Forgeability of Modified AZ and ZK Wrought Magnesium Alloys

Transcription

1 Proceedings of the 8th International Conference on Magnesium Alloys and their Applications; Wiley-VCH, Germany (2010): Forgeability of Modified AZ and ZK Wrought Magnesium Alloys Gerrit Kurz 1, Wim H. Sillekens 2, Robert J. Werkhoven 2, Dietmar Letzig 1 1 GKSS Research Centre Geesthacht Magnesium Innovation Centre; Geesthacht, GERMANY 2 TNO Science and Industry Materials Technology; Eindhoven, NETHERLANDS 1 Abstract Wrought magnesium alloy products have favourable attributes compared to castings with regard to mechanical properties. In addition they provide a complementary class of shapes and geometries. These are the current drivers for developments within the field of magnesium forging technology. Within this context, the European research project MagForge is being conducted with the overall goal to provide tailored and cost-effective technologies for the industrial manufacturing of forged magnesium alloy components. One of the present impediments to magnesium forging is the availability of suitable alloys: Forgeability in terms of the robustness of processing windows and the resulting product quality in terms of mechanical performance need to be improved. Hence this is one of the main topics of the project. This paper reports on the progress made in this particular area. The strategy adopted for advancement consists in inducing grain refinement and stabilisation of the microstructures by rare-earth and complementary modifications of the commercial AZ80 and ZK60 forging alloys. In line with common practice, forging slugs were prepared from cast billets by preextrusion. The quality of the modified alloy slugs is assessed on the basis of laboratory-scale cone-forging trials. The results show a significant improvement over the existing alloys. 2 Introduction Forging is an industrial manufacturing method in which metal parts/components are shaped from feedstock (slugs) by applying compressive forces through various tools and dies. Whereas closed-die forging is the most common variant (see Figure 1), this metalworking category also includes such processes as swaging and impact extrusion. Shapes can be intricate, and as such it is a competitive process to casting. Although forging yields (near) netshape products, most forgings require some kind of finishing such as machining for reasons of accuracy and surface quality, heat treating to modify mechanical properties, and/or coating to enhance appearance or resistance to corrosion and wear. During processing, metal flow and grain structure can be controlled, so forged components generally have good strength and toughness compared to their cast counterparts as well as no porosity.

2 Figure 1: Principle of closed-die forging (left) and some typical products (right; courtesy of Leiber) The main market for forged components is automotive; the forging industry is thus faced with some particular trends that relate to developments within this sector [1, 2]. The general platform strategies as well as the increase in diesel-engine powered cars and four-wheel drive sports-utility vehicles imply volume increases for forged components. On the other hand, there is continuing price pressure: where costs (for materials, labour, energy and so on) increase, the main customers and their system suppliers do not accept that these are passed on to deliveries. Furthermore, the automotive industry has committed itself to substantially reduce fuel consumption and exhaust emissions (amongst others CO 2 ), for which weight saving at all levels is crucial. Priorities in lightweight structure design are the un-sprung mass (wheels and their suspension), the front end before the front axle and the mass between the front axle and instrument panel. All these are typical areas where forged components can be used. The behaviour of magnesium alloys during closed-die forging has already been investigated in several tests [3, 4, 5]. It was demonstrated that the forging process offers the possibility to produce parts with a complex geometry. However, commercially produced forgings of the aluminium alloy AA6082, for example, have yield strengths of ~340 MPa and elongations of ~10 % [6], whereas current high-strength magnesium alloys have yield strengths of only about MPa [2, 7]. Possible alloy modifications leading to improved mechanical properties of the magnesium forgings are therefore of interest.. 3 Feedstock Material The main target of the MagForge project is to forge a magnesium component that fulfils all requirements of the same shaped aluminium component. Literature data on the basic alloys selected, AZ80 and ZK60, show a very small gap between the mechanical properties of the forged components and the requirements for aluminium components. In order to fulfil all the requirements of the aluminium component it is necessary to improve the strength of the two alloys. Therefore the wrought magnesium alloys AZ80 and ZK60 were modified by alloying with rare earth elements, yttrium and calcium. The AZ80 and ZK60 alloys were modified with a small (AZ80 Cer1 / ZK60 Cer1) and an elevated (AZ80 Cer2 / ZK60 Cer2) amount of cerium. Both amounts were smaller than 1 wt. %. The alloy composition of AZ80 Y is similar

3 to AZ80 Cer2, but a small quantity of yttrium (<1 wt. %) was added. A small quantity of calcium (<1 wt. %) was combined with ZK60 Cer2 to produce the ZK60 Ca. In order to obtain homogeneous, fine-grained microstructures all the cast materials were indirectly extruded with a billet temperature of 280 C and an extrusion ratio of A typical extrusion texture of the feedstock can be assumed which orients the basal planes parallel to the extrusion direction [8]. The feedstock was machined to forging slugs of 25 mm diameter and 50 mm length. Following extrusion, samples were subjected to microstructural analysis, which was carried out on polished and etched central sections of the slugs and forgings using optical microscopy. For sample preparation an etching solution based on picric acid was applied [9]. In Figures 2 and 3 the microstructures of the modified AZ80 and ZK60 feedstock are shown. The average grain size was determined using several micrographs of each alloy. The microstructures of the alloys AZ80 Cer1 and AZ80 Cer2 are quite similar as are the grain sizes of 12 and 14 µm. The microstructure of the AZ80 Y extrusion was more inhomogeneous than that of the two alloys AZ80 Cer1 and AZ80 Cer2. In all three alloys, lath-shaped (Mg 17 Al 12 ) intermetallic particles were also observed. AZ80 Cer 1 AZ80 Cer 2 Average grain size: 14 µm Average grain size: 12 µm AZ80 Y Average grain size: 16 µm Figure 2: Microstructure of the extruded modified AZ80 feedstock material. In contrast to the AZ80 alloys, the modified ZK60 alloys were more inhomogeneous. Micrographs of ZK60 Cer1 and ZK60 Cer2 revealed inhomogeneous microstructures containing grains with a wide variation of sizes. Very small grains with sizes in the range 2 4 µm coexist with large unrecrystallised grains. Average grain sizes of about 13 µm in ZK60 Cer1 and 18 µm in ZK60 Cer2 were found. The microstructure of ZK60 Ca is more homogeneous and the grain size of 9 µm is smaller than in the other modified ZK60 alloys.

