Scholarly article on topic 'Temperature Influence on Bond Formation in Multi-material Joining by Forging'

Temperature Influence on Bond Formation in Multi-material Joining by Forging Academic research paper on "Materials engineering"

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Procedia Engineering
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Keywords
{"Cold forging" / "Warm forging" / "Bond forging" / "Pressure welding" / Steel / Aluminum / Bonding}

Abstract of research paper on Materials engineering, author of scientific article — Simon Wohletz, Peter Groche

Abstract Cold welding, e.g. by cold forging, is a smart manufacturing technology, enabling novel multi material designs. A combination of steel and aluminum is particularly attractive for manufacturing multi-material products but challenging to handle in a pressure welding process. Forging at elevated temperatures could provide promising benefits to overcome limitations in cold welding with regard to process design and bond formation. The transition from cold to warm forging effects the covering layer and the bulk properties. The paper at hand focusses on the effect of temperature increase on bond strength and bond formation on the microscopic scale.

Academic research paper on topic "Temperature Influence on Bond Formation in Multi-material Joining by Forging"

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Procedía Engineering 81 (2014) 2000 - 2005

Procedía Engineering

www.elsevier.com/locate/procedia

11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014,

Nagoya Congress Center, Nagoya, Japan

Temperature influence on bond formation in multi-material joining

by forging

Simon Wohletz*, Peter Groche

Technische Universität Darmstadt, Institute for Production Engineering and Forming Machines, 64287 Darmstadt, Germany

Abstract

Cold welding, e.g. by cold forging, is a smart manufacturing technology, enabling novel multi material designs. A combination of steel and aluminum is particularly attractive for manufacturing multi-material products but challenging to handle in a pressure welding process. Forging at elevated temperatures could provide promising benefits to overcome limitations in cold welding with regard to process design and bond formation. The transition from cold to warm forging effects the covering layer and the bulk properties. The paper at hand focusses on the effect of temperature increase on bond strength and bond formation on the microscopic scale.

©2014 The Authors. Published by ElsevierLtd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University Keywords: Cold forging; Warm forging; Bond forging; Pressure welding; Steel; Aluminum; Bonding

1. Introduction

Joining technologies based on plastic deformation offer promising benefits with regard to the joint properties and accuracy in comparison to other joining techniques, like thermal welding. A comprehensive overview on joining by plastic deformation, in particular the mechanisms on metallurgical welding can be found in [1] and [2]. The demanding tribological contact conditions in cold forging processes can be utilized to establish a metallurgical weld between two or more deformable workpieces. As reported in [3] and [4] this processing technology combines

* Corresponding author. Tel.: +49-6151-16-76982; fax: +49-6151-16-3021. E-mail address: wohletz@ptu.tu-darmstadt.de

1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University doi: 10.1016/j.proeng.2014.10.271

the advantages of cold forging and multi-material design even if the hardly weldable workpiece material steel is utilized. Wagener and Haats [5] presented a cost-saving utilization of corrosion resistant material by welding titanium and an aluminum alloy during cup extrusion. Furthermore, multi-material compounds offer the possibility to combine lightweight and high strength material for the sake of weight reduction by an optimal strength to weight distribution. The usage of forging for the production of multi-material compounds entails further benefits like an excellent material utilization, a good surface quality, a high strength and short processing times.

According to the phenomenological model presented by Bay et al. [6] cold pressure welding is activated by two mechanisms which require a characteristic plastic deformation of the contacted surfaces. Technical surfaces are usually covered with a contaminant film and can also provide a relatively brittle covering layer resulting from a broad variety of surface treatments like brushing, chemical plating or others. The first mechanism is based on the fragmentation of these covering layers. If two surfaces are in contact with each other and these covering layer exceed their critical formability due to a certain level of plastic deformation, juvenile material extrudes between the cracked covering layers. If the pressure and surface exposure are sufficient, a metallic compound is built which can be stronger than the yield stress of the weaker material. The second mechanism is based on the thinning of the surfaces' contaminant film. After exceeding a certain level of surface enlargement, this contaminant film breaks down and clean surface elements are in contact which also leads to a metallurgical weld like the first mechanism. As presented by Zhang and Bay [7] brushing is one of the effective surface preparation methods for cold welding of steel to other metallic materials. According to Yan et al. [8] and Peng et al. [9] roll bonding at elevated temperatures can be beneficial for the bond strength by activating diffusion in the contact area. However, if the bonding surfaces are covered by largely distributed oxide layers as also reported in [9] or if the increased interface temperature leads to a critical formation of thick intermetallic phases as investigated in the case of lateral extrusion of aluminum and magnesium in [10], the bond strength can be affected negatively. Behrens et al. [11] investigated hot forging of aluminum and steel compounds at a starting temperature of ~ 970 °C and ~ 520 °C for steel and aluminum, respectively. In this case, intermetallic phases (~ 4 ^m) are built up at the weld interface. However, the effect of the forming temperature on the material flow, the bond occurrence and strength are not investigated. In particular, the conditions for the weld formation in a temperature range from room temperature to approximately 600 °C in case of steel are not analyzed yet. However, for the layout of new pressure welded components forming at elevated provides promising benefits as follows:

