Additive Manufacturing (AM), also known as 3D printing, using high-temperature thermoplastics has been proposed for the production of structures in space. To determine whether this is feasible, a systematic study investigating the impact of the space environment on the AM process must be conducted. Existing investigations have focussed on AM in microgravity and AM of either low-temperature thermoplastics or individual high-temperature thermoplastics at low pressure. This study examines the mechanical characteristics of high temperature, space compatible thermoplastic parts produced by AM in a low-pressure environments. Four different thermoplastic filaments were investigated—polyether ether ketone (PEEK), polyether ketone ketone (PEKK), and two types of polyetherimide (PEI) (ULTEM 9085 and ULTEM 1010). Their mechanical performance, internal structures, and crystallinity were compared to those manufactured under atmospheric conditions. It was observed that the low pressure enabled the manufacture of ULTEM 1010 specimens, which could not be achieved in ambient atmosphere. The low pressure additionally improved the interlayer adhesion of PEEK and ULTEM 9085 specimens. PEEK, PEKK, and ULTEM 1010 specimens exhibited bubbling when printed under low-pressure conditions, most noticeable in the compression and H-90° samples, resulting in worse mechanical properties than samples manufactured in atmosphere. The mechanical performance of bending and H-0° specimens remained consistent irrespective of the printing environment; this was thought to be due to conduction to the print bed being the dominant form of heat transfer, resulting in similar thermal histories.

In the automotive industry, structures and components are becoming increasingly multi-material. Materials such as steel, aluminum, and carbon fiber composite materials, among others, are used in a composite manner to improve the mechanical utility and performance of each component. The most traditional technique for assembling dissimilar materials is mechanical fastening. However, mechanical fastening requires numerous manufacturing processes, affects the inherent mechanical properties of the base material, and increases weight due to extra metal parts. Adhesive bonding is increasingly being used in more and more industrial fields as a method that avoids the above drawbacks. One disadvantage of adhesive bonding is the long-life durability of the joint when exposed to harsh conditions such as the presence of water or corrosive environments^[1]. To ensure this long-life durability of the joint, surface pretreatment of the material is always necessary. Among the many surface pretreatments, laser surface treatment is an efficient method that corrects both the physical and chemical properties of the surface at once, with numerous advantages such as requiring only electrical energy, being easy to integrate into the production process, and not using chemicals. Laser surface treatment of aluminum alloys has been shown to improve the durability of joints through the formation of passivation oxide films and roughening of the material surface^[2,3,4]. The laser-treated aluminum alloy surfaces are expected to undergo physicochemical changes over time due to absorption of moisture and organic compounds in the storage atmosphere as well as storage temperature, and the above anticorrosion effects of laser treatment are expected to change as well. In industry, material surfaces are subjected to a certain storage period after laser treatment before they are joined, but the effects of storage methods on the corrosion of treated surfaces and joint interfaces have not yet been fully investigated. In this study, laser-treated aluminum alloys were stored in a high-temperature, high-humidity environment, then joined and DCB tests were performed to investigate how the corrosion protection effect on the material surface given by the laser treatment changed during storage and how it affected joint strength. The storage period was varied from none to one month, and the results of indoor storage are also shown for comparison. Through the above, the appropriate storage environment and storage period after laser treatment will be discussed. This presentation is based on results obtained from a project, JPNP2005, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). [1] Johannes M. C. Mol, “Interface strength and degradation of adhesively bonded porous aluminum oxides,” npj Materials Degradation, Volume 1, Article number 8, 2017, DOI: 10.1038/s41529-017-0007-0 [2] Elisabeth Stammen, “Surface Modification with Laser: Pretreatment of Aluminium Alloys for Adhesive Bonding,” Tenth International Conference on Plasma Surface Engineering (PSE2006), Volume 4, Issue S1, Pages S39-S43, 2007, DOI: 10.1002/ppap.200730303 [3] Rico Rechner, “Influence on the strength and aging resistance of aluminium joints by laser pre-treatment and surface modification,” International Journal of Adhesion and Adhesives, Volume 30, Issue 7, Pages 595-601, 2010, DOI: 10.1016/j.ijadhadh.2010.05.009 [4] Jianping Lin, Junying Min, “Effect of laser ablation treatment on corrosion resistance of adhesive-bonded Al alloy joints,” Surface and Coatings Technology, Volume 345, Pages 13-21, 2018, DOI: 10.1016/j.surfcoat.2018.03.087

