When heat treating aluminum and aluminum alloys, it is important to understand the challenges we face and why absolute control of process and equipment variability is so very critical. To aid the heat treater in this regard, The Doctor has gathered together in one place the most commonly reported process problems along with some recommendations on how to resolve them. Let’s learn more.
The most commonly reported problems with heat treatment of aluminum include concerns over:
- Improper racking of parts – This results in distortion due in large part to the inability of the quenchant to extract the heat at a sufficient rate to achieve the desired mechanical properties. Racking may also create thermally induced distortion (since the creep strength of aluminum is poor). Proper racking (Fig. 1) avoids these issues.
- Excessive heating/ramp rates – These contribute to thermal distortion and should be avoided. Proper racking of parts helps uniform heating.
- Higher-than-anticipated residual-stress levels – Heat treatment not only affects mechanical properties but also directly influences residual-stress levels. Some of the various causes are differential cooling during quenching between surfaces and interior regions (including post-solidification cooling of castings), ramp rates and changes in temperature at intermediate steps. Residual stress is a function of (large) differential cooling rates, section thickness, abrupt changes in section size and material strength. It is important to understand that stresses induced by quenching are many times more pronounced than stresses from other types of processes (including casting).
Variation in time/temperature/quench parameters – These ultimately result in changes to mechanical and/or physical properties, both part-to-part and load-to-load. These include slow part transfer times, improper (slow) quench, overheating, underheating or changes in time-temperature parameters during precipitation hardening. For example, larger particles or precipitates result from longer times and higher temperatures.
- Overheating – The concern is incipient or eutectic melting (Fig 2). For example, solution heat treatment involves temperatures close to the melting point of many aluminum alloys (especially the 2xxx series, often only a few hundred degrees below their melting point). Proper temperature is needed to promote dissolution in the solid state of the alloying elements present.
- Underheating – This results in loss of mechanical properties due to inadequate supersaturation. When the aging temperature is low and/or the aging time is too short, solute atom gathering zones (GP zones) are not easily formed, which leads to low strength after aging.
- Inadequate quenching resulting in distortion – The problem/challenge is the movement of part(s) into the quench, especially when manual quenching is critical. Parts must enter smoothly. (The heat-treat shop term is that you want to avoid “slapping” the quench with the part.) Uniform heat transfer across the part avoids differential cooling and differential thermal strain. Horizontal changes in heat transfer are often more insidious than vertical ones. Keeping the quenchant at the proper temperature, controlling its rate of rise, ensuring uniform flow and optimizing the process for the chosen quenchant (e.g., air, water, polymer) are critical. For example, the cooling rate of polymers can be varied to suit a specific application by changing concentration, temperature and the degree of agitation, which affects both uniformity of heat transfer and quench rate during the nucleate boiling stage. Maintenance of the quenchant is also important. Parts of complex shapes such as forgings, castings, impact extrusions and components produced from sheet metal may be quenched at slower quenching rates to improve distortion characteristics.
- Surface blistering/high-temperature oxidation – These conditions are best described in The Heat Treat Doctor column “High Temperature Oxidation – A Case Study,” Industrial Heating, February 2016.
- Overaging – This may result in loss of mechanical properties. When the aging temperature is high and/or the aging time is long, the critical nucleus size of the phase precipitated from the supersaturated solid solution can be increased, resulting in low strength properties after aging.
Underaging – This may also result in loss of mechanical properties.
- Improper natural aging – Times may vary from around five days for the 2xxx series alloys to around 30 days for other alloys. The 6xxx and 7xxx series alloys are considerably less stable at room temperature and continue to exhibit changes in mechanical properties for many years. With some alloys, natural aging may be suppressed or delayed for several days by refrigeration at -18°C (-1˚F) or lower. It is common practice to complete forming, straightening and coining before aging changes the material properties appreciably. For example, conventional practice allows for refrigeration of 2014-T4 rivets to maintain good driving characteristics.
- Improper artificial aging – Artificial aging (aka precipitation heat treatment) is a lengthy, low-temperature process. Temperature control is critical along with maintaining a minimum temperature uniformity of ±6˚C (±10˚F). The targeted uniformity should be ±4˚C (±7˚F).
- Inadequate time at temperature – The consequence is failure to achieve mechanical properties. Too short a time will result in inadequate supersaturation; too long a time often manifests itself in part distortion.
- Inadequate temperature uniformity – This may result in an inability to achieve or may alter mechanical properties. The typical process temperature uniformity requirement is ±6˚C (±10˚F), with ±3˚C (±5˚F) preferred for most aerospace applications.
- Improper cold working after solution treating – This often results from a lack of understanding of the response of the alloy in question. For example, cold working of 2xxx series alloys in the as-quenched condition greatly increases its response to later precipitation treatment.
- Inadequate cooling rate if annealing solution-heat-treated product – A maximum cooling rate of 20˚C (40˚F) per hour must be maintained until the temperature is reduced to 290˚C (555˚F). The cooling rate is unimportant below this temperature.
It should be mentioned in passing that there are a number of ingot defects in the mill state that can influence subsequent heat treatment and mechanical properties. Some of these include:
- Porosity/midline porosity – due to lack of mass feeding, hydrogen segregation or surface oxide layers (often due to air pockets)
- Inclusions – casting impurities (due to grain refiners or air pockets) in the form of carbides, borides and oxides to name a few
- Macro or microsegregation – inhomogeneous solute composition and hard intermetallic and second-phase particles. Proper homogenization helps negate this concern.
- Deformation/shrinkage – due to stress/strain induced by cooling conditions
- Hot tears – due primarily to feeding issues
- Issues related to rolling (sheet or plate) or stretching (extrusion, bar and plate) to produce higher mechanical properties. If the higher properties are required, however, reheat treatment should be avoided.
Other Related Issues
Issues that are closely related to or are influenced by heat-treatment operations include such items as:
- Damage from metal finishing causes pitting or aggressive (intergranular) attack of the aluminum surface from sources such as excessive deoxidize etching or cleaning operations. Better control of these post-heat-treat operations avoids this issue.
- Overheating during welding results in loss of mechanical properties (e.g., tensile strength). Controlling the weld sequence, technique and joint design minimizes this concern.
- Changes to temper condition produces damage from excessive heat from operations such as welding or hydrogen-embrittlement relief baking.
- Part damage from aggressive machining. It is generally believed that heat treatments that increase hardness improve machinability of aluminum alloys. However, care must be taken to understand that cutting force is just one of many parameters (e.g., condition of the tool, tool life, the surface finish, cutting energy and chip formation mode). For example, aging of 6061 aluminum influences the cutting force at low speeds but not at high speeds.
The solution to most aluminum heat-treatment-related problems is to understand what can go wrong; establish good practices and procedures; be consistent (and repeatable) in the execution of these procedures; monitor the process in as close to real-time as possible; and maintain furnace records and time-temperature charts to confirm that the intended practices were indeed performed.
Finally, be sure that the testing methods used to confirm that the parts are good are robust and adequate to ensure proper performance in the field. Heat treaters have heard this all before, but nowhere is it more critical than in the heat treatment of aluminum and aluminum alloys.
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