• control of the material flow

• increase of the material's formability

• combination of materials with entirely different flow stresses in the cold state

Due to the lowered flow stress, forming at elevated temperature might also be beneficial in consideration of the lower forming forces. A reduced flow stress can also promote the bonding mechanism by an eased material extrusion between the covering layer's cracks. Elevated forming temperatures can also promote the bond occurrence by increased diffusion which is strongly temperature dependent. On the other hand, the elevated workpiece temperature might be disadvantageous for the bond formation. According to multi-material forging investigations diffusion can limit the bond strength as a result of excessive width of intermetallic phases [12]. The surface layer's ductility tends to increase with its temperature. Furthermore, oxide layers are increasingly built up at higher temperatures. Due to the reduced flow stress the contact stress tends to decrease at elevated temperature. In summary, the question rises, whether forge bonding at elevated temperatures is favorable with regard to the bond formation and bond strength or if the increased ductility, recovery and oxidation oppose to the formation of a sound bond.

2. Experimental setup

In order to determine the forming temperature's effect on the bond formation, billets of the two workpiece materials steel (C 15) and aluminum (AW 6082 T6) are heated by inductive heating prior to forming by combined forward and cup extrusion at a designated temperature. According to former investigations carried out by the authors [4, 13], the steel billets have been soft annealed at 690 °C for one hour and the aluminum billets are

processed in the artificially aged state (T6). Fig. 1 illustrates the processing sequence for the warm forge bonding of aluminum and steel (a), the measured starting temperatures (b) and exemplarily the temperature decrease for the last 10 s of handling prior to extrusion for the two warmest forming states of the steel and aluminum billets (c).

Fig. 1. Experimental setup and tool geometry a), starting temperatures b) and exemplary thermography images of the last 10 s prior to forging.

The initial forming temperatures of the steel billets are adjusted to room temperature, 100 °C, 200 °C, 450 °C and 600 °C. They are extruded with aluminum billets which possess starting temperatures of room temperature, 100 °C and 200 C. Until the start of the extrusion process the billet's preparation takes about 40 s after the end of the heating cycle. In order to form the billets with its destined temperature the preparation cycle has been controlled by an infrared camera (ThermaCAMTM S65). If both billets are processed at an elevated temperature, the aluminum billet is heated firstly and remains in a temperature controlled heater after brushing until both parts are transferred to the extrusion die. A pre-correction of the billets' heating temperature is necessary in order to reach the desired temperature value after 10 s of handling, 20 s of brushing and additional 10 s of handling. Therefore, the emissivity values for the aluminum and steel surface in the condition machined (prior to brushing) and brushed have been determined for each temperature combination and controlled during the duration in the die which is only about 3 s before the forming starts. The billets are formed by a direct driven 2.500 kN servo press with a constant ram velocity of 180 mm/s and the stroke is kept constant (29 mm) for all samples.

3. Experimental results

3.1. Formed geometries

The formed samples are checked for a sound geometry for all temperature combinations. Fig. 2 illustrates a set of geometrical representatives for all formed parts.

Fig. 2. Geometric representatives for all tested temperature combinations.

It can be seen that there is a broad variety of temperature sets in which sound form filling and a weld can be generated. Up to an initial temperature of 450 °C for the steel billet and 100 °C for the aluminum billet, sound bonding and form filling occurs (c). Furthermore, the combination of a 600 °C steel billet with a 200 °C aluminum billet leads also to a similar geometry (d). However, if the initial temperature of the billets is not adjusted well, two kinds of failures can be observed. If the flow stress of the aluminum workpiece is too low (a), the extruded steel cup ruptures at the end of the punch's calibration zone due to exceeding the ultimate tensile stress. If the flow stress of the first part (e.g. the steel part in this setup) is nearly at the same level than the second part's flow stress, the first part is primarily formed and lifts off (b). If the flow stress of both materials is adjusted well, like in the case of the 600 °C steel billet and 200 °C aluminum billet sound form filling and bonding occurs (d).

3.2. Microscopic analysis of the weld formation

According to Fig. 2, there are several temperature sets, which lead to a sound geometry. However, the question arises how the bond formation is affected by the elevated forming temperatures. For a deeper insight into the weld interface, bonded samples are cut in half and investigated with a scanning electron microscope (SEM, Jeol JSM 6610 LV) after polishing. Fig. 3 illustrates the weld interface between a steel aluminum combination formed at room temperature in comparison to a combination of steel and aluminum with a starting temperature of 600 °C and 200 °C, respectively. In the backscatter electron contrast the material's density determines the structure's brightness. In the regarded case, heavier material (steel) is illustrated brighter than the lighter material (aluminum).

