Effect of Post Heat Treatment on the Microstructure and Microhardness of Friction Stir Processed NiAl Bronze (NAB) Alloy

Abstract

NiAl bronze (NAB) alloy is prepared by using friction stir processing (FSP) technique at a tool rotation rate of 1200 rpm and a traverse speed of 150 mm/min. A post heat treatment is performed at the temperature of 675 °C. The effect of heat treatment on the microstructure and microhardness is studied. The results show that the microstructure of the FSP NAB alloy consists of high density dislocations, retained β phase (β′ phase) and recrystallized grains. When annealed at 675 °C, discontinuous static recrystallization (DSRX) takes place. The content of β′ phase gradually decreases and fine κ phase is precipitated. After annealing for 2 h, both the microhardness of the FSP sample in the stir zone (SZ) and the difference in hardness between the SZ and base metal decrease due to the reduction of the dislocation density and β′ phase, accompanying recrystallized grain coarsening. With further increasing of the annealing time to 4 h, the aforementioned difference in hardness nearly disappears.

Effect of FSP process parameters with air blowing on microstructure and hardness of NiAl Bronze alloy

“…These results confirm previous studies in applying FSP on NAB alloy. Lv reported that in addition to the rise in peak temperature, the strain rates also increased within SZ causing dissolution of κ precipitates and significant increase in the formation of β′ martensite phase [15]. Therefore, increasing heat exposure generally increased the rate of phase transformation thereby the dissolution of more κ particles and formation of more β′ martensite phase.…”

“…Lv also reported that the thermo-mechanical exposure attributed to increasing the rotation speed will also lead to the dissolution of more κ hard precipitates. In addition, increasing rotation speed on FSP treated NAB alloys is expected to display higher content martensite nanotwins and to produce more dislocations within the microstructure [15]. Hence, increasing the rotation speed caused the strengthening mechanisms to gradually change from grain refinement and secondary phases strengthening to solid solution, dislocations and nano-twin strengthening [15].…”

“…Previous literature discussed the effect of including a cooling system during FSP on magnesium alloys, such as; AZ31 [14], and AZ91 [15]. However, studying the effect of cooling on the properties of NAB alloy was rarely studied.…”

Effect of post heat treatment on the microstructure and microhardness of diffusion coupled gray cast iron and low carbon steel

“…Heat treatment allows the change in the microstructure and properties of investigated diffusional welded cast iron within a wide range of parameters [8]. Post heat treatment was found to have a strong influence on the micro-structure and micro-hardness of friction stir processed NiAL Bronze (NAB) alloy [9]. Furthermore heat treatment time and temperature were found to have a strong influence on the structure, thickness, inter-diffusion coefficients, composition and hardness of the diffusion layer of diffusional welded couples produced [10].…”

Casting Alloy Design and Modification

“…The introduction of new processes also shifts the boundary between processes that used to be made in fully solid state and now move towards casting and more liquid processing resulting in hybrid processes [6][7][8]. Independent of this, post treatment processes for bulk and surface properties are still needed to be used together with the novel processing property capability [8,9]. Last but not least, it is also critical to understand the application in which the part is to be used and its implications for the stability of its properties [10].…”