Material-hybrid designs made of metal and plastic are becoming increasingly important in the automotive and aerospace industries. They place new demands on joining technology in particular. One promising process is the fusion bonding of metals and fiber-reinforced thermoplastics. In this process, the thermoplastic matrix material of the fiber-reinforced composite component is melted and a direct adhesive bond is created with the metallic joining partner. This joining technology has already been extensively studied in the past. In particular, the interface between the joining partners has been identified as a critical factor for the long-term quality of the joint. The interface can be altered by influences during production, such as e-coat process or the effects of harsh environmental conditions during operation like corrosion and thermomechanical loads. Determining the extent of these influences on the interface condition and thus the quality of the joint is difficult, but it is essential to support the use of the joining method in safety-critical areas. Previous investigations of this joining method have mostly been carried out destructively using tensile shear tests and non-destructively using fiber optic and piezo ceramic sensors, but all approaches have only shown limited significance with regard to the metal-plastic interface in detail. Electrochemical impedance spectroscopy (EIS) as a very sensitive measurement method could represent an almost non-destructive and at the same time very sensitive alternative which is currently not reported for this use case in the literature. In previous internal investigations, we found that EIS can be used to measure layer thicknesses of up to 3.5 mm, contrary to the usual use of this test method for characterizing the properties of thin coatings, corrosion behavior and battery performance. In fusion bonding applications, typical material thicknesses of thermoplastics above the interface are around 1.5 mm and are thus well above the usual layer thicknesses of approx. 20 µm to 100 µm for typical EIS applications. This study aims to examine how EIS can be used advantageously for the interface characterization of fusion bonded specimens and up to which extent the EIS can provide a statement about the metal-plastic interface. In this study, a joint of a laser-pretreated steel and a glass fiber-reinforced PA6 is investigated. We measured specimens directly after production, after a treatment similar to an automotive e-coat process and after certain intervals of a salt spray test. Using the Kramers-Kronig transformation, the EIS data was validated and qualitatively analyzed with Nyquist and Bode diagrams, the impedance at 0.1 Hz, the maximum impedance and equivalent circuits. Tensile shear and edge shear test serve as destructive reference tests to determine strength parameters of the joints and evaluate the respective fracture patterns. This opens up the possibility to correlate the results of EIS and destructive measurements. In summary, EIS is suitable for characterizing the metal-plastic interface, even with polymer thicknesses of 1.5 mm, which are comparatively high for EIS. Due to the very low phase angle and the resulting low dielectricity of the glass fiber-reinforced thermoplastic, changes in the impedance data are directly related to the interface condition. The measured impedance spectra could be differentiated with Nyquist and Bode diagrams, the impedance at 0.1 Hz and the maximum impedance.

