EBSD
measurements on the weld seam area of differently extruded
ME21
hollow profiles
Authors: Fahrenson Christoph , Gensch Felix , Gall Sven , Berger Dirk 
Published: December 2016

The production of hollow profiles using porthole dies is a widespread manufacturing process for aluminum alloys. However, the extrusion of magnesium and its alloys into hollow profiles has not yet been established due to several reasons. One thereof is the lack of knowledge about the interdependencies between process parameters, die design and the product quality (microstructure, texture, mechanical properties etc.). Therefore, different extrusion experiments were carried out using the magnesium alloy ME21 (2.1 wt% Mn, 0.7 wt% Cer, Mg balance). For the realization of hollow profiles with varying wall thickness and thus a varying extrusion ratio (ER) a modular porthole die (Fig. 1) has been used. It consists typically of a mandrel part and a die part. The die part defines the outer shape of the extrudates and includes the welding chamber. The mandrel part consists of a mandrel that forms the inner shape of the hollow profile while the mandrel itself is kept in place by three bridges. Through the usage of three differently dimensioned mandrel parts the variation of the wall thickness (and the ER) can be done while the outer dimensions are kept constant for each profile. Extrusion ratios of ER = 8:1, 16:1 and 30:1 were applied.

During the extrusion process the billet material is split up into three separate metal streams which flow around the bridges of the mandrel part and successively weld under solid‐state conditions behind the bridges in the so‐called welding chamber. These welds are called longitudinal weld seams. It is known that there is an additional amount of straining of the material that forms these weld seams due to the friction. Hence, these weld seams always pose a potential anomaly when compared to the weld free material.

First analysis using polarized light‐optical microscopy indicated that both sides of the weld seam (representing each metal stream) have different textures. In order to quantify that observation EBSD is the appropriate technique to gather exact information of the weld line, which displays the separating line between the two visible parts of the weld seam and of the material in the vicinity. The EBSD measurements were performed on a ZEISS DSM 982 with Gemini optic and the EDAX Hikari camera. EDAX also supplied the necessary “OIM Data collection” and “OIM Data analysis” software. The resulting maps have the dimensions of 800 µm × 650 µm with a resolution of 0.5 µm. For better focusing at low magnification and the largest aperture the samples were tilted to 67° instead of 70°. Beforehand the samples were ground with SiC abrasive paper and then polish with 6 µm, 3 µm and 1 µm diamond‐based suspension following a chemical polish with CP2 agent. The IPF maps were taken at half wall thickness on a plane parallel to the profile surface.

The first EBSD measurements reveal for all extrusion ratios a pronounced weld line dividing the weld seam in two parts featuring sub‐textures whereby the orientation of the {0001}‐planes of one metal stream is mirror‐inverted to the other. The texture is also significantly different from that of the weld free material.

In future work the focus will be among others on the die design, especially the shape of the bridges, in order to modifiy the local weld seam texture and to optimize the overall properties of the hollow extrudates.