A20X parts since 2017

The new reduced solution treatment

Although the standard ST cycle for A20X AM parts has been shown to be very successful, there has always been a desire to reduce this cycle to allow for a shorter, more amenable process time. The problem to solve was how to achieve this without causing negative side effects, such as incipient melting, or potentially grain coarsening – bearing in mind that one of the unique properties of the A20X alloy is its very fine equiaxed microstructure.

Extensive investigations involving many trials at several different temperatures, and holding AM parts for varying times have allowed us to find a tightly controlled temperature range that is ideal for the A20X AM alloy. This new cycle involves just 2 steps, compared to the previous 3, and holding at much shorter times than previously believed possible. The main advantage is that the full heat treatment cycle, including the aging process, can be achieved in a single day. This then allows for the same level of end to end process time as for any other alloy that is just stress relieved, or directly alloy from the as-built parts*.

Tensile testing, using samples made from bars that were produced up to 15mm thick and manufactured to the ASTM E8 standard, was carried out to verify the new reduced ST cycle. AM parts built on a number of different laser powder bed fusion systems, and with different batches of powder were also tested. Furthermore, on-going detailed metallographic analysis is being carried out to further understand the effects of the solution treatment on the aging performance of the alloy. To date no significant differences have been found, compared to the previous standard 24 hour ST cycle.

Details of the new recommended solution heat treatment cycle, carried out in ovens meeting as a minimum  AMS2750 Equipment class 2, are given below:

  1. Place parts in a cold oven.
  2. Ramp to 505°C in not less than 100 minutes.
  3. Hold at 505°C for 2 hours (minimum); up to 4 hours for larger, more complex sections.
  4. Ramp to 530°C, no delay.
  5. Hold at 530°C for 4 hours (minimum); up to 6 hours for larger, more complex sections.
  6. Quench in 20-25% glycol (polymer) solution, max quench delay 15 seconds; cold water quench may be used where maximum strength is required and there is little risk of distortion.

Age hardening precipitation treatment

The AM parts can be aged to the final T7 temper, after they have been solution treated and quenched, by following the recommended cycle:

  1. Place parts in an oven.
  2. Heat to 190°C.
  3. Hold for between 4-6 hours.
  4. Cool to room temperature; it is acceptable to turn off the oven, open the doors and for the parts to be left to cool.

 

* The processing of other Al based AM alloys at temperatures in the range of 250-400°C, and for shorter times, is said to require a standard work day under normal sub-contracted conditions at typical external suppliers of such heat treatment processes.

A20X sintered powder by FAST at University of Sheffield