4 ZK60 Cer 1 ZK60 Cer 2 Average grain size: 13 µm Average grain size: 18 µm ZK60 Ca Average grain size: 9 µm Figure 3: Microstructures of the extruded modified ZK60 feedstock materials. 4 Forging Trials The forging experiments were carried out on a hydraulic press with a nominal force of 1000 kn and a ram speed of 10 mm/s. The component of choice was a stylised wheel hub. It is a small-size forging of symmetrical shape and the upper die had got a de-aeration borehole. The print of this borehole on the forged part was a good indicator of die filling. All forgings were produced with pre-heated tools. The experiments (slug dimensions: Ø 25 mm, length 50 mm), were carried out at temperatures of 175, 200, 250, 300, 350, 400 and 450 C. All parts were forged in one step. At the beginning of the forging process the slug was placed vertically in the die and then compressed. In order to avoid material failure during the process, both the upper and lower dies were pre-heated to the slug temperature. An oil-based, graphite-pigmented compound was used as lubricant. After forging, the parts were cooled down with ambient air. These experiments were performed to determine the best alloy modification for die forging. In order to find the best forging alloy the following selection criteria were chosen: Good formability at low temperatures Broad process window For comparison purposes, identical forging trials were performed on the commercial alloys AZ80 and ZK60. Figures 4 and 5 display the forging results. Only the alloy ZK60 and its modifications showed sufficient filling of the die at the lowest forging temperature of 175 C. Only the forgings of the alloys ZK60 and ZK60 Cer2 showed no evidence of cold cracking in the flange. Cold cracks appeared in the flange of the forgings of ZK60 Cer1 and ZK60 Ca. In comparison to the ZK60 alloys, acceptable die filling of the AZ80 Cer2 and AZ80 Y alloys was obtained at the higher temperature of 200 C.

5 200 C 450 C AZ80 Cer1 AZ80 Cer2 AZ80 Y Figure 4: Forging results of the modified AZ80 alloys 175 C 450 C ZK60 Cer1 ZK60 Cer2 ZK60 Ca 400 C Figure 5: Forging results of the modified ZK60 alloys. In all the modified AZ80 alloys cold cracks in the flange were observed at a forging temperature at 200 C. However, the components of all alloys forged in the temperature range between 250 C and 350 C showed no cracks and sufficient filling of the die. At the forging temperature of 400 C hot cracks appeared in the ZK60 Ca forging and destroyed the part (Figure 5). The forgings of all the other alloys showed sufficient die filling up to a forging temperature of 450 C. No hot cracks were visible in the forgings of the alloys ZK60 Cer1 and Cer2 at this process temperature. The forged components of ZK60 and AZ80 Y displayed mild hot cracking. Stronger hot cracking appeared in the forgings of the other AZ80 alloys.

6 5 Conclusions The behaviour of conventional and modified magnesium alloys during closed-die forging was investigated. It has been demonstrated that the forging process offers the possibility to produce parts with a complex geometry. Especially the forging tests at temperatures of 175 C for ZK60 Cer2 and 200 C for AZ80 Y showed good results. Both modified alloys showed good formability at low temperatures. The results of the forging trials show that the alloys AZ80 Y and ZK60 Cer2 have the best formability at low temperatures as well as the broadest processing window. Due to the fact that magnesium is about 35 % lighter than aluminium, it offers potential for the substitution of aluminium alloys in automotive components. 6 Acknowledgments The MagForge project is being conducted for the European Commission within the framework of the programme Horizontal research activities involving SMEs under the contract number CT The financial support is gratefully acknowledged. 7 References [1] B. Viehweger, in Proceedings MIA Magnesium im Automobilbau, 3rd WING- Seminar, (2004). [2] J. Becker, G. Fischer, Manufacture and properties of extruded and forged components in magnesium wrought alloys, Proceedings of the 11th Magnesium Automotive and End User Seminar, [3] G. Kurz, B. Clauw, W.H.Sillekens, D. Letzig, Die-Forging of the Alloys AZ80 and ZK60, TMS, Magnesium Technology 2009, [4] J. Swiostek, G. Kurz, P. A. Beaven, D. Letzig, Die-Forging of Commercial and Modified ZK60 Magnesium Alloys, International Conference Magnesium Broad Horizons, Saint Petersburg, 6-8 June [5] G. Kurz, J. Swiostek, P.A. Beaven, J. Bober, D. Letzig, Die-Forging of Magnesium Materials, SAE World Congress 2008, Magnesium Technologies, Detroit, April 2008, [6] [7] J. Swiostek, J. Bohlen, D. Letzig, K.U. Kainer, Comparison of microstructure and mechanical properties of indirect and hydrostatic extruded magnesium alloys, Proceedings of the 6th International Conference on Magnesium Alloys and their Applications, (2003), [8] J. Bohlen, J. Swiostek, H.-G. Brokmeier, D. Letzig, K.U. Kainer, Low temperature hydrostatic extrusion of magnesium alloys, TMS, Magnesium Technology 2006, [9] V. Kree, J. Bohlen, D. Letzig, K.U. Kainer, Practical Metallography 5, (2004),