Correlation between the Liquid Fraction, Microstructure and Tensile Behaviors of 7075 Aluminum Alloy Processed by Recrystallization and Partial Remelting (RAP)

Dong Yang,Kaikun Wang,Jinlong Fu

doi:10.3390/met8070508

Abstract

The recrystallization and partial remelting (RAP) method was applied to obtain the semisolid 7075 aluminum alloy with different liquid fractions. The effects of liquid fraction on the microstructure and tensile properties were determined in detail. The results show that during the semisolid isothermal treatment, the number of the intra-granular liquid droplets increased initially with the melting of the eutectic phases. Extension of isothermal soaking led to the coarsening and spheroidization of the intra-granular droplets. Finally, these liquid droplets merged and moved towards the grain exterior. The room temperature tensile strength of the RAP-processed AA7075 alloy, which were isothermally soaked at 600 and 610 °C, increased with the holding time from 5 to 15 min and then decreased dramatically from 15 to 25 min, whilst that soaked at 620 °C decreased monotonously. The fracture morphology exhibited intra-granular fracture mode at low liquid fractions. However, it transformed to a completely brittle and inter-granular type at high liquid fractions and the cohesive force of the liquid-solid interfaces at the grain boundaries determined the strength of the alloys. The transfer of the intra-granular liquid droplets into the inter-granular liquid phase played a significant role for the different fracture behaviors of the RAP-processed AA7075 alloy. The paper provides some reference for better controlling the microstructure and mechanical properties in semisolid processing.

Highlights

Semisolid processing (SSP) supplies an effective near-net shape-forming method for the steels [1,2]and nonferrous alloys [3,4,5] and composites [6,7]
It has already been proved that both thixoforming and rheoforming provide the components with excellent forming ability and considerable mechanical properties [8,9]
Slurry or feedstock with spherical grains and uniform liquid film is essential for obtaining thixotropic behavior in the semisolid state [13]

Summary

Introduction

Semisolid processing (SSP) supplies an effective near-net shape-forming method for the steels [1,2]and nonferrous alloys [3,4,5] and composites [6,7]. Semisolid processing (SSP) supplies an effective near-net shape-forming method for the steels [1,2]. It has already been proved that both thixoforming and rheoforming provide the components with excellent forming ability and considerable mechanical properties [8,9]. The mechanical properties of the thixoformed components are close to the forged ones [10,11,12]. Slurry or feedstock with spherical grains and uniform liquid film is essential for obtaining thixotropic behavior in the semisolid state [13]. The RAP process involves the warm working of the materials below the recrystallization temperature and subsequent reheating to the semisolid temperature range to obtain the equiaxed grains surrounded by the liquid phase

Methods

Results

Conclusion