Aliminum Heat Treating Basics
Steven Carr - Alfe Heat Treating, Inc.This evening we will take a quick overview of Aluminum Heat Treating Basics, Equipment, and Standards.
Heat Treating can be defined as "Sequential Heating and Cooling of Metal to achieve either homonenization, hardness, specfic mechanical properties, improved machinability, or dimensional stability". In the Aluminum Heat Treating World the most common "Tempers" are:
Solution Anneal (T4)
Solution Anneal and Age (T6)
Age Only (T5)
Annealing (T0)
In the above the "T" refers to "thermal treatment", and the number refers to a particular type of temperature sequence.
Solution Annealing is achieved by heating the metals to 940 - 1000 deg F for a specified perior of time, and is followed by a rapid cooling (or "quench") to freeze metallic structure as close as possible to its high temperature state of solid solution.
The heating is usually performed in either a continuous, drop bottom, or pit furnace. Quenching is performed by immersion of the load in a tank of a specified media such as cold water, or hot water. In some particular situations polymer media or even forced air (or other gas) is used for quenching when distortion must be minimized. The "drop time" measured as the time from when the load is at full "solution temperature" until it is fully immersed in the quench media. This is a parameter is minimized. Less than 30 seconds is a commonly accepted maximum drop time.
Aging is a process that occurs naturally in Aluminum Alloys that have been placed into the solution annealed state. Aging will occur at room temperature, but it is commonly desirable to accelerate the process by heating, to achieve desired properties in less time that natural aging would permit. Most aluminum alloys are aged by exposing then to a temperature between 330 and 350 F. for a designated time period. Aging is performed in either continuous or batch ovens.
Aluminum heat treatable alloys include:
Conventional Cast aluminum (such as 206, 319, 332 354, 355, 356, 357 and 359).
Die Cast Alloys (seldom heat treated)
Wrought Alloys (such as 2000, 4000, 6000, and 7000 alloy series.
In terms of developing and controlling the Aluminu Heat Treating Process there are several inputs required:
ALLOY SELECTION - including chemistry, grain refinement, melt quality, heat treatability, economics.
CASTING METHOD - sand or permanent mold, core or no core, porosity issues, and microstructure.
PART DESIGN - tolerances, thicknesses, thin/thick transitions, residual stresses
LOADING FOR HT - racks, baskets, air/water flow considerations
TIME/TEMP - dependent on alloy
PROCESS CONTROL - PLC control parameters, Alarms, Data acquisition issues
On the output side the consideration include:
SPEED - minimizing process time, temperature ramping issues, preheat requirements, product mix issues
PRECISION - (accuracy of temp control [commonly +/- 5 deg F], sometimes compromises
RELIABILITY - Final testing requirements, Hardness and mechanical properties, analysis of control data.
At Alfe continuous solution furnaces are used for high volume jobs. The conitnuous furnaces have multiple heat zones with multiple controls. Low volume jobs are done in drop bottom batch furnaces (single heat zone).
Aging of volume jobs is performed in dual level continuous aging ovens. Low volume jobs are performed in batch age ovens.
Automated stackers and conveyors are used to transfer jobs from quench tanks to aging facilities.
Control measures include: burner and process controls, conveyor speed control, and alarms for each control parameter. Data aquisition continuously gather data on temperature and speed and data can subsequently be searched by part number, rack or batch or parts.
Sand core products present special challenges, because the binders of the core sand are broken down by the heat treating processes, which allows for the removal of the core sand during heat treat. Handling this sand present a whole additional set of equipment issues.