How Does Aluminum Alloy 6061 Compare to 7075 for Heating Plate Core Material?

Aluminium is a common material for heating plate cores due to its light weight and good thermal conductivity. However, not all aluminium alloys are alike. Common grades, such as 6061 and 7075, are a trade-off of strength, corrosion resistance and manufacturability. Engineers constructing heating plates, whether laboratory hot plates, industrial process heaters or semiconductor wafer heating stages, need to understand these variances so the material selected can meet thermal performance and long-term reliability requirements.

6061 vs 7075 Aluminium Alloys Comparison
6061 and 7075 are both heat treatable wrought aluminium alloys. However, their alloying ingredients and consequent characteristics are very different.

6061 Aluminium Alloy 6061 is a versatile, heat-treatable alloy with magnesium (0.8–1.2%) and silicon (0.4–0.8%) as the principal alloying constituents. It is often referred to as the "workhorse" of the aluminium family due to its well-balanced mix of strength, corrosion resistance, machinability and weldability. 6061 in the T6 temper (solution heat treated and artificially aged) has a yield strength of about 40 ksi (276 MPa) and ultimate tensile strength of 45 ksi (310 MPa).

7075 Aluminium Alloy 7075 is a high strength alloy with the principal alloying ingredient being zinc (5.1-6.1%), with small amounts of magnesium and copper. In the T6 temper it has a yield strength of about 73 ksi (503 MPa) and an ultimate tensile strength of 83 ksi (572 MPa) -- values similar to some mild steels. However, this strength is obtained at the expense of reduced corrosion resistance and inferior weldability.

Heating Plate Applications: Key Differences
In the aluminium 6061 vs 7075 heating plate comparison, four important factors are compared: mechanical strength, corrosion resistance, machinability and fabrication, and thermal performance.

Mechanical Strength 
When plates are to be heated under large loads, or are required to resist bending under pressure, or to be flat under clamping forces, strength is a prime factor.

The 6061-T6 is suitable for many heating plate applications such as industrial process heaters and laboratory hot plates. Its yield strength of 276 MPa is adequate to prevent permanent deformation under normal operating pressures and mounting stresses.

7075-T6 has almost double the yield strength (503 MPa). This alloy is suited for heating plates which are subject to intense mechanical loads such as high pressure presses, structural parts of big heating plates or applications where the plate has to be very thin but nevertheless robust.

In reality, the vast majority of heating plate designs do not really need the full strength of 7075. The higher strength of 7075 is only advantageous if the plate will be exposed to significant bending moments, impact loads or it is important to save weight (and hence a thinner plate can be used for the same load bearing capacity).

Corrosion Resistance 
The aluminium 6061 vs 7075 heating plates choice frequently comes down to corrosion resistance, especially if the heated plate is used in humid, maritime or chemically harsh conditions.

6061 has good resistance to air corrosion, including salt spray and moderate chemical exposure. This is because of the creation of a persistent adhering aluminium oxide layer. 6061 also has a very good resistance to stress corrosion cracking (SCC) even under continuous tensile loads.

7075 is far less corrosion resistant. Due to its copper concentration (1.2-2.0%) and zinc-rich composition it is sensitive to intergranular corrosion and more seriously to stress corrosion cracking. In situations containing chloride ions (such as the coastal or industrial atmosphere) or when the plate is under residual tensile stresses from fabrication or thermal cycling, 7075 can break without warning. Hence, 7075 is commonly utilised with protective coatings (anodising, painting or cladding) where corrosion exposure is foreseen.

It should be noted that heating plates are often exposed to high temperatures, which might exacerbate corrosion processes. The gap in corrosion resistance between 6061 and 7075 is significantly more pronounced above 100° C. 6061 is the safest and more durable alternative for most heating plate applications, especially in contact with water, cleaning solutions or humid air.

Fabrication and Workability
Heating plates generally require machining procedures such as drilling holes for cartridge heaters, milling grooves for temperature sensors or fly cutting the surface for flatness. Weldability may also be needed for mounting brackets or sealing edges.

6061 machines with ease. It produces continuous chips which are readily broken up and may be machined to fine surface finishes. It is also easily welded by typical TIG or MIG techniques with little loss of strength in the heat affected zone.

7075 is harder to manufacture. It is more tough and more brittle causing more tool wear and tends to create stringy difficult to handle chips. More importantly 7075 is not a weldable alloy by conventional means. It is quite vulnerable to hot cracking and a substantial strength drop in welding. Brazing or friction stir welding can be employed but these are speciality processes.

This is a huge plus in working with 6061 for most heating plate fabricators. The ability to drill, tap, grind and weld without specific tooling or post weld heat treatment minimises manufacturing cost and lead time.

