A Detective Story! Sustaining Member. Surface Technology Support

Transcription

1 A Detective Story!

2 And you, the audience, can be the Jury

3 Every good detective story has the following; A Victim A Crime! A Scarlet Woman An MO modus operandi The Crime Scene The Evidence A Suspect A Perpetrator

4 The Victim My client s 7075 T651 aluminium alloy impeller housing for a hydraulic pump used in a commercial aircraft. Machined from 14 cm diameter extruded bar. Type II sulphuric acid anodised to microns, dyed black and hot water sealed

5 The Crime! Anodising flaking off but only after heating to 150 o C

6 A Scarlet Woman

7 The MO Has this been seen before? A literature search found that in the forums at finishing.com and pfonline.com similar problems have been reported Some We do 7075 Type parts III (hardcoat) were type II anodizing. anodized Occasionally, 20 micron thick we (.0008") get parts per to anodize milspec. are made They from were 7075-T6 subsequently alloy. cleaned When these in a cleanroom, parts are anodized in ultrasonic they sometimes that cleaning exhibit what stations looks with like D.I blisters and (supposedly) in the anodic anodize-safe coating. detergents, dips in isopropyl alcohol and D.I., and a vacuum bake approaching 100 deg. C. When the tape was repositioned, the anodized finish came off. Based on a lit search and preliminary theories, ultrasonic cleaning was blamed for crazing the parts, and alcohol was accused of dehydrating the parts.

8 The Crime Scene In this case, the crime scene is on the victim The directionality of flaking is not consistent with the lay Note the elongation of the flakes in one direction Direction of the bar extrusion

9 On faces transverse to the extrusion direction No direction to the flaking fewer flakes

10 The Preliminary Evidence Examination of the anodising process showed no faulty processing Pitting of the faces transverse to the extrusion direction sometimes seen after machining in which case no adhesion failure observed on these faces Areas under the flakes insulating or electrically conductive - variable Adhesion failures variable sometimes bad, sometimes slight, sometimes no failure at all even from the same material batch TIME FOR A MICROSECTION or two!!!

11 Microsection of the 7075 material Direction of the bar extrusion

12 What are the particles? SEM EDX analysis showed Al 7 Cu 2 Fe An intermetallic compound Other particles of Mg 2 Si also found within the clusters

13 Microsection of the anodised coating - unheated Aluminium Particles Anodised coating

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15 Microsection of the anodised coating heated at 150 o Mount Anodised coating Aluminium

16 Microsection of the anodised coating heated at 150 o Particles Vertical cracks Horizontal cracks

17 Microsection of the anodised coating heated at 150 o Vertical cracks Mount Anodised coating Horizontal cracking via particles Aluminium

18 Microsection of the anodised coating heated at 150 o Section fractured off half way through coating Mount Anodised coating Aluminium

19 The Suspect It appears that bands of inter-metallic particles are becoming incorporated into the anodised coating. Heating an anodised coating causes dehydration and vertical cracking, causing a dried mud appearance but does not usually lead to poor adhesion. However, incorporated particles allow horizontal cracking to occur leading to affected regions flaking off. Therefore the 7075 material is the prime suspect

20 Source of intermetallic particles Original cast billet of 7075 alloy with centre cored build up of intermetallics in centre. During extrusion, the core, rich in intermetallics breaks up, resulting in lines of intermetallic particles being distributed along the extrusion direction.

21 Distribution of intermetallic particles The extruded bar will now show bands of intermetallics along its length but the ends will only show individual particles. Solution heat treatment of the bar is not very effective at re-dissolving the intermetallics in 7000 series alloys. The end faces on the housing can exhibit pitting if care is not taken with machining as the individual hard particles can be pulled out, leaving pits in the surface.

22 During anodising Anodic coating During anodising, the coating grows both inwards as well as outwards. If rows of intermetallics are close to the surface they will not be dissolved by pre-treatments but become incorporated in the coating.

23 Proposed crack and flaking mechanism When the anodic coating is heated, it cracks because it dehydrates. Usually cracking is vertical and does not affect the adhesion of the coating. The presence of the intermetallics causes horizontal cracks to form and join up as it is a path of least resistance. When the horizontal crack meets vertical cracks, the affected section can flake off.

24 Proposed crack and flaking mechanism 2 If the intermetallic particles are present at the aluminium/coating interface, then horizontal cracking occurs along the interface and all the coating can flake off Hence, regions under the flakes can be conducting or non-conducting

25 Summary of the Evidence The anodising process was sound Bands of intermetallics were found throughout the alloy The intermetallics can become incorporated into the anodic coating unchanged Heating causes dehydration and vertical cracking but horizontal cracks occur through incorporated particles This can lead to flaking of the anodic coating either within the coating or at the coating/substrate interface Changes in cleaning/etching would have little effect as the coating grows inwards towards the bands of intermetallics Variability in adhesion is a function of the density of the bands of intermetallics

26 Summary of the Evidence Because the banding is directional along the extrusion path, then the flaking seen in the direction of extrusion will also be directional Faces transverse to the extrusion direction will not show banding because only individual particles are present at the surface

27 It appears that the perpetrator of the crime was The 7075 T651 aluminium alloy

28 How can this crime be prevented in future??? That is what my client paid for... Any Questions?? And so its over to you members of the Jury