All of the internal pieces that do the work are metal (with some metal-based mesh). The only plastic piece is "smoothie" attachment, which I've, honestly, never used - as I'm a much bigger fan of just plain and simple fresh-pressed juice. But I love this machine. It's durable. Clean up is quick and easy. And the juice that gets extracted is so tasty! I highly recommend this product.
Since it is a rotary machine, it juices with speed. I use all of those ingredients, but it does take a lot of kale or other leafy greens to make enough. I cram everything that fits through my juicer and it grinds through it with speed and ease. Clean up is a breeze. From my research, the masticating ones are more efficient, but slower and more parts to clean. Neither were acceptable to me, so chose this one and am very happy.
I don't know if the metal is aluminum. Breville may be able to tell you if you contact them. Juice does come in contact with metal inside the juicer via the juice drain that pours into the big pitcher that comes with the juicer.
It takes a lot of greens to get a little juice. I've heard masticating ones are better, but I preferred the speed and ease of cleaning. There is lots of fibrous "waste," but I love the juice. This is a great product. Easy to clean. Fast.
The amount of foam it produces is based entirely on the speed that you have it set at. I always keep my speed on the lowest setting unless I need it higher to cut through tougher items, but then I turn is back down. You maximize the amount of juice you get that way. If it goes to fast, it slings more product into the hopper before it has a chance to extract it all.
Foam does not bother me as it dissipates rather quickly. It just makes the drink more frothy, which I prefer anyway. When you juice multiple items in the same batch, you will need to stir it. The foam does not sit on top, like say a beer.