Warm negative incremental forming of magnesium alloy AZ31 Sheet: New lubricating method

Abstract

The present study has been undertaken in order to investigate the suitable lubricants and lubricating methods, which can be employed to form a magnesium alloy AZ31 sheet by warm negative incremental forming (NIF). For the intended purpose, Nano-K₂Ti₄O₉ whisker and organic binder were employed to improve the bonding strength at lubrication coating/sheet interface and lubricating properties at elevated temperatures. The Nano-K₂Ti₄O₉ whisker enhanced solid lubrication film and the solid graphite or MoS₂ powder-coated porous ceramic coatings by pulsed anodic oxidation (PAO) almost have the same coefficient of the initial friction about 0.07–0.1 at room and elevated temperatures, which was satisfied with friction and lubrication condition of warm incremental forming (IF) of the metal sheet. Solid graphite or MoS₂ powder-coated ceramic coatings possessed the remarkable lubrication and self-lubrication effect. These suggested lubricating methods gave an excellent solid lubrication performance and good surface quality of the formed parts in warm IF process of the metal sheet.

Introduction

It is difficult to form hard-to-form sheet metals (for example, magnesium alloy and titanium alloy) by the single point incremental forming (SPIF) process due to the low formability at room temperature. However, the rolled AZ31 alloys offer good mechanical properties with good balance between strength and ductility, and excellent formability at elevated temperature because non-basal slip systems become active in addition to basal slip such as hot drawing forming (Hai et al., 2008), gas bulging forming (Kim et al., 2008) and warm SPIF process (Ji and Park, 2008, Ambrogio et al., 2008).

A thorough deliberation may reveal that the lubrication is not so simple when the magnesium alloy sheet is formed. For applying the lubricant to the surface of sheet metal and ensuring its presence throughout the whole forming process in warm SPIF of magnesium alloy sheet, the lubricant must be special, which will withstand the local deformation and high temperature, and will not be squeezed out from the mating surfaces. If the lubricant is not selected by keeping in view the specific characteristic of the SPIF process, the metal sheet may peel off.

In the cold incremental forming of soft metals, such as pure aluminum and low carbon steel, mineral oils can provide sufficient lubrication to produce parts with an acceptable surface quality, as shown in literature (Kim and Park, 2002) However, the preliminary experiments were carried out by employing mineral and machine oils as lubricants in the hot forming conditions which thinned out and did not serve the purpose well due to low viscosity and low heat-resistance. In this category, solid graphite and MoS₂ (these lubricants have smaller coefficient of friction than the other solid lubricants such as boron nitride, etc.) were investigated in detail. As the solid lubricant, the graphite has been applied for a long time. In the sol–gel graphite solid lubrication coating, the graphite was as solid lubricant and Zn was as a platform for graphite powder that enhanced the bonding strength with substrate (Wang et al., 2005). Solid lubricants are highly effective because of their excellent lubricating ability in the wide temperature range, from room temperature to 400 °C. However, the solid lubricants are generally coated with a binder on the workpiece by conventional spraying, their original solid-lubricating abilities are lost with a lubricating life (Hanada et al., 2005). Properties of MoS₂ coatings can be improved by the codeposition of titanium (Jing et al., 2004). The parts of pure titanium sheet in NIF with good surface can be realized by using the anodic oxidation (AO) and the paste of molybdenum disulphide (MoS₂) with petroleum jelly (Hussain et al., 2008). The cone parts of titanium alloy (TiAl₂Mn_1.5) sheet with AO were processed successfully by electric hot method and using solid MoS₂ powder dusted on the surface of the sheet metal as lubricant in SPIF (Fan et al., 2008).

Well known, the ceramic coating by micro-arc oxidation or PAO is classified into a porous loose over-coating and a compact bulk coating (Guo and An, 2006, Jin et al., 2006). Therefore, porous ceramic coating on the metal sheet may be coated by application of the solid lubricant so that the lubricant particles can be retained in the pore of the coating or absorbed by the porous coating. But up to the present-day, no literatures report the research on the solid lubricant film or coating prepared by the PAO method for warm SPIF of the magnesium alloy sheet.

In this present paper, sheet blank with and without PAO ceramic coating was used to investigate the effect of lubrication coating and its categories on the surface quality of the formed parts. The main objective of the research contained the proper lubricants, lubricating properties and novel lubrication method for warm negative incremental forming of the magnesium alloy sheet.