Fig. 3. Comparison of the weld interface of a cold and warm forged component.

A metallurgical weld is established at the interface of the cold and warm forged component. However, the unsteady extrusion conditions and the lack of contact stress at the positions 4 and 8 lead to the absence of a weld. At the other positions a metallurgical weld is established. A typical cold welded interface is generated at the positions 1-3. Here, the material welds without the presence of largely scaled intermetallic phases which would lead to a greyscale contrast at the interface. At position 1 fragments of the brushed covering layer can be identified. These fragments, which provide the same material contrast as steel, can be found randomly distributed all over the weld interface. Due to the high punch velocity and the short part's cooling time at air, no largely scaled intermetallic phases can be identified for the warm forged components either. However, significant differences can be made between a cold and warm forged component. Due to rapid reoxidation after brushing scales build up and remain at the weld interface as additional fragments of the covering layer. The different contrast of the oxides (5) in comparison with the steel fragments in (1) illustrates the different composition of the particles, which act as a covering layer at the start of the forging process.

Fig. 4 illustrates the weld interface formation at different starting temperatures in combination with an energy dispersive X-ray (EDX) acquisition of that region. The acquired position corresponds to position 2 and 6 in Fig. 3.

Starting temperatures:

Fig. 4. Elementary composition of the weld interface at different starting temperatures.

The samples, which are joined at room temperature (a), do not show any noticeable accumulation of oxygen at the weld interface. As soon as the steel's starting temperature exceeds 200 °C a 2 - 4 ^m thick seam is built, in which a significant accumulation of oxygen is detected. At a starting temperature of 450 °C considerable scale formation sets in. For the starting temperature combination Tsteel 450 °C and Taluminum 100 °C (d) a very thick (~22 ^m) seam can be detected. This seam at the weld interface does not consist of a homogenous composition. Fragments of the steel's covering layer, scale fragments and precipitations of the aluminum's alloying elements are surrounded by aluminum. However, samples at even higher starting temperatures do not present such a relatively largely scaled seam at the weld interface. Here (e), only fragments of the relatively brittle scale can be found at the interface. Fragments of the steel's brushed covering layer are not visible at a large scale. However, the question arises, whether the oxide layers interfere with the bond formation negatively or if higher temperatures promote the bond strength.

3.3. Determination of the joint strength

In order to determine how the joint strength is affected by different initial forming temperatures, the steel part is extracted from the aluminum cup. Therefore, samples with a thickness of 3 mm are cut out from the forged parts. Due to the parts' geometry, the metallurgical weld is superposed by a mechanical joint. In order to determine the proportion of mechanical joint to metallurgical weld, the samples are extracted as illustrated in Fig. 5. The samples are extracted as whole samples and half samples. The extraction force for the whole samples contains the mechanical and metallurgical joint. The measured forces for these samples are divided by two for the sake of a better comparability to the other samples. Furthermore, the samples were sliced according to the illustrated regions to determine which region provides the highest bond strength

mechanical + metallurgical I II III

metallurgical weld weld (I | II | III) Fig. 5. Evaluation of joint strength by extraction.

The metallurgical weld provides about 72.1 % to 94.5 % to the overall bond strength. Furthermore, region II is characterized by comparatively high extraction forces, whereas region I and III provide only little extraction forces. The initial temperature sets of 200 °C | room temperature and 450 °C | room temperature needed very low forces until the parts are extracted. However, there are two sets, 450 °C steel and room temperature aluminum as well as 600 °C steel and 200 °C aluminum providing nearly as good or even better joint strengths as the cold forged samples. However, the deviations for these extraction forces are very high, so that the illustrated graphs only represent a tendency.

4. Conclusion

Cold and warm forging of two aluminum and steel billets can be utilized to establish a metallurgical weld. In addition to cold forging, warm forging enables the equalization of both deformable parts' flow stress. However, forming of steel at elevated temperatures leads to an increased oxidation of the surface near regions. These oxides, which act as a covering layer can be found at the weld interface. Largely scaled intermetallic phases do not occur in the investigated setup. The extraction tests reveal that sound bonding can also be established by forming at higher temperatures. Despite relatively high deviations, the bond strength seems to be increased at certain temperature levels.

Acknowledgements

The authors want to express their gratitude to the DFG (Deutsche Forschungsgemeinschaft) for the support of the project "Investigation and enhancement on bonding by cold bulk metal forming processes" (GR 1818/48-1).

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