Microstructural evaluation and phase transformation of recast layers in electrical discharge machined dual phase Fe-Mn-Al alloy

Abstract

The electrical discharge machining process is an effective way to machine hard alloys. A homogenized dual phase Fe-24.0%Mn-8.3%Al-5.0%Cr-0.38%Si-0.34%Mo-0.45%C (wt%) alloy casting piece was employed to study the phase transformation and microstructure phenomena of recast layers by means of electrical discharge machining. The wave-like recast layers appear with micro cracks. The thickness of the recast layer increases with pulse duration. When the pulse duration ranges from 200 to 300 μs, the discharge current has no influence on the thickness of the recast layer. The copper concentration is high in the recast layer caused by the melting of the copper electrode. The copper contents of the recast layer increase with the pulse duration and the discharge current.

The novel solidification microstructures of the recast layer can be classified as (1) a dense and fine dendritic outermost sublayer, (2) a coarse dendritic intermediate sublayer and (3) an innermost γ recast zone. The average chemical composition of the recast sublayers is Fe-19.007%Mn-8.377%Al-2.981%C-4.505%Cr-0.627%Cu-0.377%Mo-0.376%Si (wt%). The novel structure of the recast sublayer is identified as (Fe,Mn)₃AlC_x, which was formed due to the re-melting of the matrix and the carburization of the cracked dielectric fluid. The precise lattice parameter, a _o, of the (Fe,Mn)₃AlC _x phase measured by extrapolation against cos²θ/sinθ is 0.3801 nm. The microhardness of the recast layers is as high as Hmv574 due to the existence of (Fe,Mn)₃AlC_x carbide.