how to make the ball expand on contact?

The code.

local enabled = true
Player = script.Parent.Parent
character = Player.CharacterAdded:wait();
mouse = Player:GetMouse()

function onKeyDown(key)
    key = key:lower()
    if key == "x" then
        b = Instance.new ("Part")
        b.BrickColor = BrickColor.new("Bright violet")
        b.Shape = "Ball"
        b.Size = Vector3.new(3, 3, 3)
        b.TopSurface = "Smooth"
        b.BottomSurface = "Smooth"
        b.CanCollide = false
        b.Anchored = false
        b.CFrame = Player.Character.Torso.CFrame *CFrame.new(0, 0, -12)
        b.Parent = workspace
        game.Debris:AddItem(b,6)
        b2 = Instance.new("Part")
        b2.BrickColor = BrickColor.new("Lavender")
        b2.Shape = "Ball"
        b2.Size = Vector3.new(5, 5, 5)
        b2.TopSurface = "Smooth"
        b2.BottomSurface = "Smooth"
        b2.Transparency = 0.7
        b2.CanCollide = false
        b2.Anchored = false
        b2.CFrame = Player.Character.Torso.CFrame *CFrame.new(0, 0, -12)
        b2.Parent = b
        game.Debris:AddItem(b2,6)
        y = Instance.new("BodyVelocity")
        y.maxForce = Vector3.new(math.huge, math.huge, math.huge)
        y.velocity = Player.Character.Torso.CFrame.lookVector*80
        y.Parent = b
        f = Instance.new("Fire")
        f.Size = 5
        f.Heat = 15
        f.Color = Color3.new(101, 101, 152)
        f.SecondaryColor = Color3.new(170, 170, 255)
        f.Parent = b        
        weld = Instance.new("Weld")
        weld.Part0 = b2
        --weld.C0 = b2.CFrame:inverse()
        weld.Part1 = b
        weld.Parent = b
    end
end

mouse.KeyDown:connect(onKeyDown)

I'm making fighting moves so you can imagine the effect i want.