PT Unknown
AU Hanke, S
TI Microstructural Alterations of Commercial Metallic Alloys by Friction Surfacing
PD 08
PY 2014
LA en
DE solid-state joining; thermomechanical treatment; recrystallisation
AB The process of friction surfacing has been known in its basic arrangement for around 70 years, but only a limited amount of scientific studies has been carried out, and industrial use is marginal. Lately, the emerging shortness of energy and resources draws attention to alternative production routes, and interest in the friction surfacing process has revived. The process’ advantages base on the fact that the deposited material does not undergo the liquid state, but instead is “forged” onto the substrate. Little is understood of the metallurgical mechanisms involved.
This work aims at revealing some of these mechanisms by electron-microscopy investigations, and at evaluating the microstructure resulting from friction surfacing as to the coatings’ behaviour under tribological loading. The materials tested are 6082 aluminium alloy, NiAl-bronze (CuAl10Fe5Ni4), tool steel (≈X50CrMoV5-1) and the alloy Cr60Ni40.
The aluminium alloy, which is most easily processed by friction surfacing, undergoes dynamic and static recovery. It is a precipitation hardening alloy, and since the temperatures occurring during friction surfacing do not match the thermal sequence for artificial ageing, the material’s hardness is adversely affected. The other three alloys undergo major phase transformations during processing. The bronze assumes a high temperature bcc structure while being deposited. During cooling, small globular or acicular fcc α grains crystallise, followed by precipitation of second (κ-) phases in spherical shape, due to decreasing solubility. The tool steel completely converts into its fcc austenite state, followed by martensitic transformation during cooling. When several layers are deposited overlapping each other, both tempering and secondary hardening effects arise. The non weldable and non deformable Cr60Ni40 alloy consists of a Cr- and a Ni-rich phase during processing. In the coatings, an fcc supersaturated solid solution and a bcc phase rich in Cr, incorporating fine Ni-precipitates, prevail.
Supersaturation of solid solution, spheroidisation of second phases and low grain size due to recrystallisation were found to be very beneficial for the wear resistance of all alloys against surface fatigue under cavitation. Sliding wear tests revealed comparable behaviour of the conventional and the friction surfaced material state.
For the feasibility of friction surfacing, a material’s properties at the temperatures reached during the process are essential. In particular, it must be possible to transfer a sufficient volume of stud tip material into an adequate plasticised state. This opens up possibilities to join novel material combinations and achieve coatings with advantageous microstructures compared to conventional techniques.
ER