Metal-polymer-joints can be found in multiple today’s major applications like in hybrid car parts in terms of lightweight construction or molded electrical contacts for high-voltage applications. Common to both is the harsh environment they are exposed to including elevated temperature changes inducing thermomechanical stresses based on the differences of the CTE values as well as corrosive media like salty water, solvents or electrolytes. Each promotes its own corrosion mechanism that in most cases comes along with a degradation of the material’s and therefore the part’s performance in service. Especially, two mechanisms have to be considered when dealing with corrosion of metal-polymer-joints. Firstly, corrosion processes of metal and polymer can influence each other as well as the metal-polymer interface layer depending on the material combination. Secondly, thermomechanical stress due to temperature changes can also have an effect on the material’s corrosion behavior. Studying both mechanisms would help to improve the service time of hybrid parts. The work introduces a new method to investigate corrosion processes in metal-polymer-joints combining the influence of corrosive media and thermomechanical load. A hybrid specimen consisting of a specially formed metal inlay fully surrounded by a potting polymer is used for the measurements. When exposed to cyclic temperature load in a test chamber mechanical stress is induced in the polymer due to the different CTE of the materials. For the measurements typical material combinations used in lightweight construction and high-voltage applications are used. The hybrid specimens are exposed to thermal shocking cycles while being fully surrounded by corrosive media. Throughout the experiment, the specimens are constantly monitored using electrochemical impedance spectroscopy (EIS). In addition, a visual inspection of the specimens takes place after certain times to analyze the overall condition and possible cracks. As a reference, the same setup is applied only to the metal inlays used in the hybrid specimens. By interpreting the results of the EIS measurements, possible corrosion processes of metal and polymer as well as their correlation can be characterized in detail. Additionally, the comparison of hybrid specimens and raw metal inlays reveals the influence of the thermomechanical load on the corrosion process. The experimental part is supported by FE-simulations, which are also supplementary presented in this investigation.

When performing vibration-based measurement techniques, e.g., for structural health monitoring, piezo-electric elements are often used as the sensor and/or actuator part due to their durability. The connection between the piezo-electric element and the test structure plays a decisive role in the quality of the results obtained in the vibration-based measurement process. In addition to stable and mode-independent vibration transmission, further requirements such as reversibility or temperature resistance can be imposed depending on the application. In many preliminary studies, bonding by means of a two-component epoxy resin was practically and simulatively validated for a large amount of use cases. Nevertheless, the limited reversibility and the preparation time of the adhesion process can limit the flexibility of the vibration-based measurement method. The aim of this work is to evaluate different joining technologies of piezo-electric elements on metal and polymer substrates, especially with respect to reversibility. Different techniques for joining piezo elements are collected, considering previous work as well as newly developed approaches within the scope of the work. In the next step, frequency spectra of simple circular blanks are obtained using an EMI setup of piezo elements joined to the blanks with the appropriate joining technique. Joining by means of a two-component epoxy is considered as the reference method. All joining techniques are evaluated, especially based on the degree of reversibility, transmission quality, effort for implementation and durability, in comparison to the reference method. Finally, recommendations regarding the proper joining technique for different experimental conditions are given based on the results.

Additive Manufacturing (AM), also known as 3D printing, using high temperature thermoplastics has been proposed for the production of structures in space. To determine whether this is feasible a systematic study investigating the impact of the space environment on the AM process must be conducted. Existing investigations have focussed on AM in microgravity and AM of either low temperature thermoplastics or individual high temperature thermoplastics at low pressure. This study examines the mechanical characteristics of high temperature, space compatible thermoplastic parts produced by AM in a low pressure environments. Four different thermoplastic filaments were investigated - poly ether ether ketone (PEEK), poly ether ketone ketone (PEKK), and two types of polyetherimide (PEI) (ULTEM 9085 and ULTEM 1010). Their mechanical performance, internal structures and crystallinity were compared to those manufactured under atmospheric conditions. It was observed that the low pressure enabled the manufacture of ULTEM 1010 specimens, which could not be achieved in ambient atmosphere. The low pressure additionally improved the inter-layer adhesion of PEEK and ULTEM 9085 specimens. PEEK, PEKK and ULTEM 1010 specimens exhibited bubbling when printed under low pressure conditions, most noticeable in the compression and H-90° samples, resulting in worse mechanical properties than samples manufactured in atmosphere. The mechanical performance of bending and H-0° specimens remained consistent irrespective of the printing environment, this was thought to be due to conduction to the print bed being the dominant form of heat transfer, resulting in similar thermal histories.