Thermal Conductivity and Thermal Expansion 
The alloying components are present in modest concentrations such that both alloys have similar thermal characteristics. But there are little differences:

Property 6061-T6 7075-T6 Thermal Conductivity (25°C) ~167 W/m·K ~130 W/m·K Coefficient of Thermal Expansion (20-100°C) ~23.6 µm/m·K ~23.4 µm/m·K Electrical Resistivity ~32 nΩ·m ~52 nΩ·m 6061 has better thermal conductivity (167 W/m·K vs 130 W/m·K) and transfers heat better. A heating plate with enhanced thermal conductivity has a more uniform surface temperature and a faster response to power fluctuations. The difference of around 22% is apparent in precision applications, but not dominant. The thermal stress management is comparable for both alloys as the coefficients of thermal expansion are virtually equivalent.

Comparison Table 6061 versus 7075 Heating Plate Cores
6061-T6 Alloy 7075-T6 Alloy
Yield Strength (0.2% offset) 40 ksi (276 MPa) 73 ksi (503 MPa) 
Tensile strength45 ksi (310 MPa) 83 ksi (572 MPa)
Brinell hardness 95-150
Corrosion resistance (overall)Good in marine/chemical environments Very goodBad (prone to intergranular corrosion and SCC)
Resistance to Stress Corrosion Cracking HighLow (needs protective covering in hostile situations)
Machinability Good (regular tooling, good chip formation)Fair to Poor (more tool wear, harder chips)
Weldability Excellent (TIG/MIG with 4043 or 5356 filler) Very Poor (not for structural welds)
Thermal Conductivity (W/m·K) ~167 ~130 
Typical applications of heating platesGeneral industrial hot plates Laboratory heating stages Process tank heaters Semiconductor chuck basesHigh load compression platens thin high strength heater plates (with protective covering)
Cost Lower (widely available)Higher (speciality alloy)
Application Specific Recommendations
When 6061 Is the Material of Choice
The most common alloy used for heating plate applications is 6061. Some examples of situations are:

Laboratory hot plates. Resistance to corrosion is favoured by spill resistance, cleaning chemicals and humid air.

Industrial process heaters - Good corrosion resistance should resist contact with water, steam or mild chemical solutions.

Heated platens, for laminating or pressing – The plate can be subjected to moisture from adhesives or resins 6061 resists pitting

Welded assembly - Thermowells, brackets or mounting flanges must be weldable.

Machined heating plates - Complex patterns for cartridge heaters or sensor channels are easier to make in 6061.

Cost sensitive tasks - 6061 is more easily available and less costly per pound.

In practice, 6061 has the right combination of qualities - sufficient strength, strong corrosion resistance, decent thermal conductivity, and ease of production - to make it the default option for heating plate cores.

When to Use 7075 7075 is only advised when the heating plate must support mechanical loads above the capability of 6061. Examples are:

High pressure Compression moulding - The platen should not bend under several tonnes of clamping force without permanent distortion.

Thin, lightweight heating plates - Aerospace or portable equipment when weight is at a premium and a thinner plate (due to 7075's increased strength) saves mass.

Very high static loads - The plate carries hefty components or has large bolt preloads.

If you choose 7075, there are other measures you must take:

The plate should be anodised (ideally hard anodised or coated with a corrosion inhibiting coating) to prevent corrosion.

Stress relieving heat treatment should be done before final machining to minimise residual tensile stresses from machining.

The operational environment should be dry or protected (e.g., in a sealed enclosure with desiccant).

Do not weld. Use mechanical fastening.

It should be noted that even with protective coatings, 7075 is still susceptible to stress corrosion cracking during lengthy service lives, especially at elevated temperature. Therefore, many engineers prefer 6061 for any heating plate that is expected to last more than a few years in an uncontrolled environment.

Heating Plates – Special Thermal Considerations
Often the heating plates have to be heated and cooled in cycles. 6061 and 7075 have similar coefficients of thermal expansion therefore differential expansion between the plate and embedded heaters (usually steel or Incoloy) is similar. But 6061 has better thermal conductivity (167 versus 130 W/m·K) which has two practical advantages:

Faster temperature responsiveness - Heat from cartridge heaters or cast-in elements distributes more rapidly around the plate.

Improved temperature uniformity - Less temperature differences between the heater positions and the margins of the plate.

For precision heating applications like as semiconductor wafer chucks or laboratory reaction blocks, the greater thermal conductivity of 6061 makes it the preferred choice, irrespective of strength considerations.

To conclude
Comparing aluminium 6061 vs 7075 for heating plate core material, 6061 is the preferable alloy for the great majority of applications. It offers a superb combination of sufficient strength (40 ksi yield), high corrosion resistance, good thermal conductivity (167 W/m·K), and outstanding machinability and weldability. These qualities lead to a long service life, reliable performance and cheaper fabrication costs.

The 7075 is almost twice as strong (73 ksi yield) therefore is good for heating plates under intense mechanical loads. However, its poor corrosion resistance, sensitivity to stress corrosion cracking, problematic machinability and unweldable character, severely limit its usage. The protective coatings and controlled surroundings are necessary when 7075 is the selection.

The selection of a material for the heating plate core should be based upon the general analysis of the thermal, mechanical and environmental elements involved. Most designers find 6061 to be the safe, reliable and economical choice. Use 7075 only when the mechanical requirements are really more than what 6061 can provide-and even then, pay close attention to corrosion protection.