The base material was an EN-AW 6016 sample with temper designation T4 (solution heat treated - naturally aged to stable temper). The material has been treated by a diode laser (P= 1000 W, v= 100 mm/s, d(focus)= 5 mm). Besides that, a reference sample has been artificially aged for four hours, at 155°C to give a temper designation T6. Those samples and another untreated reference sample (EN-AW 6016-T4) were exposed to a tensile test.
The base material (T4) and the laser treated sample has shown totally different forces / strengths and elongations. It is noticeable, that the laser treated sample sheared off near bye the treated zone. However, there is no remarkable change in both hardness and microstructure visible.
The artificially aged reference sample, however, has shown increased shear forces, but reduced elongation compared to the T4 reference sample and greater elongation than the laser treated T4-sample.
It is questionable if the short treat input of the laser treatment (2s) is sufficient to change the temper designation?
There is an error, where How mechanical properties change without microstructure change, in addition to the mechanical properties associated with each one of them Climate change means that all properties and the basis of the change is the microscopic structure.
Let us start with fundamentals. Firstly, what was the surface of laser beam on the surface of your sample ? Than calculate the power density= 1000/this surface. If the density was more than 1 kW/cm^2 you have probably melted the surface of Al samples. I say probably, because it depends of the reflexion from the surface.
Second point. Observe, the surface after laser treatment. If it shows the traces of melting, then your laser melted the Al sample. And, consequently, modified its microstructure.That is why some properties of laser treated samples have changed.
I guess you start with experimentation. Slow down, take a breath, and think it slowly over. There are no miracles such as change of properties without changing of microstructure.
Did you induce residual stresses? They don't necessarily show up during indentation. b
From your comment: "...there is no remarkable change in both hardness and microstructure visible." You may need to examine the micorstructure at a higher magnification than usual. If your hardness measurement is "macro", the indenter may be completely penetrating a micro surface layer. Please see the link to a photomicrograph from TWI.
From Christoph's comment above: if the surface has melted during the laser treatment, the bulk of your part acts as a large heat sink, creating an "as-quenched" characteristic on the modified surface. Since your part is aluminum, the observable effect may diminish with time/aging.
I agree with Lech, properties cannot change without changing the microstructure.
http://www.twi-global.com/EasysiteWeb/getresource.axd?AssetID=13586&type=full&servicetype=Inline
http://www.twi-global.com/technical-knowledge/published-papers/laser-welding-of-aluminium-alloys-using-different-laser-sources-june-1998/
Dear all,
thanks for the advices. Yes, there is a higher laser density than the mentioned one (~5,3 W/cm² with P=1000W, d=5 mm) but ther is no molten surface visible (cross section, top view; each treated with Dix/Keller, Kroll and color atching, mag 500x).
Moreover, base material was not exposed to any additional residual stresses during processing.
Our intention was that the effect is attributed to a change in the time-temperature- ratio (TTT diagram; high energy in short time ~ less energy in long time).
With about 5 kW/cm2 you had to melt sample. Maybe the reflexion was very high. If you try to paint it black or so to see the difference?
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