The 6061 T6 and 6061 T651 are two different heat treatment conditions of the 6061 aluminum alloy, and their main differences and connections are as follows:

1. Heat Treatment Process:
   • 6061 T6: It undergoes solution heat treatment and artificial aging to achieve high strength.
   • 6061 T651: In addition to the T6 treatment, it includes a stretching process to reduce residual stress and improve the flatness of the material.
2. Mechanical Properties:
   • Both 6061 T6 and T651 have the same tensile strength and density, thus offering a high strength-to-weight ratio.
   • T651 has better dimensional stability and lower residual stress compared to T6 due to the additional stretching step.
3. Processing Performance:
   • 6061 T6: It has a higher internal stress, which can lead to deformation during processing.
   • 6061 T651: The additional stretching process effectively eliminates internal stress, making it more suitable for processing and less prone to deformation after processing.
4. Application Fields:
   • 6061 T6: Suitable for applications requiring high strength and rigidity, such as aerospace, automotive manufacturing, and machinery manufacturing.
   • 6061 T651: Suitable for applications requiring high strength and toughness, such as mold manufacturing, shipbuilding, railway vehicles, and precision machining, high-stress applications.
5. Cost:
   • T651 typically costs more than T6 due to the additional heat treatment steps, such as stretching.

In summary, the main connection between 6061 T6 and 6061 T651 is that they are both based on the 6061 aluminum alloy, with different heat treatment processes to meet different application needs. The T6 condition provides high strength, while the T651 condition maintains high strength while improving dimensional stability and processing performance through additional stretching steps, making it suitable for more precise machining requirements.